The microstructures and low-cycle fatigue behaviors of steels with different Mn contents subjected to continuous cooling heat treatment processes (from Ms +10 °C to Ms −20 °C) are examined. Phase transformation testing results show that the steel without Mn exhibits short transformation time and short incubation period. When Mn content increases from 1.8% to 3.2%, bainite transformation time is prolonged, especially incubation period. The microstructure of the steel without Mn is mainly grain boundary allotriomorphic ferrite, while the other three steels with Mn consist of bainitic ferrite plate and retained austenite. Mn is an essential element used to obtain lower bainite. The low-cycle fatigue of the steels undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. Higher elongation of the steel without Mn is beneficial to improve fatigue life under plastic strain amplitudes. The Coffin–Manson formula and damage hysteresis model are then used to evaluate the fatigue performances of the steels. It shows that the steel with 2.3% Mn exhibits higher fatigue damage capacity (W0) with a considerable damage transition exponent (β) than the other three steels. This result attributes to the largest distribution of high-angle misorientation of in the bainitic steel with 2.3% Mn.

Interest in using aluminum in nano-bainite steel, especially for high-carbon bearing steel, is gradually growing. In this study, GCr15SiMo and GCr15SiMoAl steels are introduced to investigate the effect of Al alloying on the hot deformation behavior of bearing steel. Results show that the addition of Al not only notably increases the flow stress of steel due to the strong strengthening effect of Al on austenite phase, but also accelerates the strain-softening rates for its increasing effect on stacking fault energy. Al alloying also increases the activation energy of deformation. Two constitutive equations with an accuracy of higher than 0.99 are proposed. The constructed processing maps show the expanded instability regions for GCr15SiMoAl steel as compared with GCr15SiMo steel. This finding is consistent with the occurrence of cracking on the GCr15SiMoAl specimens, revealing that Al alloying reduces the high-temperature plasticity of the bearing steel. On the contrary, GCr15SiMoAl steel possesses smaller grain size than GCr15SiMo steel, manifesting the positive effect of Al on bearing steel. Attention should be focused on the hot working process of bearing steel with Al.

The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 °C followed by tempering within 240–450 °C. Results show that the sample tempered at 340 °C occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 °C and 360 °C with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 °C. When the temperature is increased to 450 °C, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 °C, followed by decreasing with further increase in the temperature. This behavior is related to the competition between retained austenite further transforming into bainite and decomposing into carbide during tempering.

Low-cycle fatigue behaviors of the pre-hardening (PH) and the water-quenching (WQ) Hadfield steel were studied using optical microscopy, transmission electron microscopy, and electron backscatter diffraction technique. The effect of the PH treatment on low-cycle fatigue behavior of the Hadfield steel was analyzed through comparing the cyclic hardening/softening behaviors and the changing regulations of stress amplitude, internal stress, and effective stress at different total strain amplitudes. Results showed obvious differences in fatigue behaviors between the PH (with a cold rolling deformation degree of 40%) and the WQ Hadfield steels. Transient hardening followed by cyclic stability behavior occurred in the PH Hadfield steel under cyclic loading, whereas cyclic softening behavior was barely observed. The fatigue life of the PH Hadfield steel was higher than that of the WQ Hadfield steel at relatively low strain amplitudes, while a contrary result was obtained at relatively high strain amplitudes. At low strain amplitudes, the deformation twins induced in the PH Hadfield steel could enhance the multiplication and slip process of dislocations, which actually improved the deformation uniformity. The long-range motion of dislocations was intensified at high strain amplitudes. However, the dislocation motion was also blocked by twin boundaries. As a result, the interactions between dislocations and deformation twins enhanced, finally causing severe dislocation accumulation. These two effects of deformation twins on dislocation motion eventually resulted in different low-cycle fatigue behaviors of the PH Hadfield steel.

The microstructures and tensile behaviors of traditional Hadfield steel, named Mn12 steel, and Hadfield steel alloyed with N+Cr, named Mn12CrN steel were studied through optical microscopy, transmission electron microscopy, and scanning electron microscopy, among others. Three different tensile strain rates of 5×10−4, 5×10−3, and 5×10−2 s−1 were selected in the tensile test. The deformation microstructures and fracture morphologies of the two steels after fracture in the tensile test were observed to analyze the tensile deformation response to different tensile strain rates. Results showed that the grain size of Mn12CrN steel was evidently refined after alloying with N+Cr. The grain would not become abnormally coarse even with increasing austenitizing temperature. During tensile deformation, the strength and plasticity of Mn12CrN steel were superior to those of Mn12 steel at the same strain rate. With increasing the strain rate, the changes in strength and plasticity of Mn12CrN steel were less sensitive to tensile strain rate compared with Mn12 steel. The effects of grain refinement and N+Cr alloying on dynamic strain aging and deformation twining behaviors were responsible for this lack of sensitivity to strain rate.

