Co-reporter:Li-Jing Zhang;Xi-Ping Guo
Rare Metals 2017 Volume 36( Issue 3) pp:174-182
Publication Date(Web):2017 March
DOI:10.1007/s12598-017-0881-1
Nb–Ti–Si-based alloy powders were prepared by mechanical alloying (MA) of elemental particles. The evolutions of morphology, size, phase constituents, crystallite size, lattice strain, composition and internal microstructure, etc., of the alloy powders were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), laser particle size analyzer and transmission electron microscope (TEM) analyses. The alloy particles are gradually refined and their shapes become globular with the increase in milling time. The diffraction peaks of Nb solid solution (Nbss) phase shift toward lower 2θ angles during ball milling from 2 to 5 h, and after that Nbss diffraction peaks shift toward higher 2θ angles with the increase in milling time from 5 to 70 h, which is mainly attributed to the alteration of the lattice parameter of Nbss powders due to the solution of the alloying element atoms into Nb lattice to form Nbss. During ball milling process, the decrease in crystallite size and increase in lattice strain of Nbss powders lead to continuous broadening of their diffraction peaks. A typical lamellar microstructure is formed inside the powder particles after ball milling for 5 h and becomes more refined and homogenized with the increase in milling time. After 40-h-ball milling, the typical lamellar microstructure disappears and a very homogeneous microstructure is formed instead. This homogeneous microstructure is proved to be composed of only supersaturated Nbss phase.
Co-reporter:Yanqiang Qiao, Xiping Guo, Yuxiang Zeng
Intermetallics 2017 Volume 88(Volume 88) pp:
Publication Date(Web):1 September 2017
DOI:10.1016/j.intermet.2017.04.008
•Zr has little affect on the microstructure and phase constituents of the alloy.•Zr improves both the room-temperature toughness and the high-temperature strength.•The alloy with 8 at.% of Zr shows a high fracture toughness of 15.0 MPa m1/2.•The compressive strengths at 1250 °C are improved to 279–293 MPa by Zr alloying.•The oxidation resistance of the alloys is obviously improved with Zr addition.The effects of alloying with Zr on the microstructure, mechanical and oxidation properties of Nb-Ti-Si based ultrahigh temperature alloys have been investigated in this study. The microstructures of the all alloys were comprised of primary γ(Nb,X)5Si3 blocks, Nbss and eutectic colonies, and the additions of Zr do not affect the microstructure and phase constituents of Nb-Ti-Si based alloys. Zr improves both the room-temperature toughness and the high-temperature strength. The alloy with addition of 8 at.% Zr shows the highest fracture toughness of 15.01 Mpa·m1/2. The compressive strengths of the alloys are improved to 278.89–293.08 MPa for the Zr-containing alloys when compare with the Zr-free alloy (194.23 MPa). The oxidation resistance of the alloys was also obviously ameliorated with Zr addition, showing a reduced weight gain with the increase of Zr content.
Co-reporter:Xiao Ma, Xiping Guo, Maosen Fu, Haisheng Guo
Scripta Materialia 2017 Volume 139(Volume 139) pp:
Publication Date(Web):1 October 2017
DOI:10.1016/j.scriptamat.2017.06.035
Crystallographic characteristics have great effects on the performance of directionally solidified Nb-Ti-Si based in-situ composite, which is mainly composed of parallel Nbss/γNb5Si3 eutectic cells along the longitudinal axis of the specimens. Nbss and γNb5Si3 grow up in a coupled manner and exhibit preferred growth orientation along [111]Nb and 0001¯γ zone axes, respectively. Under these orientations, two types of coherent interfaces have been formed between the two phases, corresponding to the different morphologies of γNb5Si3. The preferred growth orientations and coherent interface structures might promote the coupled growth and enhance the stability of the eutectic cells of this in-situ composite.Download high-res image (352KB)Download full-size image
Co-reporter:Yan Junjie, Guo Xiping
Rare Metal Materials and Engineering 2017 Volume 46, Issue 1(Volume 46, Issue 1) pp:
Publication Date(Web):1 January 2017
DOI:10.1016/S1875-5372(17)30067-X
With the melting temperature of 2323 K and withdrawing rate of 100 μm/s, Nb-Ti-Si based ultrahigh temperature alloy has been directionally solidified with the use of crucibles. The directionally solidified (DS) specimens were subsequently heat-treated in two ways: either at 1723 K/50 h (HT1) or at 1623 K/50 h + 1723 K/50 h + 1373 K/50 h (HT2). XRD, SEM and EDS have been employed to investigate the influence of heat-treatments on the microstructures and fractography of the directionally solidified alloys. The results show that after heat-treatment, the volume fraction of the large-size primary silicides decreases. Both kinds of heat-treatments could effectively alleviate or even eliminate the segregation in the alloys. The original boundaries between the (Nb, X)5Si3 + Nbss eutectic cells in the DS specimens have thoroughly disappeared after heat-treatments. Specifically, silicides have been more evenly distributed after HT2 than HT1. Compared with DS specimens, the average room temperature fracture toughness of the specimens has increased by 12.3% (19.2 MPa·m1/2) while the average tensile strength has increased by 26.6% and the maximum value reaches 933.2 MPa after HT2. These improvements could be mainly attributed to the more effective dispersion strengthening of (Nb, X)5Si3 silicide particles and the shape change as well as size increase of the ductile Nbss phases after HT2.
