Shulong Xiao

Microstructures and mechanical properties of TiAl alloy prepared by spark plasma sintering

Abstract A fine-grained TiAl alloy with a composition of Ti-47%Al(mole fraction) was prepared by double mechanical milling(DMM) and spark plasma sintering(SPS). The relationship among sintering temperature, microstructure and mechanical properties of Ti-47%Al alloy was studied by X-ray diffractometry(XRD), scanning electron microscopy(SEM) and mechanical testing. The results show that the morphology of double mechanical milling powder is regular with size of 20−40 μm. The main phase TiAl and few phases Ti 3 Al and Ti 2 Al were observed in the SPS bulk samples. For samples sintered at 1000 °C, the equiaxed crystal grain was achieved with size of 100−250 nm. The samples exhibited compressive and bending properties at room temperature with compressive strength of 2013 MPa, compression ratio of 4.6% and bending strength of 896 MPa. For samples sintered at 1100 °C, the size of equiaxed crystal grain was obviously increased. The SPS bulk samples exhibited uniform microstructures, with equiaxed TiAl phase and lamellar Ti 3 Al phase were observed. The samples exhibited compressive and bending properties at room temperature with compressive strength of 1990 MPa, compression ratio of 6.0% and bending strength of 705 MPa. The micro-hardness of the SPS bulk samples sintered at 1000 °C is obviously higher than that of the samples sintered at 1100 °C. The compression fracture mode of the SPS TiAl alloy samples is intergranular fracture and the bending fracture mode of the SPS TiAl alloy samples is intergranular rupture and cleavage fracture.

Microstructure, mechanical properties and dry wear resistance of β-type Ti–15Mo–xNb alloys for biomedical applications

Abstract In order to study the effect of element Nb on the microstructure and properties of the biomedical β -type Ti-Mo based alloys, Ti-15Mo- x Nb ( x =5, 10, 15 and 20 in %) alloys were investigated. The dry wear resistance of β -type Ti-15Mo- x Nb alloys against Gr15 ball was investigated on CJS111A ball-disk wear instrument. Experimental results indicate that crystal structure and morphology of the Ti-15Mo- x Nb alloys are sensitive to their Nb contents. Ti-15Mo- x Nb alloys match those for β phase peaks and no any phases are found. The Vickers hardness values of all the Ti-15Mo- x Nb alloys are higher than HV200. The compression yield strength of the Ti-15Mo-5Nb alloy is the lowest and that of the Ti-15Mo-10Nb alloy is the highest. For all the Ti-15Mo- x Nb alloys, the friction coefficient is not constant but takes a higher value. In dry condition, SEM study reveals deep parallel scars on the wear surfaces of all the Ti-15Mo- x Nb alloys under different loads. The friction coefficient of the Ti-15Mo-5Nb alloy under 1 N is the lowest. The wear principal mechanism for Ti-15Mo- x Nb alloys is adhesive wear.

Microstructure and dry wear properties of Ti-Nb alloys for dental prostheses

The microstructure and properties of a series of binary Ti-Nb alloys for dental prostheses with niobium contents ranging from 5% to 20% were investigated. The experimental results indicate that the crystal structure and morphology of Ti-Nb alloys are sensitive to their niobium contents. When Nb content is 5%, the acicular α crystal grain is observed. When Nb content is 10%, the coarse equiaxed crystal grain and the fine, acicular α crystal grain are observed. When Nb content is 15%, only the α equiaxed crystal grain is observed. When the alloy contains 20%Nb, the equiaxed and dendritic α crystal grain are observed. For Ti-Nb alloys, the increase of Nb content modifies the microstructure of Ti-Nb alloys significantly and decreases their compression elastic modulus, in which Ti-20Nb alloy shows the largest compression strength and Ti-5Nb alloy shows the best plasticity. The dry wear resistance of Ti-Nb alloys against Gr15 ball was investigated on CJS111A ball-disk wear instrument. For Ti-Nb alloys, Ti-10Nb alloy shows a smallest steady friction coefficient, Ti-5Nb alloy shows the smallest wear depth and best wear resistance, and Ti-15Nb alloy shows the largest wear depth and worst wear resistance. The phenomenon of furrow cut happens and furrows form during wear tests.

Microstructures and mechanical properties of hot-pack rolled Ti-43Al-9V-Y alloy sheet

Abstract Ti-43Al-9V-Y alloy sheets with dimensions of 300 mm×100 mm×(1.5–2) mm were produced by hot-pack rolling. After rolling, the microstructure of Ti-43Al-9V-Y alloy sheet becomes near gamma(NG), which is comprised of γ+ B 2 phases. After heat treatment(HT) at 1 200−1 320 °C for 30 min followed by furnace cooling(FC), network shape structure of B 2 phases in as-rolled microstructure is retained on the whole. Moreover, with increasing the HT temperature, precipitation of B 2 phase lamellae in equiaxed γ grains is increased. Equiaxed γ grains transform partly to α 2 /γ/ B 2 lamellar structure after the heat treatment at 1 320 °C for 30 min. Tensile test results show that room-temperature yield strength(YS) and ultimate tensile strength(UTS) of the as-rolled material are 509 and 612 MPa, respectively. With the test temperature increasing, the YS and UTS of the as-rolled are decreased, but the elongation is improved. After HT at 1 200 °C, both yield strength and fracture strength of Ti-43Al-9V-Y alloy sheet are the lowest. With HT temperature increasing, fracture strength is increased obviously, but yield strength of the sheet after HT at 1 280 °C is the highest, about 869 MPa.