•The effects of high-temperature deformation and cooling process on the microstructures and mechanical properties of a pearlite steel were researched.•A new pearlite steel containing high Cr and high Si was obtained.•A fully pearlite microstructure with an interlamellar spacing of approximately 100 nm was obtained.•A full pearlite steel with a hardness of HRC50, a tensile strength of 1715 MPa and an elongation of 18% can be obtained.In this study, a new type of pearlite steel, 80CrSiV, is designed and the effects of the cooling rate, final cooling temperature, deformation temperature and level of deformation on the microstructures and mechanical properties of the steel are determined through thermal simulation, scanning electron microscopy, transmission electron microscopy, hardness and tensile tests. Results show that the microstructures and mechanical properties of the steel are significantly affected by the cooling rate after deformation and the final cooling temperature but are only slightly affected by the deformation temperature and level of deformation. A full pearlite microstructure with a hardness of HRC50.3, a tensile strength of 1715 MPa and an elongation not less than 17.9% can be obtained under the condition of a cooling rate of 120 °C·min− 1 and a final cooling temperature of 550 °C.Download high-res image (115KB)Download full-size image

In the paper, different morphologies of bainite were obtained through isothermal quenching at 320 °C and 395 °C in a medium-carbon carbide-free bainitic steel. The cyclic deformation mechanism was explored by using low cycle fatigue testing. The volume fraction of retained austenite was measured by X-ray diffraction and the space partitioning of the solute atoms was constructed by three-dimensional atom probe. Results showed that the fatigue life at 320 °C was always higher than that at 395 °C under low and high total strain amplitude. The cyclic softening at the early fatigue stage increased the plastic strain of the sample which was responsible for the reduction of the fatigue life at 395 °C. Strain-induced retained austenite to martensite contributed to initial cyclic hardening, but almost having no effect on the subsequent cyclic stable/softening behaviors. The finer bainitic ferrite sheaves obtained at 320 °C changed the small fatigue crack propagation direction and delayed the crack propagation rate, which was beneficial for the fatigue properties. In addition, the substitutional atoms did not redistribute between the retained austenite and bainitic ferrite before and after cyclic deformation.

The effects of deformation on the microstructures and mechanical properties of carbide–free bainitic steel for railway crossing were studied in detail, with an aim of analyzing the hydrogen embrittlement characteristics of the deformed bainitic steel. A small deformation could cause a notable increase in the strength and a slash decrease in uniform plasticity. High density of dislocations accumulated during the deformation and the newly formed high strength martensite should be responsible for the changes of mechanical properties. Results from the slow strain rate tests reveal that the hydrogen embrittlement sensitivity is higher for the deformed bainitic steel as compared with the undeformed steel, which also increase with increasing reduction. It should be attributed to the newly formed martensite phase, the accumulated dislocations and the heterogeneous residual stress in the deformed specimens. Fractography observations reveal a changed fracture mode from ductile to brittle with increasing hydrogen charging time.

A novel process, quenching to a low temperature far below Ms followed by a short austempering treatment (Q-A-T), was carried out on a low-carbon martensite steel. Results show that a notably improved toughness without losing strength was obtained on the Q-A-T specimen as compared with the oil-quenched specimen. Microstructure analyses reveal that the Q-A-T specimen consisted of low-carbon martensite, high carbon–low temperature bainite and retained austenite. The fast quenching process effectively refined the martensite plate and the austempering process introduced the high strength phase–low temperature bainite, both of which are responsible for the improved mechanical properties.

Amicromechanicsmodel composed of a three-phase confocal elliptical cylinder of the nanocompositeswith interface effect is proposed.Ageneralized self-consistent method for the piezoelectric nanocomposites accounting for fiber section shape under far-field mechanical–electrical loads is presented based on the model. By using the theory of Gurtin–Murdoch surface/interface and the conformal mapping technique, a closedform solution of the effective electroelastic constants is obtained. The present solution can be degenerated into the existing solution. The results show that the effective electroelastic constants are dramatically size dependent when the size of the fiber is on the order of nanometers. The effective electroelastic constants decrease monotonically with the fiber section aspect ratio \({\gamma}\) increasing from 0 to 1. The elastic constant and dielectric constant obtained by the present solution are very similar to the results obtained by the classical electroelastic theory, whereas the piezoelectric coupling constant obtained by the present solution is very different from the results obtained by the classical electroelastic theory.

•Constitutive model showed an r of 0.982 between tested and predicted stress.•Flow localisation was found in the instability regions.•Deformation mechanism in the stability regions is dominated by DRX.•The optimum processing region were at 930–980 °C and 0.001–0.014 s−1.The hot deformation behaviour of low carbon bainitic steel, 29MnSiCrAlNiMo steel, was studied at a temperature range of 800–1100 °C and a strain rate range of 1 × 10−2 to 1 × 101 s−1. The flow curves display two features, with steady-state region and without it. After a comprehensive consideration of the deformation temperature, strain rate and strain, a constitutive model was proposed, exhibiting a correlation coefficient of 0.982 between the experimental and predicted stress values. The result shows a decreased deformation activation energy from 460 to 267 kJ/mol with increasing strain level from 0.1 to 0.8. Processing maps at different strain levels were also constructed, which exhibit an expanded instability region with increasing strain. Microstructural observations show that the flow localisation occurred when the hot working was performed on the instability regions, and partial dynamic recrystallisation occurred along the grain boundaries and deformation bands, resulting in a mischcrystal structure. The optimum hot working processing parameters for the studied steel are at 930–980 °C and 0.001–0.014 s−1, and a full dynamic recrystallisation structure with a fine and homogeneous grain can be obtained. Microstructure observation verifies the applicability of the processing maps for optimising the processing parameters of the 29MnSiCrAlNiMo steel.

