α+β固溶处理对Ti65合金薄板的影响:硅化物析出、力学行为和新型{1011}孪晶系

为应对航空领域对轻质和优异高温性能钛合金部件的迫切需求,对经不同热处理的新型高温钛合金Ti65合金薄板的显微组织演变、硅化物析出和力学行为进行系统研究。结果表明:在固溶空冷后的合金中析出亚微米级硅化物(Ti,Zr)_(6)Si_(3),导致拉伸性能变差和剧烈波动。在950℃固溶炉冷后的合金中发现穿晶界(Ti,Zr)_(5)Si_(3)和近晶界(Ti,Zr)_(6)Si_(3)硅化物的共存。当固溶温度升高至1010℃时,(Ti,Zr)_(3)Si硅化物在位错附件出现。薄板固有尺寸效应与硅化物析出的协同效应导致拉伸性能的明显波动,塑性显著下降,甚至发生脆性断裂。硅化物析出受低温固溶处理的保温时间的影响较弱。此外,在990℃热轧获得的薄板中观察到新型{1011}孪晶,其轴角对为65°<8443>,仅存在于粗大的基面织构α晶粒中。

Effect of mold temperature and casting dimension on microstructure and tensile properties of counter-gravity casting Ti-6Al-4V alloys

The cast Ti-6Al-4V alloy bars with different section sizes were fabricated by investment casting at counter-gravity condition with the mold temperatures of 300 °C and 650 °C, respectively. The microstructure of the alloy was observed by means of OM and SEM, and the effect of mold temperature and casting dimension on tensile properties was studied. Results show that equiaxed grains are obtained regardless of the casting dimension. β grain size tends to increase with an increase in mold temperature. Hot isostatic pressing of the alloy was carried out for tensile properties' comparison. Room temperature tensile test results show that Ti-6Al-4V alloy produced via counter-gravity casting has good balance of strength and ductility after hot isostatic pressing(HIP). The alloy shows higher ductility due to the elimination of porosity. In both cast and HIP status, the tensile strength is inclined to decrease with an increase in mold temperature, while the ductility is prone to slightly increase. Both the strength and ductility tend to decrease with an increase in the casting dimension.

Limitation of the Johnson-Mehl-Avrami equation for the kinetic analysis of crystallization in a Ti-based amorphous alloy

The primary crystallization of the Ti40Zr25NisCu9Bc18 amorphous alloy was studied by isochronal differential scanning calorimetry （DSC）. The activation energy was determined by the Kissinger-Akahim-Sunose method. Trying to analyze the crystallization kinetics of the Ti40Zr25NigCu9Be18 amorphous alloy by two different methods, it was found that the crystallization kinetics did not obey the Johnson-Mehl-Avrami equation. A modified method in consideration of the impingement effect was proposed to perform kinetic analysis of the isochronal crystallization of this alloy. The kinetic parameters were then obtained by the linear fitting method based on the modified kinetic equation. The results show that the isochronal crystallization kinetics of the amorphous Ti40Zr25Ni8CugBe18 alloy is heating rate dependent, and the discrepancy between the Johnson-Mehl-Avrami method and the modified method increases with the increase of heating rate.

Evolution of microstructure and hardness in a dual-phase Al0.5CoCrFeNi high-entropy alloy with different grain sizes

Cold-rolling with subsequent annealing was carried out to produce recrystallized structures with different grain sizes in an Al0.5CoCrFeNi high-entropy alloy to systematically investigate the grain growth behavior and varying properties.The results show that recrystallized microstructures can be achieved through an annealing process at 1200℃for 75 min to 16 h,and the average grain size in this study ranges from 5.33 to 30.03 μm.The hardness shown to be affected through grain coarsening was then measured as a function of the grain size,and it is found to follow the classical Hall-Petch strengthening.

Solidification characteristics of high Nb-containing γ-TiAl-based alloys with different aluminum contents

The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron microscopy （SEM） equipped with energy dispersive spectroscope （EDS）, and transmission electron microscopy （TEM）. The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47A17Nb2.5V1.0Cr （at%） alloy solidifying through the peritectic reaction. The β-solidifying Ti46A17Nb2.5V1.0Cr （at%） alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48A17Nb2.5V1.0Cr （at%） alloy is equivalent to that of Ti46A17Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.

Improved tensile properties of Al_(0.5)CoCrFeNi high-entropy alloy by tailoring microstructures

The effect of heat treatment on microstructure and tensile properties of as-cast Al_(0.5)CoCrFeNi high-entropy alloy was investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM) equipped with energy-dispersive spectroscopy(EDS), and tensile tests.The results show that heat treatment strongly affects the microstructure, particularly the morphology of bcc phases,and improves tensile properties. Microstructure analysis reveals that rod-shaped and elliptoid phases appear in the matrix after heat treatment at 1150 ℃. Besides, under850 ℃ heat-treated condition, ultimate tensile strength increases by about 60% without sacrificing much plasticity,which can be attributed to the content of bcc phases and fine precipitates dispersed in the dendrites. For other heattreated conditions, tensile ductility increases by at least30%, especially 60% for heat treatment at 450 ℃, and strength also improves. Fracture analysis indicates that the fracture mode of heat treatment at 850 ℃ is a mixture of quasi-cleavage and ductile fracture, while the other heattreated conditions show the mode of ductile fracture.

