AbstractAl-Si coated ultra-high strength steel (UHSS) has been commonly applied in hot stamping process. The influence of austenitizing temperature on microstructure of Al-Si coating of UHSS during hot stamping process and its tribological behavior against H13 steel under elevated temperature were simulatively investigated. The austenitizing temperature of Al-Si coated UHSS and its microstructual evolution were confirmed and analyzed by differential scanning calorimetry and scanning electron microscopy. A novel approach to tribological testing by replicating hot stamping process temperature history was presented. Results show that the hard and stable phases Fe2Al5 + FeAl2 formed on Al-Si coating surface after exposure to 930 °C for 5 min, which was found to be correlated to the tribological behavior of coating. The friction coefficient of coated steel was more stable and higher than that of uncoated one. The main wear mechanism of Al-Si coated UHSS was adhesion wear, while abrasive wear was dominant for the uncoated UHSS.

Serpentine is usually added into oil for tribological applications. To explore the performance and mechanism of serpentine, three different kinds of pins were tested under lubricated/dry sliding conditions. The result shows that a surface layer with an excellent anti-friction property was formed on the surface of worn steel. The binding ability between the matrix elements in the wear-induced transition zone and the unsaturated bonds released by serpentine is the key factor to form the anti-friction surface layer. Meanwhile, the hardness of the pairing material and frictional contact time determine the thickness and surface roughness of the anti-friction surface layer. In all, this work is hoped to be helpful in designing and researching a new industrial anti-friction material which can be used for dry sliding conditions.

P91 steel is an important bearing material used in nuclear power plants. The study of its mechanical degradation behavior is important for ensuring safe operation. The relationship between the dislocation density of P91 steel under different strains and the corresponding nonlinear ultrasonic parameter β was studied. The dislocation density of strained samples was estimated by X-ray diffraction. Nonlinear ultrasonic testing was conducted to evaluate β, showing that this value increased with increasing dislocation density induced by different tensile elongations. It was shown that the ultrasonic second-harmonic generation technique can effectively evaluate the degradation behavior of metallic materials, and the prediction of the residual life of bearing parts in service can be made based on β and the dislocation density.

•The dry sliding friction-induced deformation layer can be divided into two layers.•The cementite particles of about 200 nm were observed in worn surface of the disc.•The pin atoms diffused into the friction-induced deformation layer of the discs.Evolution of microstructure and compositions in worn surface and subsurface of 45 (0.45 mass% carbon) steel disc slid against tin-alloy-pin was analyzed by SEM, TEM and SIMS. The mechanical alloying layer and plastic deformation layer were formed in the sliding friction-induced deformation layer (SFIDL) of 45 steel. Ultra-refine and nano grains were detected in the worn surface layer. Elements of Sn, Cu and Sb, originated from the mating tin-alloy-pin, with diffusion depth of 35 μm, 11 μm and 4 μm, respectively, were detected in its SFIDL. Mechanisms accelerating atom diffusion in SFIDL were subsequently propounded.

Transformation Induced Plasticity (TRIP) steels exhibit a better combination of strength and ductility properties than conventional high strength low alloy (HSLA) steels, and therefore receive considerable attention in the automotive industry. In this work, the tensile mechanical behaviors of TRIP-aided steels were studied under the condition of the quasi-static and high deformed rates. The deformed specimens were observed by scanning electron microscope (SEM) along the tensile axis. The threshold strain of microvoid formation was examined quantitatively according to the evolution of deformation. The results showed that: the yield and tensile strengths of TRIP steels increase with the strain rate, whereas their elongations decrease. However, the threshold strain for TRIP steels at high strain rate is larger than that at low strain rate. Comparing with the deformed microstructure and microvoids formed in the necking zone of dual phase (DP) steel, the progressive deformation-induced transformation of retained austenite in TRIP steels remarkably increases the threshold strain of microvoid formation and furthermore postpones its growth and coalescence.Highlights► The tensile mechanical behaviors of TRIP steels were studied under high rate deformation conditions. ► The threshold strain of microvoid formation was examined quantitatively. ► The effects of retained austenite of TRIP on suppressing microvoid formed during tensile process have been discussed.

The intermetallic compounds (IMC) in the solder and at the interface of Sn–3.0Ag–0.5Cu (SAC)/Cu and Sn–3.0Ag–0.3Cu–0.05Cr (SACC)/Cu joints were investigated after isothermal aging at 150 °C for 0, 168 and 500 h. Different shaped Ag3Sn phases were found near the IMC layer of the latter joint. Interestingly, fine rod-shaped and branch-like Ag3Sn were detected near the interface after soldering and long Ag3Sn changed into shorter rods and small particles during aging. It is investigated that the Cr addition and thermal aging have effect on the evolution of Ag3Sn morphologies and it is controlled by interfacial diffusion. Energy minimization theory and the redistribution of elements are used to explain the morphological evolution of Ag3Sn. Small Ag3Sn particles were also found on the IMC layer after aging, unlike the large Ag3Sn at that of SAC/Cu joints. In conclusion, a favorable morphology of the joint interface leads to better bonding properties for SACC/Cu joints against thermal aging than that for SAC/Cu.Highlights► Different shaped Ag3Sn particles were found at interfaces of Pb-free solder joints. ► Fine rod-shaped and branch-like Ag3Sn existed near the IMC layer after soldering. ► Long Ag3Sn changed into shorter rods and small particles during aging. ► The evolution of Ag3Sn morphologies is controlled by interfacial diffusion. ► Cr along with Ag and Cu in Sn matrix was detected through TEM and SEM.

The microstructure evolution of the worn surface and sub-surface layer of SUS 304 austenitic stainless steel (ASS) disc against Al2O3 ceramic ball were studied on the basis of the tribological behaviors in the tests performed using a Cameron-Plint TE67 pin-on-disc tester. The microstructure after friction test was observed by optical and scanning electron microscope. The possible phase transformation of meta-stable austenite to martensite was detected by X-ray diffractometer. Results showed that friction-induced deformation led to finer grain at the subsurface beneath the worn surface. Furthermore, white layer was observed on some worn surface layers after higher normal loads. Transformed martensite from the austenite appeared on the worn surface under both low and high normal-loading conditions. Absence of transformed martensite was detected at the site about 25 μm below the worn surface although the grains at the site were still intensive and fine. In addition, the specific wear rate of SUS 304 stainless steel specimens was measured, and the possible reasons affecting the wear behavior were analyzed and discussed.

Multi-phase transformation-induced plasticity (TRIP) steels exhibit a combination of good ductility and high strength, and can be good candidates for automotive applications in improving crashworthiness of car bodies. Understanding of the mechanical properties of the TRIP steels under high strain rate is essential. In this paper, the tensile deformation behavior of cold-rolled TRIP-aided DP steels over the large range of the strain rates (400–1600 s−1) was studied using a pneumatic indirect bar–bar tensile impact tester based on one-dimensional elastic stress wave principle. The volume fraction of transformed retained austenite was analyzed by XRD. The strain rate effect on the amount of transformed retained austenite was also analyzed and discussed. The void initialization and morphology of fracture surfaces at different strain rates were observed by SEM. The results indicate that with increasing strain rates, both the yield strength and tensile strength of TRIP-aided steels increase, but the elongations decrease. The stability of retained austenite is strain rate dependent because the adiabatic temperature rises under high deformation rate. Increased strain rate results in the decrease of the amount of transformed retained austenite. The site of initialized void is located in the ferrite matrix resulting from TRIP effect. Furthermore, the morphology of fracture surface at high strain rate is different from that at the static or quasi-static tensile conditions.