•A strain-induced phase transition from β to α″ occurred during the rolling process.•A parallel orientation relationship [0 0 1]α″//[110]β between β and α″ phase was found.•Lots of β phase made the AC specimens exhibit a low strength and high ductility.•Lots of α phase made the FC specimens exhibit a high strength but low ductility.•A growth law of β grain from 700 to 850 °C was found: D = 5.7 × 105exp(−1.78 × 104/T).A kind of β-Zr40Ti5Al4V alloy was studied in detail on the cold rolling and the subsequent heat treatment process (including furnace cooling (FC) and air cooling (AC)). The analysis of the phase composition and microstructure of the rolled alloy indicated that a strain-induced phase transformation process from body-centred cubic β to orthorhombic α″ occurred during the cold rolling process and there was a parallel orientation relationship [0 0 1]α″//[1 1 0]β between the two phases. The transitional α″ phase transformed back to β or further transformed to α phase via a process of α″ → β → α during heat treatment. After the FC treatment, the α phase dominated the microstructure, and the alloy exhibited high strength but low ductility properties. In comparison, more β phase (∼67.1%) was retained after the AC treatment at 600 °C and a full β phase microstructure was produced when the temperature exceeded 670 °C. The amount of β phase made the AC specimens exhibit low strength and high ductility properties. The statistical results showed an increased β grain size with increasing temperature and the calculated results revealed that the activation energy for grain growth between 700 °C and 850 °C was ∼147.9 kJ/mol. Then a relationship between β grain size D and temperature T was obtained: D = 5.7 × 105exp(−1.78 × 104/T).

The cyclic deformation characteristics and fatigue behaviors of Hadfield manganese steel have been investigated by means of its ability to memorize strain and stress history. Detailed studies were performed on the strain-controlled low cycle fatigue (LCF) and stress-controlled high cycle fatigue (HCF). Initial cyclic hardening to saturation or peak stress followed by softening to fracture occurred in LCF. Internal stress made the dominant contribution to the fatigue crack propagation until failure. Effective stress evolution revealed the existence of C–Mn clusters with short-range ordering in Hadfield manganese steel and demonstrated that the interaction between C atoms in the C–Mn cluster and dislocation was essential for its cyclic hardening. The developing/developed dislocation cells and stacking faults were the main cyclic deformation microstructures on the fractured sample surface in LCF and HCF, which manifested that fatigue failure behavior of Hadfield manganese steel was induced by plastic deformation during strain-controlled or stress-controlled testing.

The effects of retained austenite and hydrogen on the rolling contact fatigue (RCF) behaviours of a new carbide-free bainitic steel (CFBS) were studied by means of the RCF testing, electrolytic hydrogen charging, transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the new carbide-free bainitic steels (CFBSs) exhibited very good RCF performance under the high contact stress of 1.7 GPa, and pitting and spalling were the main mode of the RCF failure. The RCF performance of the new CFBS was improved by the retained austenite content increasing, while obviously decreased by hydrogen.

The microstructures and the mechanical properties of 30MnSiCrAlNiMo low-carbon steel were systematically optimized by a series of heat-treatment processes, and the heat-treatment process of low-temperature bainite in low-carbon steel was explored. Results showed that the microstructure of low-temperature bainite in the low-carbon steel, containing a fine plate of carbide-free bainitic ferrite and a thin film of retained austenite, could be produced by continuous cooling transformation around the Ms temperature from Ms+10 °C to Ms−20 °C at a cooling rate of 0.5 °C min−1. A new model was proposed to evaluate the comprehensive mechanical properties of steel, which found that the low-temperature bainite had the best comprehensive mechanical properties compared to any other microstructures for the low-carbon steel. The higher dislocation density and finer bainitic ferrite plate in the low-temperature bainite resulted in the higher yield strength and the higher toughness, but relatively lower ultimate tensile strength owing to the lower work-hardening rate caused by the higher initial dislocation density. There were some very fine particles in the bainitic ferrite of the steel after isothermal treatment at higher temperature. The ultimate tensile strength and the low-temperature impact toughness of the steel decreased with the volume fraction of the retained austenite increasing, while the elongation initially increased with an increase in the volume fraction of the retained austenite (<10%) and then remained constant.

