TiB、TiC和Y_(2)O_(3)增强钛基复合材料的蠕变行为和组织演变

采用感应熔炼法制备(TiB+TiC)和(TiB+TiC+Y_(2)O_(3))增强钛基复合材料。蠕变试验在650℃和120~160 MPa下进行,通过XRD、SEM和TEM对铸态试样和蠕变试样的显微组织演变进行详细表征。结果表明,两种复合材料的铸态组织均为网篮组织。在相同的蠕变条件下,添加Y_(2)O_(3)的复合材料具有更低的稳态蠕变速率。蠕变后,在α/β界面、溶解的β相和增强体周围有硅化物析出,这些硅化物对位错有很强的钉扎作用。因为存在贯穿TiB晶须的层错结构,所以TiB周围的硅化物尺寸明显大于TiC和Y_(2)O_(3)周围的硅化物尺寸。蠕变变形主要受溶质阻力控制,位错运动还受到α/β界面、增强相和硅化物的影响。

Effect of cooling rate on solidification microstructure and mechanical properties of TiB2-containing TiAl alloy

Effects of cooling rate and 0.25 at.%TiB2 addition on solidification microstructure and mechanical properties of Ti−48Al−2Cr−2Nb alloys fabricated by the investment casting with different thicknesses were studied.The results show that with the cooling rate increasing from 37 to 2×102 K/s,the solidification path of the studied alloys is unchanged.The grain size of the matrix alloy is refined from 650 to 300μm,while the grain size of Ti−48Al−2Cr−2Nb−TiB2 is reduced from 550 to 80μm.The lamellar spacing of matrix alloy is reduced from 360 to 30 nm with increasing the cooling rate from 37 to 2×102 K/s,while TiB2 addition shows little refinement effect on the lamellar spacing.Ti−48Al−2Cr−2Nb−TiB2 sample under medium cooling rate(69 K/s)exhibits superior microhardness(HV 550)and ultimate tensile strength(570 MPa)among the studied alloys.The refined grain size,lamellar spacing and fine TiB2 particles could account for the favorable mechanical properties of the studied TiB2-containing alloy.The microstructure evolution was discussed in light of cooling rate,constitutional supercooling and borides addition.

Characterization of hot deformation and microstructure evolution of a new metastableβtitanium alloy

The hot deformation characteristics of as-forged Ti−3.5Al−5Mo−6V−3Cr−2Sn−0.5Fe−0.1B−0.1C alloy within a temperature range from 750 to 910℃and a strain rate range from 0.001 to 1 s^(-1) were investigated by hot compression tests.The stress−strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate.The microstructure is sensitive to deformation parameters.The dynamic recrystallization(DRX)grains appear while the temperature reaches 790℃at a constant strain rate of 0.001 s^(-1) and strain rate is not higher than 0.1 s^(-1) at a constant temperature of 910℃.The work-hardening rateθis calculated and it is found that DRX prefers to happen at high temperature and low strain rate.The constitutive equation and processing map were obtained.The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map,which indicates excellent hot workability.At the strain rate of 0.1 s^(-1),the stress−strain curves show an abnormal shape where there are two stress peaks simultaneously.This can be attributed to the alternation of hardening effect,which results from the continuous dynamic recrystallization(CDRX)and the rotation of DRX grains,and dynamic softening mechanism.

Interactions between Ti Al alloy and AZC/AMT binder systems in investment casting

In this work, two kinds of binders, Ammonium Zirconium Carbonate(AZC) and Ammonium Metatungstate(AMT) hydrate, and three kinds of powders(ZrO 2, Al2O3 and Y2O3) were mixed to fabricate six kinds of face coating systems. The thermal behaviors of the AZC and AMT dried binders were investigated by TG-DTA, and the phase transformation of the two binders was determined by XRD. Monoclinic ZrO 2 phase was formed from AZC at 620 °C and WO3 at 700 °C, and the phase transformation was completed at lower than 1000 °C in both binders, and therefore, the sintering temperature for the molds was selected at 950 °C. The interaction between the ceramic molds with different face coatings and the Ti-48Al-2Cr-2Nb alloy during investment casting was studied. Results showed no α-case reaction in the TiA l-mold reaction, and the AMT + Y2O3 face coating appeared to be the best choice for investment casting of TiA l alloys under the experimental conditions.

Microstructure characterization and tensile properties of a Ni-containing TiAl-based alloy with heat treatment

The effects of Ni addition on solidification micro structure and tensile properties of Ti-48Al-2Cr-2Nb alloy were investigated using differential scanning calorimetry（DSC）,X-ray diffraction（XRD）,scanning electron microscopy（SEM） equipped with energy-dispersive spectroscope（EDS） and transmission electron microscopy（TEM）.Results show that with 3 at%Ni addition,the as-cast micro structure is mainly composed of fine lamellar colonies（~50 μm）,γ grains and Ni-ridied τ_3 phase.After heat treatment at 1380℃,the Ni-containing alloy is characterized by fine fully lamellar micro structure（~90 μm）.The heat-treated Ni-containing specimen exhibits superior room temperature tensile properties than other specimens.The tensile properties are discussed in light of the microstructure evolution and role of Ni-riched τ_3 phase.

Microstructure evolution of Ti–46Al–6Nb–(Si,B) alloys during heat treatment with W addition

The cast ingots of Ti-46Al-6Nb-0.25Si-0.2B and Ti-46Al-6Nb-0.5W-0.25Si-0.2B（at%） were made by induction skull melting（ISM） technique.A series of heat treatments（HTs） were conducted to research the microstructure evolution of both alloys.Microstructure and tensile property were examined by scanning electron microscope（SEM）,X-ray diffraction（XRD）,transmission electron microscope（TEM）,and tensile testing machine.The results show that microsegregation（liquid segregation and solid segregation） is exacerbated by the addition of0.5 at%W;the addition of Nb,W in TiAl alloy makes the phase transition difficultly take place;then,the microstructures and tensile properties of both alloys are improved after certain HT processes;finally,the thicknesses of the γ/α_2 lamellae after a certain HT process are significantly affected by the number of residual γ phases before the furnace-cooling moment.