Co-reporter:Yin Wang, Xiping Guo, Yanqiang Qiao
Materials & Design 2017 Volume 116(Volume 116) pp:
Publication Date(Web):15 February 2017
DOI:10.1016/j.matdes.2016.12.033
•Y2O3 matrix mould shells with yttria face coats doped with different oxides were prepared via lost wax process.•Discontinuous Ti solid solution layer existed along the boundary of alloys re-melted in Y2O3 mould shells doped with oxides.•Two reaction layers could be found between the alloys and Y2O3 mould shells doped with oxides.•Mould shells of pure yttria and yttria doped with La2O3 + ZrO2 are fit for the casting of Nb-Si based alloys.To find the best mould shells for investment casting of Nb-Si based ultrahigh temperature alloy, Y2O3 mould shells with zirconia face coat and yttria face coats doped without and with CaO + ZrO2, MgO + ZrO2, La2O3 + ZrO2 and CeO2 + ZrO2 were prepared and their interactions with the melt of the alloy at 1850 °C for 30 min were investigated. The results show that the interfaces between the alloy and the mould shells with yttria face coats doped with CaO + ZrO2, MgO + ZrO2 and La2O3 + ZrO2 during re-melting process all comprise two types of reaction layers, which are mainly composed of HfO2 + Y2O3 and TiO + HfO2. The interface between the alloy and the mould shell with yttria face coats doped with CeO2 + ZrO2 is also composed of two types of reaction layers, which consist of HfO2 + Y2O3 and cubic HfO2 respectively. A single reaction layer consisting of HfO2 + Y2O3 constitutes the interface between the alloy and the pure yttria mould shell, and the interface between the alloy and the mould shell with zirconia face coat is also composed of a single reaction layer but consisting of cubic HfO2. Both pure yttria and yttria doped with La2O3 + ZrO2 mould shells are more suitable for the investment casting of the alloy.Download high-res image (148KB)Download full-size image
Co-reporter:Yanqiang Qiao, Jianping Kong, Qi Li, Xiping Guo
Surface and Coatings Technology 2017 Volume 327(Volume 327) pp:
Publication Date(Web):25 October 2017
DOI:10.1016/j.surfcoat.2017.07.013
•Two kinds of Si-B-Y co-deposition coatings were prepared on an Nb-Ti-Si based alloy.•The coating with higher B content shows a porous NbB2 outer layer.•The coating with less B content exhibits a dense NbSi2 outer layer with NbB2 dispersed in its upper part.•The coating with less B content exhibited a good oxidation resistance at 1250 °C.•The good oxidation protection should be ascribed to the compactness, proper B and Y contents in the coating.Two kinds of Si-B-Y co-deposition coatings on an Nb-Ti-Si based alloy were prepared by pack cementation method and their microstructure and oxidation behavior were investigated. Results indicated that the coating prepared with higher B content in the pack was comprised of a porous NbB2 outer layer, a NbSi2 intermediate layer and a transitional layer, while the coating prepared with less B content in the pack consisted of a NbSi2 outer layer with NbB2 dispersed in its upper part and a transitional layer. The oxidation tests showed that both the coatings can provide a good protection at 850 °C within 100 h. However, the coating prepared with higher B was non-protective at 1250 °C, and the formed scale was brittle and can peel off easily. The coating prepared with less B content exhibited a good oxidation resistance at 1250 °C, which should be attributed to the compactness of the coating, a proper B content and the Y elements in the coating.