•Nanoscratch technology was employed to evaluate the abrasion properties of steels.•A modified equation for calculating nanoscratch true wear volumes is developed.•The value of H/Er can help to assess the wear resistance of steels.The wear behaviours of 65Mn and 40CrMnSiW steels were investigated by subjecting them to nanoscratch, nanoindentation, and dry sliding wear processes. A simple modified equation for calculating nanoscratch true wear volumes is developed, i.e. the true volumes are closely related to the elastic recovery angle which can be given easily. When comparing the nanoscratch phenomena with macro sliding wear, it was found that the micromechanism of the macro wear was similar to those of nanoscratching. The nanoscratching and the macro sliding wear testing represented the procedures for evaluation of single-asperity and multi-asperity contact behaviours of abrasive wear, respectively. In addition, to compare wear resistance properties of steels, the mean wear rate increased with decreases in ratio of the hardness H and the reduced modulus Er in turn, namely, the value of H/Er can help to assess the wear resistance of steels in some degree.

A thick nanocrystalline surface layer with gradient nanostructure was produced on Hadfield steel through high-speed pounding (HSP) for 240 min. The grain sizes in the surface layer decreased to about 25 nm. To study the wear behaviour of the nanocrystallised Hadfield steel, three types of samples (a nanocrystallised sample (NS), an untreated sample (US) and a deformed sample (DS) had the same hardness as NS) were studied by using a pin-on-disc type tribometer under un-lubricated conditions at 25 °C, 300 °C, and 500 °C. The experimental results show that the coefficient of friction and wear weight loss of the NS are lower than for the DS and the US and that the wear resistance is remarkably improved. Compared with the US, the enhanced wear properties of the NS must be attributed to the increased hardness, which improves the resistance to plastic deformation and wear removal. However, compared with the DS, after surface nanocrystallisation of the NS, a transition occurs in the dominant wear mechanism from adhesive wear to slight abrasive wear. Therefore, the advantages realised in the wear properties of the NS may be attributed to the enhancement of the surface activity by grain nanocrystallisation, which results in improvements in the formation of the oxide layer. The roughness of the worn surfaces shows that both the wear scar width and depth of the worn surface of the NS are much smaller than those found in the DS and the US. Finally, a wear model of nanocrystallised material was established.

The damage of a Hadfield steel crossing in a railway turnout due to wheel rolling impact passages is investigated numerically. A three dimensional elastic–plastic finite element model has been developed by taking into account non-linear material properties, dynamic wheel impact and wheel–crossing contact. The effects of wheel velocity on the damage and deformation of the crossing is analysed. In the present study, four rolling velocities of the wheel, i.e., 50 km/h, 100 km/h, 150 km/h, and 200 km/h were chosen. The wheel exhibits elastic material behaviour, whereas the crossing exhibits elastic–plastic material behaviour. To represent the damage and deformation behaviours of the crossing, non-linear dynamic work hardening is further included in the calculation. Dynamic stress and strain in the crossing were obtained by the explicit finite element method, including the maximum von Mises stress, the maximum strain, the maximum horizontal stress, perpendicular stress and parallel stress. The snake-like motion of the wheel rolling on the crossing was verified by the simulation; in the meantime, there is a significant impact load on the nose rail, which caused a big von Mises stress in the nose rail. The big von Mises stress is the main reason for the nose rail fatigue spalling. The maximum von Mises stress and maximum strain in the nose rail increase with the train speed increasing. The study provides a method for the further research on the fatigue and the wear behaviours of railway crossings.

In order to predict the mechanism of the formation and propagation of rolling contact fatigue cracks in Hadfield steel railway crossings (HSRC), the residual strain, microstructure, crystal orientation, and microhardness surrounding fatigue cracks were investigated. Results show that the formation and propagation of fatigue cracks in HSRC have no correlation with the crystal orientation and boundaries of grains. The hardness and residual strain in the field around the fatigue crack are lower than those in other regions. The compressive strain around the fatigue crack is released after crack propagates, which reduces the hardness around the fatigue crack. Deformation twins and dislocations play a key role in the work hardening of HSRC.

Piezoelectric materials with a doubly periodic array of cracks and rigid-line inclusions under far-field antiplane mechanical load and inplane electric load are investigated. By employing the conformal mapping technique and the elliptical function theory, an exact solution of the whole-field stress and electrical displacement is obtained. The closed-form formulae for the stress and electrical displacement intensity factors at the tips of cracks and rigid-line inclusions and the effective electroelastic moduli of the composites are presented. Some existing solutions can be regarded as degenerated cases of the present results. Numerical examples are provided to show the interesting electroelastic interaction between multiple cracks and multiple rigid-line inclusions.

Though the resultant microstructure and mechanical properties of pure Zr after severe plastic deformation have been studied, the microstructural evolution and the variation of mechanical properties with progressive deformation of pure Zr have not been investigated in detail. Microstructure observation verified that dislocation slip dominated the plastic behaviour of pure Zr during room temperature deformation. With the increase in the rolling reduction (RZr) from zero to 90%, the various microstructural features, e.g. dislocation tangles, dislocation lamellas, dislocation cells or subgrain structure, and nano-crystal grains (∼72 nm), etc., successively occurred. Dynamic recovery played an important role on the plastic behaviour of the pure Zr with high stacking fault energy (SFE), which slowed down the work hardening rate during high strain rate rolling deformation and caused strain softening on the pre-deformed specimens during low strain rate tension deformation. The specimens annealed at 450–600 °C not only possessed higher ultimate tensile strength and yield strength, but exhibited higher elongation as compared with the undeformed specimen. The fracture mode of all annealed specimens was entirely dimple fracture.