Microstructure and mechanical properties of TiAl-based alloy prepared by double mechanical milling and spark plasma sintering

Abstract A fine-grained TiAl alloy with a composition of Ti–45Al–2Cr–2Nb–1B–0.5Ta–0.225Y (mole fraction, %) was prepared by double mechanical milling(DMM) and spark plasma sintering(SPS). The relationship among sintering temperature, microstructure and mechanical properties was studied. The results show that the morphology of double mechanical milled powder is regular with size in the range of 20–40 μm and mainly composed of TiAl and Ti3Al phases. The main phase TiAl and few phases Ti3Al, Ti2Al and TiB2 were observed in the SPSed alloys. For samples sintered at 900 °C, the equiaxed crystal grain microstructure is achieved with size in the range of 100–200 nm. With increasing the SPS temperature from 900 °C to 1000 °C, the size of equiaxed crystal grain obviously increases, the microhardness decreases from HV658 to HV616, and the bending strength decreases from 781 MPa to 652 MPa. In the meantime, the compression fracture strength also decreases from 2769 MPa to 2669 MPa, and the strain to fracture in compression increases from 11.69% to 17.76%. On the base of analysis of fractographies, it shows that the compression fracture transform of the SPSed alloys is intergranular rupture.

Microstructure and mechanical properties of Ti–43Al–9V alloy fabricated by spark plasma sintering

Abstract A fine-grained TiAl alloy with the composition of Ti–43Al–9V was prepared by mechanical milling and spark plasma sintering (SPS). The relationship among sintering temperature, microstructure and mechanical properties was studied. The results show that the morphology of mechanical milling powder is regular with size in a range of 5–30 μm. Main phases of γ-TiAl, α2-Ti3Al and few B2 phase are observed in the SPS bulk samples. For samples sintered at 1150 °C, equiaxed crystal grain microstructure is achieved with size in a range of 300 nm-1 μm. With increasing SPS temperature to 1250 °C, the size of equiaxed crystal grains obviously increases, the microhardness decreases from HV592 to HV535, and the bending strength decreases from 605 to 219 MPa. Meantime, the compression fracture strength also decreases from 2601 to 1905 MPa, and the strain compression decreases from 28.95% to 12.09%.

Effect of particle size distribution on properties of zirconia ceramic mould for TiAl investment casting

The effect of particle size distributions (PSDs) on the properties of zirconia ceramic mould for TiAl investment casting was described. The relationship between the zirconia powder characteristics and properties of the ceramic moulds was investigated. The particle size distribution, morphology of particles, viscosity of slurries, mechanical properties and fracture surfaces of ceramic moulds were examined. The effect of PSDs on the viscosity was observed through the measurement of slurries prepared from zirconia and binder. The morphology of the fracture surface of the zirconia moulds with different PSDs was also investigated. The measurement of bend strength shows that the mechanical behaviors of the green and fired zirconia ceramic moulds are comparable for all systems. The preliminary results illustrate that the PSDs play an important role in determining the quality of ceramic moulds and thus on the metallurgical quality of TiAl components produced by investment casting process.

Constitution modeling and deformation behavior of yttrium bearing TiAl alloy

Abstract The deformation flow behaviors of Ti-45Al-5.4V-3.6Nb-0.3Y alloy at different temperatures and strain rates were studied by isothermal compressing simulation test. The apparent activation energy of deformation was calculated to be 402.096 kJ/mol and constitutive equation was established to describe the flow behavior. Microstructure and flow softening observations exhibited that Ti-45Al-5.4V-3.6Nb-0.3Y alloy had bad hot workability at low temperature (lower than 1 100 °C) and high strain rate (higher than 0.5 s −1 ) characterized by localization deformation and instability. With deformation temperature higher than 1 150 °C and strain rate lower than 0.01 s −1 , the alloy owned good hot deformability, and plenty of dynamic recrystallized grains could be observed in the deformed microstructures.

Effect of Nb addition on microstructure, mechanical properties and castability of β-type Ti–Mo alloys

Abstract To develop novel β -type biomedical titanium alloys, a series of Ti–15Mo– x Nb alloys( x =0, 5, 10 and 15, mass fraction in %) were designed and prepared by using vacuum arc melting method. The present study focused on the effect of Nb addition on the microstructure, mechanical properties and castability of Ti–15Mo alloy. Phase analysis and microstructure observation show that all the alloys consist of single β phase and the equiaxed β grain is refined with increasing Nb content. These β -type Ti–15Mo– x Nb alloys exhibit good plasticity and rather low compression elastic modulus (in the range of 18.388–19.365 GPa). After Nb addition, the compression yield strength of the alloys increases. With increasing Nb content, the micro-hardness of the alloys decreases. The alloys exhibit obvious fibrous strip microstructure after cold compression deformation. The castability test shows that the castability of the alloys after Nb addition decreases and that of the Ti–15Mo alloy is the highest (92.01%).

Fracture characteristics of notched investment cast TiAl alloy through in situ SEM observation

TiAl alloys were produced by investment casting method combined with induction skull melting (ISM) technique. In situ scanning electron microscopy (SEM) was utilized to study the fracture characteristics and crack propagation of a notched investment cast TiAl specimens in tension under incremental loading conditions. The whole process of crack initiation, propagation and failure during tensile deformation was observed and characterized. The results show that the fracture mechanism was sensitive to not only the microcracks near the notched area but also lamellar orientation to loading axis. The high tensile stress leads to the new microcracks nucleate along lamellar interfaces of grains with favorable orientation when local stress intensity reaches the toughness threshold of the material. Thus, both plasticity and high tensile stress are required to cause notched TiAl failure.