Co-reporter:Xuan LI, Xi-ping GUO, Yan-qiang QIAO
Transactions of Nonferrous Metals Society of China 2016 Volume 26(Issue 7) pp:1892-1901
Publication Date(Web):July 2016
DOI:10.1016/S1003-6326(16)64267-X
Zr-Y jointly modified silicide coatings were prepared on an Nb-Ti-Si-Cr based ultrahigh temperature alloy by pack cementation process. The wear behaviors of both the base alloy and coatings were comparatively studied at room temperature and 800 °C using SiC balls as the counterpart. The Zr-Y jointly modified silicide coating is mainly composed of a thick (Nb,X)Si2 outer layer and a thin (Ti, Nb)5Si4 inner layer. The coatings possess much higher microhardness than the base alloy. The wear rates of both the base alloy and coatings increase with increasing the sliding loads. However, the coatings have much lower wear rates than the base alloy under the same sliding conditions. The coatings have superior anti-friction property, and can provide effective protection for the base alloy at both room temperature and 800 °C in air.
Co-reporter:Xiao Ma, Xiping Guo, Maosen Fu, Yanqiang Qiao
Intermetallics 2016 70() pp: 17-23
Publication Date(Web):March 2016
DOI:10.1016/j.intermet.2015.11.001
•Metastable fcc-Ti grains precipitate from Nbss in Nb–Ti–Si based ultrahigh temperature alloys.•Fcc-Ti precipitates partially transform into twinned hcp-Ti lamellas during homogeneous heat-treatment.•Stacking faults (SFs) play an important role in the fcc-Ti → hcp-Ti martensitic transformation.•The scale and stress state of metastable fcc-Ti might be critical to its stability.The morphologies, crystallographic characteristics and phase transitions of Ti precipitates developed in arc-melted and heat-treated Nb–Ti–Si based ultrahigh temperature alloys have been investigated by high resolution transmission electron microscopy (HRTEM). Rod-like Ti precipitates with fcc structure could only be found at interfaces in the arc-melted specimens, while ultrafine fcc-Ti precipitates formed inside Nbss in the heat-treated specimens. The fcc-Ti precipitates at interfaces partially transformed into hcp-Ti lamellas through a martensitic transformation during heat-treatment, and twins composed of different directional hcp-Ti lamellas formed in the fcc-Ti matrix with the orientation relationship (OR): [110]fcc−Ti//[112¯0]M//[1¯1¯20]T. The precipitation mechanisms of fcc-Ti grains in Nbss and hcp-Ti lamellas in fcc-Ti matrix have been analyzed.Different directional hcp-Ti lamellas precipitated inside fcc-Ti matrix through a martensitic transformation during heat-treatment. The newly born hcp-Ti martensites have an orientation relationship (OR) of [110]fcc−Ti//[112¯0]hcp−Ti and coherent interface with fcc-Ti matrix. Twins have been formed between different directional lamellas with OR of [110]fcc−Ti//[112¯0]M//[1¯1¯20]T and parabolic incoherent twin boundaries (ITBs).
Co-reporter:Tao Yang, Xiping Guo, Yucheng Luo
International Journal of Refractory Metals and Hard Materials 2016 Volume 56() pp:35-43
Publication Date(Web):April 2016
DOI:10.1016/j.ijrmhm.2015.11.012
•Dissoltion of B in Mo under present condition can be negligible.•Morphology and internal structure of powder particles change markedly with MA.•After 40 h of milling, almost strain-free Moss with grain size of 6.5 nm forms.•Mo lattice constant changes with the dissolution of Si and Zr atoms.•Thermodynamic analysis shows preferred formation for Moss than amorphous phase.Elemental powder mixtures with compositions of Mo–13.8Si, Mo–20B and Mo–12Si–10B–3Zr–0.3Y (at.%) were respectively milled in a high energy planetary ball mill at a speed of 500 rpm. Microstructural evolution of powder particles during milling processes was evaluated. The results show that B can hardly be dissolved into Mo under present milling conditions and the additions of B and Si both accelerate the refining rate of Mo crystallites. For Mo–12Si–10B–3Zr–0.3Y system, the morphology and internal structure of powder particles change significantly with milling time. After 40 h of milling, an almost strain-free super-saturated molybdenum solid solution with a grain size of about 6.5 nm forms. The grain refinement mechanism and dissolution kinetics of solute atoms are highlighted. Both thermodynamic calculation and experimental results reveal that for the present alloy composition it is more favorable to form solid solution than amorphous phase.Microstructural evolution of mechanically alloyed Mo–Si–B–Zr–Y powders is evaluated from the points of particle morphology and internal structure, phase constituent, gain size, microstrain, and dissolution of solute atoms.