The deformation, fracture, and wear behaviours of two new C+N enhancing alloying austenitic steels (CNEASs) for railway crossings under high speeds and heavy loads were investigated by tensile, Charpy impact, and sliding wear tests, in comparison with the traditional Hadfield austenitic steel. The main plastic deformation mechanism of the CNEASs was deformation twinning due to its low stacking fault energy (SFE). The enhanced strength and plasticity resulted from the large amounts of ultra-fine nano-twinning that occurred during plastic deformation, while the oversaturation effect of nitrogen on CNEASs further increased the hardness and work hardening capacity. Contrary to the increased ductile to brittle transition temperature (DBTT) caused by nitrogen in austenitic steel, the new studied steel exhibited a lower DBTT as compared to the Hadfield steel, because the combined alloying with C+N enhanced the metallic character of the interatomic bonds, increasing the fracture-resistance under cryogenic temperatures. Sliding wear tests showed that abrasive wear dominated the wear behaviour of the CNEASs. The remarkable improvement of wear resistance in the steels enhancing alloyed with C+N, particularly at high temperatures, was attributed to the formation of thick tribo-oxides on the worn surface under the effect of Cr and nitrogen. Therefore, the results demonstrated that the experimental steels possessed a higher strength, plasticity, hardness, impact toughness at cryogenic temperatures, work hardening capacity, and wear resistance as compared to the Hadfield steel. This makes the new steel an ideal material for railway crossings under high speeds and heavy loads.

Though the deformation behaviour of Zr–Nb alloys has been widely investigated, the studies about the deformation behaviour of the α phase, and the contribution of the α phase, β phase and rotation process to the deformation during rolling have not reached an agreement. When increasing the deformation from zero to 94%, the average thickness of α and β plates monotonously decreased from 701 to 42 nm and from 116 to 14 nm, respectively, whilst the plates were gradually rotated to an orientation parallel to the rolling direction. When increasing the rolling reduction from 42% to 94%, the contribution to the rolling deformation changed from 74.6% to 93.8% for α plates in thickness reduction, from 4.7% to 6.2% for β plates in thickness reduction, but from 20.7% to approximately zero for plate rotation. The refinement of α and β plates and the increment of dislocation density in α plates resulted in a significant increase in the hardness and tensile strength. The rolled Zr705 showed a strain softening at low plastic strains, which may be attributed to the dynamic recovery. This resulted in an early appearance of necking and fracture.Highlights► The α and β plates decreased from 701 to 42 nm and 116 to 14 nm, respectively. ► The contribution to the rolling reduction increased from 74.6% to 93.8% for Rα. ► The contribution to the rolling reduction increased from 4.7% to 6.2% for Rβ. ► The contribution to the rolling reduction reduced from 20.7% to 0% for plate rotation. ► Rolled Zr705 showed a strain softening due to the dynamic recovery.

Hydrogen embrittlement and secondary cracks of bainitic steels were studied by means of the slow strain rate test (SSRT), in situ tension in transmission electron microscopy (TEM) analysis and scanning electron microscopy (SEM). The results showed that the microstructure of the bainitic steels became finer, the phase interfaces as irreversible hydrogen traps significantly were increased, the nano-scale carbides were precipitated from retained austenite, and hydrogen embrittlement was decreased greatly with Al addition. Lots of secondary cracks were formed with Al addition. The stress concentration was relaxed and the hydrogen embrittlement was reduced significantly because of the presence of secondary cracks. Due to hydrogen, the dislocation emission and motion were enhanced and the formation of secondary cracks was reduced.Highlights► HE was reduced and secondary cracks were increased with Al addition. ► HE was increased and secondary cracks were reduced after H-charged. ► Due to hydrogen, the dislocation emission and motion were enhanced.

Phase boundaries play significant roles in the crack initiation and propagation behavior of duplex phase materials such as nanoscale bainite–austenite (α/γ) microstructures. Simulation of α/γ-iron phase boundary interacting with crack propagation was carried out using molecular dynamics method. Bi-phase systems consisting of α-iron with body-centered cubic structure and γ-iron with face-centered cubic structure, were constructed. Phase boundaries were created using Nishiyama–Wasserman and Kurdjumov–Saches (K–S) orientation relationships, respectively. Eight orientations of cracks in α- or γ-phase were analyzed to illustrate the orientation dependence of crack growth. Finnis–Sinclair potential for Fe fit by Mendelev was used to describe interatomic potentials. Tensile load was applied normal to the crack surface. Simulation results reveal that in most cases phase boundary provides an obstacle to slip and dislocation motion during crack propagation, except in two orientations with K–S relation. New-cracks always nucleate phase boundary as a result of slip blocking and dislocation pileups. Bcc/fcc iron phase boundary is therefore demonstrated to be a great contribution to the strengthening of nanoscale duplex steel.Research highlightsWe simulated the effect of K-S and N-W phase boundary on micro-crack propagation. Initial crack parallel to phase boundary is inclined to be integrated. K-S bi-phase system has higher ductile behavior than N-W one. Slip is able to traverse the K-S boundary along (0 1 -1)γ/(-1 1 1)α.