Soldering of Zr-based bulk metallic glass and copper by Au-12Ge eutectic alloy

Zr-based bulk metallic glass and copper with different surface roughness were soldered using low temperature eutectic Au-12 Ge(wt%) solder on a thermomechanical simulator. The cross-sectional microstructures of the brazed joints were analyzed by scanning electron microscopy(SEM) and transmission electron microscope(TEM) in detail, and the compositional distribution along the interface was analyzed by energy-dispersive spectrometer(EDS). Results show that the surface roughness of base metals plays an important role in the quality of the brazed joint because the surface roughness can enlarge the effective contact area, which can improve the brazing surface quality between two materials. A moderate roughness of treated Zr-based metallic glass of 18 μm is shown to be the best for the soldering, while the surface roughness has a weak effect on the soldering behavior of Au-12 Ge solder on copper. After soldering, long-range diffusion of atoms occurs between the base metal and solder, and five distinct regions are formed at the joint region.

Microstructure Evolution and Phase Transformation of Ti–6.5Al–2Zr–Mo–V Alloy During Thermohydrogen Treatment

Thermohydrogen treatment （THT） is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys. In the current work, as-cast Ti-6.5Al-2Zr-Mo-V alloy was hydrogenated with different hydrogen contents and processed solution aging. Accordingly, the microstructure evolution and phase transformation were analyzed. Results show that during solution aging, eutectoid decomposition occurs and the product is a mixture of coarse primaryα, fine eutectoid product and undecomposed βH. The size of primary α is closely dependent on the hydrogen content, and large primary α can be obtained at medium hydrogen content. Further, the influence of hydrogen content on the growth of primary α phase was revealed. The primary α is much fine, and the eutectoid product is relatively homo- geneous with 0.984 wt% H. After THT, the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.

Physical properties of WSTi3515S burn-resistant titanium alloy

Development of burn-resistant titanium alloys is the most direct way of mitigating the ignition and propagation of titanium fires in jet engines. WSTi3515S alloy（Ti–35V–15Cr–0.3Si–0.1C） is a new high alloying beta type burn-resistant titanium alloy, belonging to Ti–V–Cr type alloys which have been made significant progress in engineering technology in the past 5 years. The physical properties of WSTi3515S burn-resistant titanium alloy such as the elastic properties and thermal properties were measured and analyzed in different conditions. The results show that both the Young＇s modulus and shear modulus of WSTi3515S alloy decrease slightly with the temperature increasing at the tested temperature range. The Poisson＇s ratio of WSTi3515S alloy is around 0.36. However, the thermal properties such as the specific heat, thermal diffusivity, thermal conductivity and thermal expansion increase with the temperature increasing, which results from the strengthening of lattice heat vibration at elevated temperature. And the room temperature density of WSTi3515S alloy is 5.295 gácm^（-3）.

Solidification microstructure characteristics of Ti–44Al–4Nb–2Cr–0.1B alloy under various cooling rates during mushy zone

Beta-solidifying TiAl alloy has great potential in the field of aero-industry as a cast alloy.In the present work,the influence of cooling rate during mushy zone on solidification behavior of Ti-44Al-4Nb-2Cr-0.1B alloy was investigated.A vacuum induction heating device combining with temperature control system was used.The Ti-44Al-4Nb-2Cr-0.1B alloy solidified from superheated was melted to β phase with the cooling rates of 10,50,100,200,400 and 700 K·min^（-1）,respectively.Results show that with the increase in cooling rate from 10 to 700 K·min^（-1）,the colony size of α_2/γ lamella decreases from 1513 to48 urn and the solidification segregation significantly decreases.Also the content of residual B2 phase within α_2/γlamellar colony decreases with the increase in cooling rate.In addition,the alloy in local interdendritic regions would solidify in a hypo-peritectic way,which can be attributed to the solute redistribution and enrichment of Al element in solidification.

Dry-sliding tribological properties of AlCoCrFeNiTi_(0.5)high-entropy alloy

High-entropy alloys(HEAs),with a new alloying concept,could possess many unique mechanical and functional properties.The current work investigated whether one such alloy offers potential for bearing surfaces under dry conditions.The dry,reciprocating sliding wear characteristics of AlCoCrFeNiTi0.5alloy were investigated under various applied loads and sliding speeds.Transmission electron microscopy(TEM) and scanning electron microscopy(SEM) were utilized to characterize internal structure and wear surfaces of the alloy,respectively.It is found that the AlCoCrFeNiTi0.5alloy preserves better wear resistance than Fe77Ni23solid solution alloy,Ti-46Al-2Cr-2Nb intermetallic alloy,or a wear-resistant steel AISI52100,especially under higher loads.The wear rate increases slowly with the applied loads increasing and keeps steady under different sliding speeds.The wear mechanisms are abrasive wear,adhesive wear and oxidative wear.The nano-sized Fe-Cr solid solution and Al-NiTi rich intermetallic phase precipitated in the dendritic regions and the formation of oxidation play important roles in the good wear resistances of this high-entropy alloy.