Co-reporter:Song Zhang, Xiping Guo
Intermetallics 2016 70() pp: 33-44
Publication Date(Web):March 2016
DOI:10.1016/j.intermet.2015.12.002
•B or Cr promotes hypereutectic formation and Cr exerts no influence on silicides.•The combined additions of Hf and B has a strong effect of promoting γ(Nb,X)5Si3.•Cr is detrimental to fracture toughness but beneficial to macrohardness.•Macrohardness obviously decreases for Hf-containing alloys after heat-treatment.•In the presence of B and/or Cr, Hf addition degrades the oxidation resistance.Eight multi-component Nb–Si based ultrahigh temperature alloys were prepared by vacuum non-consumable arc melting. The effects of Hf, B and Cr alloying on the phase selection, phase stability, both non-equilibrium and equilibrium microstructure, room-temperature fracture toughness, hardness and oxidation resistance at 1250 °C of the alloys have been investigated and estimated systematically. The results show that the addition of B or Cr promotes the formation of hypereutectic structures. The alloying with both Hf and B suppresses the formation of β(Nb,X)5Si3 and promotes the formation of α(Nb,X)5Si3 and γ(Nb,X)5Si3, while the alloying with Cr has no effect on the crystal structures of 5-3 silicides. The room-temperature fracture toughness of the alloys is always degraded by the addition of Cr but almost not influenced by the combined additions of Hf and B. The hardness of 5-3 silicides exhibits a tendency of γ > α > β. The macrohardness of the alloys increases with Cr addition, and it obviously reduces in the presence of Hf after 1450 °C/50 h heat-treatment. The best oxidation-resistant performance has been obtained for the alloy with both B and Cr additions. However, in the presence of B and/or Cr, the oxidation resistance of the alloys has been degraded by further addition of Hf.
Co-reporter:Song Zhang, Xiping Guo
Intermetallics 2015 Volume 64() pp:51-58
Publication Date(Web):September 2015
DOI:10.1016/j.intermet.2015.04.017
•The combined addition of B and Hf has a strong effect of promoting γ(Nb,X)5Si3.•β(Nb,X)5Si3 completely transformed into α(Nb,X)5Si3 during heat-treatment.•α(Nb,X)5Si3 can be in equilibrium with γ(Nb,X)5Si3 in the alloy with B or Hf.•In the presence of B, lower (Ti + Hf) concentration can still stabilize γ(Nb,X)5Si3.•The hardness of 5-3 silicides exhibits a trend of γ > α > β.Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures.
Co-reporter:Ping Zhang, Xiping Guo
Surface and Coatings Technology 2015 Volume 274() pp:18-25
Publication Date(Web):25 July 2015
DOI:10.1016/j.surfcoat.2015.04.016
•Silicide/aluminide composite coatings were prepared on Nb–Ti–Si based alloy.•A two-step process including siliconizing firstly and then aluminizing was applied.•Al moves across the as-siliconized coating and an Al3(Nb,X) layer forms beneath it.•A dense and double layer scale forms on the coating after oxidation at 1250 °C/50 h.Silicide/aluminide composite coatings were prepared on an Nb–Ti–Si based alloy by a two-step pack cementation process including siliconizing firstly and then aluminizing. The siliconized coating is composed of a thick (Nb,X)Si2 outer layer and a thin (Ti,Nb)5Si4 inner layer. During the aluminizing process, the original (Nb,X)Si2 outer layer retains and Al moves through it to form an Al3(Nb,X) layer beneath it. Besides, a thin Al3(Nb,X) outermost layer also forms above the (Nb,X)Si2 layer. This composite coating possesses good oxidation resistance at 1250 °C within 50 h, and a double-layered scale with an Al2O3 outer layer and a SiO2-matrix inner layer forms.