This paper presents results concerning Hadfield steel subjected to explosive treatment and compression, respectively, with the purpose of clarifying the difference between dynamic and static deformation behaviors. A Hadfield steel sample that was deformed to a lower strain at an exceptionally high strain rate exhibited the same hardness as a sample that was deformed to a higher strain at a low strain rate. A deformation model based on in situ deformation has been developed, whereby the enhanced work hardening during explosive treatment is attributed to the deformation of the grains being mainly accommodated by the curvature of the grain boundaries and shape change of the surrounding grains in their original positions, without obvious macroscopic deformation.

The problem of nanocomposite materials under far-field antiplane mechanical load and inplane electric load is investigated. Based on the theory of Gurtin–Murdoch surface/interface model, an exact solution is obtained for the inhomogeneity/matrix/equivalent medium model, in terms of which a generalized self-consistent approach is proposed for predicting the effective electroelastic moduli of nanocomposites. A closed-form solution of the effective electroelastic moduli is presented. The numerical results reveal that the effective electroelastic moduli are size dependent when the size of the inhomogeneity is on the order of nanometer. With the increase in the size of the inhomogeneity, the present solution approaches to the classical results obtained in the classical theory.

A process primarily based on carburization and successive low-temperature austempering, was proposed for the generation of low-temperature bainitic microstructure in the surface layer (∼2.5 mm in thickness) of low-carbon steel. For a potential use in the manufacture of heavy-duty gears, wear properties of this type of microstructure were studied. Comparisons were made with carburized commercial steel of 20CrMnTi subjected to quenching followed by tempering, which is widely used in the area of gear. The result showed that the experimental steel and the control steel present different wear mechanisms under the same sliding wear process. The low-temperature bainitic steel exhibits an excellent wear resistance as a consequence of the following three main factors: (i) high strength plus high toughness, (ii) carbon enriched austenite, and (iii) extremely fine α-phase microstructure. The low-temperature bainitic microstructure exhibits high strength and high toughness due to the refinement of the microstructure. Secondly, the film-like carbon enriched austenite (∼30 nm in thickness) uniformly distributed between the carbide-free bainitic ferrite plates can retard the crack propagation during sliding friction and play an advantageous role in wear resistance. Lastly, an extremely fine single α-phase microstructure was promptly induced in the dry sliding friction of the surface layer of the low-temperature bainitic steel; such fine refined microstructure lead to an increase in hardness, which can improve wear resistance.Highlights► An approach is proposed for the development of low-temperature bainitic microstructure in the surface layer of a low-carbon steel. ► Wear properties of the low-temperature bainitic microstructure were investigated comparisons were made with carburized commercial gear steel of 20CrMnTi. The low-temperature bainitic steel exhibits an excellent wear resistance as a consequence of three main factors: high strength plus high toughness, carbon enriched austenite, and extremely fine α-phase microstructure. ► This material which has low-temperature bainite in the surface layer is potentially of use in the manufacture of heavy-duty gears if the process can be further optimized.

The microstructure in the worn surfaces of a failed bainitic steel railway crossing was investigated using optical microscopy, SEM, TEM, nanoindentation and Mössbauer spectroscopy. The results indicated that a nanocrystalline layer had formed in the surface of a worn crossing during service. The formation of the nanocrystalline layer was due to the severe plastic deformation (SPD) caused by the repeated heavy loading in service by high speed train wheels. The mechanism of formation of the nanocrystalline layer was strain induced dynamic recrystallization, and the nanocrystalline grains were nucleated from the original crystals of the steel directly. The alloying elements in the worn surfaces of the steel segregated slightly by diffusion during the process of recrystallization. The nanocrystalline layer does not display the white etching layer commonly observed in ordinary railway rails, the reason may be the differences of its microstructure and carbon content with the ordinary rail steel.

Residual microstresses in particle reinforced alumina/SiC and alumina/mullite composites were calculated. The results indicated that there existed a linear relation between matrix microstresses and the particle contents in the composites. The influence of stress state on crack propagating and grain boundary strengthening was analyzed. Ratios of grain boundary toughness to grain toughness of these composites were calculated in view of microstress analysis, and percentage of transgranular fracture (PTF) that increases with the microstress in the alumina matrix was then deduced. The relationship between microstructure, component, matrix microstresses, and PTF was established. Therefore, the fracture characteristic was predicted on basis of the particle content and distribution in addition to the microstructure of the composites.