Co-reporter:Xuan Li;Yanqiang Qiao
Oxidation of Metals 2015 Volume 84( Issue 3-4) pp:447-462
Publication Date(Web):2015 October
DOI:10.1007/s11085-015-9564-1
Oxidation-resistant ZrSi2–NbSi2 bilayer coatings were prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy using a two-step (first sputtering Zr film, and then Si–Y co-deposition) process. The coatings prepared with different Zr film thicknesses possess a similar structure, consisting of a ZrSi2 outer layer, a (Nb,X)Si2 (X represents Ti, Cr and Hf) middle layer and a (Ti,Nb)5Si4 inner layer. The higher co-deposition temperature (Si–Y co-deposition at 1350 °C) would cause cracks at the interfaces between the constituent layers of the coatings. The coated specimens possess much lower mass gains than the base alloy after exposure of same static oxidation times at 1250 °C. After oxidation, a dense scale consisting of a mixture of SiO2, ZrSiO4, ZrO2, Al2O3, TiO2 and Cr2O3 formed, which could protect the specimen from oxidation at least for 100 h at 1250 °C in air.
Co-reporter:Xuan Li;Yanqiang Qiao
Oxidation of Metals 2015 Volume 83( Issue 3-4) pp:253-271
Publication Date(Web):2015 April
DOI:10.1007/s11085-014-9519-y
Zr–Y modified silicide coatings have been prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy by a pack cementation process. The effects of amount of Zr powder in the pack mixture and co-deposition temperature on the coating structure were assessed. The coatings prepared at 1,250 °C with different amounts of Zr powders in the pack mixture have similar structures, mainly consisting of a thick (Nb,X)Si2 (X represents Ti, Cr and Hf elements) outer layer, a thin (Ti,Nb)5Si4 middle layer and a 1–2 µm thick (Nb,X)5Si3 inner layer. The increased amount of Zr powders in the pack mixtures led to a significant decrease in the coating thickness. The coatings prepared at 1,150 and 1,200 °C have similar structures to that prepared at 1,250 °C. However, when the co-deposition temperature increases to 1,300 and 1,350 °C, a (Ti,Nb)5Si3 outermost layer formed in addition to the three layers mentioned above. The Zr–Y modified silicide coating can protect the base alloy from oxidation at least for 100 h at 1,250 °C in air. The good oxidation resistance of the Zr–Y modified silicide coating is attributed to the formation of a dense scale consisting of SiO2 and TiO2.
Co-reporter:Song Zhang, Xiping Guo
Intermetallics 2015 Volume 57() pp:83-92
Publication Date(Web):February 2015
DOI:10.1016/j.intermet.2014.10.007
•Four multicomponent Nb–Si based alloys with different B contents were prepared.•B addition produced significant effects on phase selection and microstructure.•B addition improved oxidation resistance and an oxidation mechanism was proposed.•B addition imposed significant influences on fracture toughness and microhardness.Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–5Cr–4Hf–3Al–xB (x = 0, 2, 5 and 10, respectively) (at%) were prepared by vacuum non-consumable arc melting. The effects of B content on the phase selection, microstructure, oxidation resistance at 1250 °C, room-temperature fracture toughness and microhardness of the alloys were investigated. The results showed that the microstructures of the four alloys were all comprised of primary silicide blocks, Nbss dendrites and eutectic colonies. However, the crystal structures or types of silicides and the amounts of constituent phases obviously varied with increase in B content in the alloys. The oxidation resistance of the alloys was significantly ameliorated by B addition. The room temperature fracture toughness of the alloys was improved with 2 at% B addition but degraded with 5 or 10 at% B addition. The microhardness of Nbss rose slightly with increase in B content in the alloys, while that of silicides was dependent on their crystal structures, types and concentrations of alloying elements.
Co-reporter:Song Zhang, Xiping Guo
Materials Science and Engineering: A 2015 Volume 638() pp:121-131
Publication Date(Web):25 June 2015
DOI:10.1016/j.msea.2015.04.003
Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Al–2B–xHf–yCr ((x, y)=(0, 0), (0, 5), (4, 0) and (4, 5)) (at%) were prepared by vacuum non-consumable arc melting. The effects of Cr and Hf additions on the phase selection, microstructure, room-temperature fracture toughness and oxidation resistance at 1250 °C of the alloys have been investigated. The results show that all the four alloys are comprised of primary silicides and Nbss dendrites together with one or two types of eutectic. The Cr addition does not change the crystal structures of 5-3 silicides but imposes a significant influence on the amount, size and morphology of the constituent phases. The Hf addition suppresses the formation of α(Nb,X)5Si3 but promotes the formation of γ(Nb,X)5Si3. The combined additions of Cr and Hf also cause the formation of Cr2(Nb,X) containing three-phase eutectic. The room-temperature fracture toughness of the alloys decreases with Cr addition while increases with Hf addition. The oxidation-resistant performance of the alloys has been ameliorated by Cr addition, while it has been significantly degraded by Hf addition in the absence of Cr. Furthermore, the formation mechanism of the scale has been illustrated.
Co-reporter:Song Zhang, Xiping Guo
Materials Science and Engineering: A 2015 Volume 645() pp:88-98
Publication Date(Web):1 October 2015
DOI:10.1016/j.msea.2015.08.006
Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Cr–3Al–2B–xHf (x=0, 2, 4 and 8) (at%) were prepared by vacuum non-consumable arc melting. The effects of Hf content on the phase selection, microstructure, room-temperature fracture toughness, compressive performance and isothermal oxidation behavior at 1250 °C of the alloys have been investigated. The results show that the formation of α(Nb,X)5Si3 is suppressed and instead the formation of γ(Nb,X)5Si3 is promoted by Hf addition. The Hf addition (especially 8 at%) improves the room-temperature fracture toughness of the alloy. The hardness of γ(Nb,X)5Si3 increases with its Hf concentration. The compression strength of the alloy is the highest at 4 at% Hf, and then obviously reduces at 8 at% Hf due to embrittlement of the alloy (especially that of γ(Nb,X)5Si3). The oxidation resistance of the alloy has been firstly ameliorated by the lower Hf addition (2 at%) and then degraded by the higher Hf addition (especially 8 at%). The details have been discussed.
Co-reporter:Baohui Guo, Xiping Guo
Materials Science and Engineering: A 2014 617() pp: 39-45
Publication Date(Web):
DOI:10.1016/j.msea.2014.08.009
Co-reporter:Yingtian Liu, Xiping Guo
Progress in Natural Science: Materials International 2013 Volume 23(Issue 2) pp:190-197
Publication Date(Web):April 2013
DOI:10.1016/j.pnsc.2013.02.003
In order to protect NbTiSi based ultrahigh temperature alloy from oxidation, pack cementation processes were utilized to prepare Ce and Y jointly modified silicide coatings. The Ce and Y jointly modified silicide coating has a double-layer structure: a relatively thick (Nb, X)Si2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Ti, Nb)5Si4 transitional layer. The pack cementation experiments at 1150 °C for 8 h proved that the addition of certain amounts of CeO2 and Y2O3 powders in the packs distinctly influenced the coating thickness, the contents of Si, Ce and Y in the (Nb, X)Si2 outer layers, and the density of cavities in the coatings. In order to study the effects of Ce and Y joint modification in the silicide coatings, both only Ce and only Y modified silicide coatings were also prepared for comparison. The mechanisms of the beneficial effects of Ce and Y are discussed. A pack mixture containing 1.5CeO2–0.75Y2O3 (wt%) powders was employed to investigate the growth kinetics of the Ce and Y jointly modified silicide coating at 1050, 1150 and 1250 °C. It has been found that the growth kinetics obeyed parabolic laws and the parabolic rate constants were 109.20 μm2/h at 1050 °C, 366.75 μm2/h at 1150 °C and 569.78 μm2/h at 1250 °C, and the activation energy for the growth of the Ce and Y jointly modified silicide coating was 197.53 kJ/mol.
Co-reporter:Zhiping Sun, Xiping Guo, Chuan Zhang
Journal of Alloys and Compounds 2012 Volume 522() pp:149-156
Publication Date(Web):5 May 2012
DOI:10.1016/j.jallcom.2012.01.142
Nb–Ti–Si based alloys are considered as candidates of next-generation high temperature materials (i.e., working temperature >1200 °C). Boron has a beneficial effect in enhancing the oxidation resistance and reducing the anisotropy ratio ac/aa of the T2 phase in the Nb–Si alloys. The liquid–solid multiphase equilibria in the Nb–Ti–Si alloys with B additions have been investigated via an approach of integrating thermodynamic modeling with designed experiments. The present study suggests that Ti and Si additions increase the stability of Nb3B2, and the primary region of Nb3B2 will appear in the Nb–Ti–Si–B quaternary, while there is no primary solidification region of Nb3B2 in the constitute binaries and ternaries. The proposed liquidus surface of the Nb-rich Nb–Ti–Si–B system is associated with eight primary solidification regions of Nbss, T2, T1, D88, (Nb,Ti)3Si, NbB, TiB and Nb3B2. The direct eutectic solidification occurs between Nbss and T2 in the Nb-rich region in the Nb–Ti–Si–B liquidus projection, which could provide new opportunities for alloy design based on the Nb–Ti–Si–B system.Highlights► The primary region of Nb3B2 exists in the Nb–Ti–Si–B quaternary, while there are no primary regions of Nb3B2 in the constitute binaries and ternaries. ► The proposed liquidus surface of the Nb-rich Nb–Ti–Si–B system is associated with eight primary regions of Nbss, T2, T1, D88, (Nb,Ti)3Si, NbB, TiB and Nb3B2 respectively. ► The knowledge of phase equilibria in the Nb–Ti–Si–B system is essential to understand the microstructure of these Nb–Nb silicide based alloys generated by liquid–solid processes.