We measured the wear resistances of alumina, alumina/silicon carbide composite and alumina/mullite composite by abrasive wear. And we studied the influence of fracture mode and worn surface pullout on wear resistance. The results are as follows: the main wear mechanisms of alumina and alumina/silicon carbide were fracture wear and plastic wear respectively, and for alumina/mullite composite, fracture wear and plastic wear mechanisms worked together. The wear resistance of the alumina/silicon carbide composite and the alumina/mullite composite was better by a factor of 1∼3 than that of the monolithic alumina. There were two main reasons for the better wear resistance, i.e., the improved mechanical properties and the more smooth worn surfaces. However, The primary reason was the reduction of area fraction of pullout on the worn surfaces induced by fracture mode transition.

In this paper the electrical contact heating (ECH) technology was applied to refine the austenitic grain size of medium carbon alloy steel. The ECH equipment was designed to reach uniform heating of uniform heat transfer in the sample. After the heat treatment of the ultra-pure 42CrMoVNb steel using the ECH equipment, a uniform ultrafine microstructure with an average austenite grain size of 1.4 μm was obtained. The mechanical properties of the ultrafine 42CrMoVNb steel were also studied. A high strength (1538 MPa) and high impact toughness (81 J/cm2) was obtained for the ultrafine 42CrMoVNb steel. For the same austenitic grain size, the strength and toughness of the steel subjected to the ECH treatment were improved compared to the steel subjected to a cyclic salt bath quenching treatment.

The nanocrystallization and α martensite formation in the surface layer of medium-manganese austenitic wear-resistant steel subjected to high-energy shot peening treatment were investigated by X-ray diffraction and transmission electron microscopy. The results show that nanograined microstructure mainly composed of strain-induced α martensite grains with the average size of ∼8 nm were produced in the shot-peened surface. However, any induced phases are not observed in compression-deformed samples with the reduction from 20 to 60%. The examination of grazing-incidence X-ray diffraction in different grazing angles indicates that with decreasing the depth from the shot-peened surface the fraction of strain-induced α martensite increases and the grain size decreases.

This study sets out to introduce the flash butt welding of high manganese steel crossing and carbon steel rail by employing an austenite–ferrite two-phase stainless steel insert. There are two flash butt welded joints for the connection of the high manganese steel and the carbon steel rail, one is the welded joint of the carbon steel and the stainless steel, and the other is that of the high manganese steel and the stainless steel. The mechanical properties and the microstructures of the welded joint are studied by means of static bending, three bend-fatigue and metallographic for the practical rail. There is no carbide precipitation on the austenitic grain boundary in the HAZ of the high manganese steel crossing subjected to jetting water cooling after the flash butt welding, and there is no martensitic transformation in the HAZ of the carbon steel rail subjected to annealing treatment by a special induction heat treatment device, which will avoid the brittleness of the welded joint effectively. The welded joint of the carbon steel rail and the stainless steel insert is annealed at 900 °C for 10 min, which will release the residual stress of the welded joint and thus enhance the strength of the welded joint. It is indicated that the flash butt welding of the high manganese steel crossing and the carbon steel rail via the austenite–ferrite two-phase stainless steel insert is feasible.

The microstructure evolution and the deformation mechanism change in 0.98C–8.3Mn–0.04N steel during compressive deformation at room temperature have been studied as a function of the reduction in the range of 20–60%. Experimental results show that with the reduction increasing the microstructure of the deformed sample changes from dislocation substructures into the dominant twins plus dislocations. This suggests that the plastic deformation mechanism changes from the dislocation slip to the dominant deformation twinning. The minimum reduction for deformation twins starting is estimated to be at between 30 and 40%. With the reduction further increases to more than 40%, the deformation twinning is operative and the thickness of deformation twins gradually decreases to nanoscale and shear bands occur. These high-density twins can be curved by the formation of shear bands. In addition, both transmission electron microscopy and X-ray diffraction examinations confirm the inexistence of deformation-induced martensites in these deformed samples.

The formation of the fatigue cracks was due to massive vacancy clusters in the subsurface layer of Hadfield steel crossing, which are induced by the accumulated plastic deformation under the conditions of impact and contact stresses from train wheels. The high concentration layers of vacancy clusters were formed parallel to the working surface of the crossing, which caused the initial rolling contact fatigue cracks to be parallel to the working surface with a laminar distribution in the depth direction. It can be predicted that metals containing elements with larger atomic diameter should have better rolling contact fatigue and wear performances.Highlights► Plastic deformation is caused by rolling contact stress on Hadfield steel crossing. ► Accumulated plastic deformation induced massive vacancy clusters. ► Massive vacancy clusters in worn surface of crossing induced the fatigue cracks. ► Metal with bigger atomic radius maybe has better rolling contact fatigue performance.