Co-reporter:Zhiping Sun, Xiping Guo, Chuan Zhang
Calphad 2012 Volume 36() pp:82-88
Publication Date(Web):March 2012
DOI:10.1016/j.calphad.2011.12.002
A thermodynamic description for the Nb–Si–Sn system has been developed on the basis of the constituent binaries and critically reviewed ternary experimental data. The published thermodynamic descriptions for the Nb–Si and Nb–Sn binaries were directly used and that for the Si–Sn binary was remodeled in the present study. A two-sublattice model, (Nb,Si,Sn)3 (Nb, Si, Sn), was applied to the A15 phase considering its crystal structure and homogeneity range. The isothermal sections at 1600 °C, 1500 °C, 1200 °C and 900 °C, the liquidus projection and the solidification path of the alloy (Nb–18Si–5Sn) were calculated accordingly based on the currently obtained thermodynamic description and compared with the experimental results. Comparison between the calculated results and the experimental measurements shows that the present modeling can provide a satisfactory account of the experimental information for the Nb-rich corner of the Nb–Si–Sn ternary phase diagram.Highlights► The Si–Sn binary has been thermodynamically remodeled. ► A thermodynamic description for the Nb–Si–Sn ternary system has been developed. ► The phase equilibrium calculations yield good agreements with experimental results. ► The liquidus projection of the Nb–Si–Sn ternary system has been predicted.
Co-reporter:Wang Mao, Xiping Guo
Progress in Natural Science: Materials International 2012 Volume 22(Issue 2) pp:139-145
Publication Date(Web):April 2012
DOI:10.1016/j.pnsc.2012.03.008
The phase compositions, microstructure and especially phase interfaces in the as-cast and heat-treated Nb–Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary phases in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6Cr–4Al–5Hf–2B–0.06Y (S2) (at%) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1.5B–0.06Y (S3) (at%) alloy. An orientation relationship between Nbss and γ(Nb,X)5Si3 was determined to be (11¯0)Nb//(101¯0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transformed into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heat treatment at 1500 °C for 50 h in the S1 alloy. Meanwhile, Ti diffused from Nbss to β(Nb,X)5Si3, which induced αTi to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newly-formed αTi and previous Nbss was (110)Nb//(11¯01¯)αTi and [001]Nb//(123¯1¯)αTi. Among the (Nb,X)5Si3 phases, the contents of Cr and Al in β(Nb,X)5Si3 are nearly the same as those in γ(Nb,X)5Si3 but obviously higher than those in the α(Nb,X)5Si3, whereas the content of Si in α(Nb,X)5Si3 is nearly the same as that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3.