•Duplex microstructures (DM) obtained at 850–925 °C were investigated.•The stable Dαp with increasing annealing time or temperature was discussed.•With increasing strain rate, an increased plasticity parameter of RA was studied.•Microvoids were more prone to be formed along the interfaces of αp/βp.•A deformation sequence of β, αs and αp to coordinate the plasticity of DM was found.The formation of duplex microstructure in Zr–2.3Nb alloys at a temperature range of 850–925 °C, its plastic behaviour at various strain rates and the accommodation of each phase in duplex microstructure during the tensile deformation process were investigated thoroughly in this paper. It is shown that with increasing temperature, the dimension of βp exponentially increased, while the dimension of αp remained nearly unchanged due to the competition of grain growth and phase transformation from α to β. The duplex microstructure obtained at 850 °C displayed a higher strength and a comparable plasticity compared with other duplex microstructures obtained at higher temperatures, due to the reinforcement of the ω phase and the uniform grain size. With increasing strain rate, the strength of the duplex microstructure increased, while the plasticity parameters – elongation (EL) and reduction of area (RA), showed opposite tendencies, a decreased tendency for EL and increased tendency for RA. Analysis indicated that the decreased EL was due to the fact that the higher dislocation annihilation rate at higher strain rate reduced the work hardening rate, resulting in an early onset of necking. The increased RA was consistent with the fracture surface observation: the increased mean size and mean depth of dimples, which revealed an increased post-necking EL and then slowed down the falling tendency of EL. Microvoids were prone to form along the interfaces of the αp and retained β phases. Finally, SEM and TEM observations revealed the deformation sequence of these phases as follows: β, αs, αp. And then a simple deformation model illustrating the deformation behaviour of each phase was proposed.

The microstructures on the damaged surfaces of two served bainitic steel crossings were investigated using optical microscope and scanning electron microscope (SEM). The tensile properties of the bainitic steel with different contents of hydrogen were measured. The results showed that the plasticity of the bainitic steel, such as the reduction of area and elongation, decreases sharply with the increase of hydrogen content. There was a critical content of hydrogen without the hydrogen embrittlement for the commercial bainitic steel used for crossing, which was 7 × 10−5 wt%. When the content of hydrogen in a bainitic steel was lower than the critical value, during the used process of the crossings, the wear failure appeared during the early stage; however, the fatigue spalling appeared in the end of the process. When the content of hydrogen was higher than the critical, brittle fracture was responsible for the failure of the crossing in a short time during use.

•We present a micromechanics model of nano composites with elliptical fiber.•We obtain a closed-form solution of the effective antiplane shear modulus.•We study the effect of fiber section aspect ratio on the elastic properties.A three-phase confocal elliptical cylinder model with interface stress is proposed. Based on the model, a generalized self-consistent method for the nano composites accounting for fiber section shape under antiplane shear is obtained. By applying the theory of Gurtin–Murdoch surface/interface and the conformal mapping technique, a closed-form solution of the effective antiplane shear modulus is obtained. Several existing solutions can be regarded as the degenerated cases. The major results are: (a) the effective antiplane shear modulus is size dependent when the size of the elliptical section fiber is on the order of nanometer, whereas the interface effect can be neglected when the fiber with large characteristic dimensions; (b) the interface effect of the nano fiber increases with the increase of the fiber section aspect ratio γ from 0 to 1; (c) the higher the fiber volume fraction is, the greater the interface effect is; (d) when the modulus of the fiber is small, the effective modulus of the nano composites depends strongly on the fiber interface effect, whereas when the modulus of the fiber is large, the fiber interface effect has little influence on the effective antiplane shear modulus.

Stress/strain analysis is the key to understanding and predicting wear and fatigue behavior of a crossing. By taking into account non-linear material properties, a three dimensional elastic–plastic finite element model, which is composed of wheel, crossing and ties, is presented for the simulation of stress/deformation in a railway crossing. The influences of dynamic wheel load and wheel–crossing contact are examined. Stress, plastic strain and vertical displacement of the simulated crossing under dynamic wheel load at different wheel–crossing contact positions are investigated. The maximum vertical displacement occurs at the wheel–crossing contact region, and decreases gradually from the wheel–crossing contact region to the toe-end and heel-end of the crossing. The maximum von Mises stress and maximum equivalent plastic strain in the crossing increase remarkably with the increase of the train speed. The maximum vertical displacement in the crossing increases obviously and varies linear approximately with the train speed. The maximum von Mises stress, maximum vertical displacement and maximum equivalent plastic strain in the crossing are very sensitive to the axle load and are linear approximately with the axle load.Highlights► We present a non-linear elastic–plastic model composed of wheel, crossing and ties. ► We study stress, plastic strain and vertical displacement of a simulated crossing. ► We examine the influences of dynamic wheel load and wheel–crossing contact positions. ► Train speed and axle load affect remarkably on mechanical properties of the crossing.

A theoretical study on piezoelectric coated nano-inclusion composites with interface effect under antiplane shear is reported. Based on the theory of Gurtin–Murdoch surface/interface theory and the generalized self-consistent method, a closed-form solution of the effective electroelastic moduli are obtained. The numerical results reveal the size dependence of the effective electroelastic moduli when the size of the coating and inclusion are on the order of nanometer. The effects of the coating thickness and inclusion radius on the effective electroelastic moduli of the composites are discussed.