Co-reporter:Ping Zhang, Xiping Guo
Surface and Coatings Technology 2011 206(2–3) pp: 446-454
Publication Date(Web):
DOI:10.1016/j.surfcoat.2011.07.056
Co-reporter:Yanqiang Qiao, Xiping Guo
Applied Surface Science 2010 Volume 256(Issue 24) pp:7462-7471
Publication Date(Web):1 October 2010
DOI:10.1016/j.apsusc.2010.05.091
Abstract
Cr-modified silicide coatings were prepared on a Ti–Nb–Si based ultrahigh temperature alloy by Si–Cr co-deposition at 1250 °C, 1350 °C and 1400 °C for 5–20 h respectively. It was found that both coating structure and phase constituents changed significantly with increase in the co-deposition temperature and holding time. The outer layers in all coatings prepared at 1250 °C for 5–20 h consisted of (Ti,X)5Si3 (X represents Nb, Cr and Hf elements). (Ti,X)5Si4 was found as the only phase constituent in the intermediate layers in both coatings prepared at 1250 °C for 5 and 10 h, but the intermediate layers in the coatings prepared at 1250 °C for 15 and 20 h were mainly composed of (Ti,X)5Si3 phase that was derived from the decomposition of (Ti,X)5Si4 phase. In the coating prepared at 1350 °C for 5 h, single (Ti,X)5Si3 phase was found in its outmost layer, the same as that in the outer layers in the coatings prepared at 1250 °C; but in the coatings prepared at 1350 °C for 10–20 h, (Nb1.95Cr1.05)Cr2Si3 ternary phase was found in the outmost layers besides (Ti,X)5Si3 phase. In the coatings prepared at 1400 °C for 5–20 h, (Nb1.95Cr1.05)Cr2Si3 ternary phase was the single phase constituent in their outmost layers. The phase transformation (Ti,X)5Si4 → (Ti,X)5Si3 + Si occurred in the intermediate layers of the coatings prepared at 1350 and 1400 °C with prolonging co-deposition time, similar to the situation in the coatings prepared at 1250 °C for 15 and 20 h, but this transformation has been speeded up by increase in the co-deposition temperature. The transitional layers were mainly composed of (Ti,X)5Si3 phase in all coatings. The influence of co-deposition temperature on the diffusion ability of Cr atoms was greater than that of Si atoms in the Si–Cr co-deposition processes investigated. The growth of coatings obeyed inverse logarithmic laws at all three co-deposition temperatures. The Si–Cr co-deposition coating prepared at 1350 °C for 10 h showed a good oxidation resistance due to the formation of SiO2 and Nb, Cr-doped TiO2 scale after oxidation at 1250 °C for 10 h.
Co-reporter:Zhiping Sun, Xiping Guo, Yongsheng He, Jinming Guo, Ying Yang, Y. Austin Chang
Intermetallics 2010 Volume 18(Issue 5) pp:992-997
Publication Date(Web):May 2010
DOI:10.1016/j.intermet.2010.01.024
Refractory metal intermetallic composites (RMICs) based on the Nb–Ti–Si–Cr–Hf quinary are candidate materials for ultra-high temperature applications beyond 1200 °C. In this investigation, minor additions of Al, B, Y and Mo were introduced into the Nb-22Ti-(15–18)Si-6Cr-4Hf (at.%) base alloy for potential enhancement of the mechanical and environmental properties. The phases, phase compositions and morphologies in the as-cast microstructure of the base alloy with or without minor Al, B, Y and Mo additions were investigated using X-ray diffraction (XRD), optical metallography (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS) and electron probe microanalysis (EPMA). The experimental results obtained from several selected alloys were found to agree with the solidification paths calculated from the Scheil model using established thermodynamic properties of all the phases in the Nb–Ti–Si–Cr–Hf–Al system.
Co-reporter:Xiaodong Tian, Xiping Guo
Surface and Coatings Technology 2009 203(9) pp: 1161-1166
Publication Date(Web):
DOI:10.1016/j.surfcoat.2008.10.018
Co-reporter:Xiaodong Tian, Xiping Guo
Surface and Coatings Technology 2009 204(3) pp: 313-318
Publication Date(Web):
DOI:10.1016/j.surfcoat.2009.07.031
Co-reporter:Wang Mao, Xiping Guo
Progress in Natural Science: Materials International (April 2012) Volume 22(Issue 2) pp:139-145
Publication Date(Web):1 April 2012
DOI:10.1016/j.pnsc.2012.03.008
The phase compositions, microstructure and especially phase interfaces in the as-cast and heat-treated Nb–Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary phases in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6Cr–4Al–5Hf–2B–0.06Y (S2) (at%) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1.5B–0.06Y (S3) (at%) alloy. An orientation relationship between Nbss and γ(Nb,X)5Si3 was determined to be (11¯0)Nb//(101¯0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transformed into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heat treatment at 1500 °C for 50 h in the S1 alloy. Meanwhile, Ti diffused from Nbss to β(Nb,X)5Si3, which induced αTi to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newly-formed αTi and previous Nbss was (110)Nb//(11¯01¯)αTi and [001]Nb//(123¯1¯)αTi. Among the (Nb,X)5Si3 phases, the contents of Cr and Al in β(Nb,X)5Si3 are nearly the same as those in γ(Nb,X)5Si3 but obviously higher than those in the α(Nb,X)5Si3, whereas the content of Si in α(Nb,X)5Si3 is nearly the same as that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3.
