Dynamic Mechanical Behavior And Deformation Mechanism Of TiAl Intermetallics At Elevated Temperatures

The current research on the mechanical behavior of TiAl intermetallics,a possible new-generation structural material serving at elevated temperatures,is mainly associated with the material's behavior under quasi-static loadings and different temperatures. However,research on the dynamic response of TiAI is still in its initial stage,mainly focused on the compressive behavior of TiAl under different temperatures and strain rates.This project,supported by NSFC(No.90505002),aims to elementarily investigate the tensile impact response of TiAl intermetallics at elevated temperatures and its deformation mechanism.In this study,the heating technique used in the tensile impact testing at elevated temperatures is improved.Consequently,the elevated temperature up to 850℃in the experiments is achieved.The tensile impact behaviors of Ti-46.5Al-2Nb-2Cr with different microstructures, Near Gamma(NG),Near Lamellar(NL) and Duplex(DP),were investigated under different temperatures ranging from room temperature(RT) to 840℃and strain rates of 320,800 and 1350 s^-1.Also,the quasi-static tensile experiments at a strain rate of 0.001s^-1 and corresponding temperatures were carried out using MTS 809 testing system.The results show that the mechanical behaviors of NG,NL and DP TiAls all exhibit similar dependence on temperature and strain rate.The Brittle to Ductile Transition Temperatures(BDTT) increase with increasing strain rates.The temperature-dependence curves of strength under dynamic and quasi-static loading conditions are divided into three regions,regionⅠ,ⅡandⅢ,by a "characteristic" temperature(about 650℃for NG TiAl and about 500℃for the other two) and BDTT.In RegionsⅠandⅢ,the strengths decrease with increasing temperatures, whereas in regionⅡ,the strengths remain nearly stable.The temperature ranges of regionⅡunder dynamic loadings are wider than those under quasi-static loadings. That is to say the temperature ranges in which the strengths remain stable with increasing temperature under dynamic loadings are wider than those under quasi-static loadings.On the whole,the strengths and unstable strainsε_b under dynamic Ioadings are higher than those under quasi-static loadings,whereas there is no significant strain-rate dependence of those under high strain rates ranging from 320 to 1350s^-1 in this study.At the same time,the work-hardening rates under dynamic loadings are independent of temperature and strain rate.It can be concluded that TiAl is a high-velocity ductile material.That is to say,the comprehensive properties of TiAl at elevated temperatures under dynamic loadings are better than those under quasi-static loadings.It must be pointed out that,with increasing volumes of lamellar crystals (NG→DP→NL),the BDTT increases under quasi-static loadings,the yield points in the true-stress true-strain curves become less and less clear and the work-hardening parts in the curves begin to deviate from linearity.The SEM fractographic observation shows that there exists similar temperature and strain-rate dependence of fracture mode for the three TiAls.Below BDTT,the materials are brittle and the fracture modes change from planar cleavage fracture to intergranular fracture with increasing test temperatures.However,above BDTT,the materials become ductile and a large number of voids appear on the fracture surfaces.TEM analysis is performed to investigate the deformation mechanisms of the three TiAls under different temperatures and strain rates.The results show that there also exists similar temperature and strain-rate dependence of deformation mechanism for the three TiAls.The dislocation density under dynamic loadings is smaller than that under quasi-static loadings,while the twinning density under dynamic loadings is higher than that under quasi-static loadings.Similarly,under dynamic loadings,the higher the temperature is,the greater the tendency for twinning to occur,which resuits in twinnings throughout the whole crystal.Nevertheless,within the rate range in the present study,the twinning density is not sensitive to the dynamic strain rates and there exists a region,similar to the aforementioned RegionⅡ,in which the twinning density remains nearly stable with the increasing temperatures.Not only is the density of the stacking faults under dynamic loadings higher than that under quasi-static loadings,but also the stacking faults become more likely to occur with increasing dynamic strain rates within the rate range in this study.To sum up,the deformation of TiAl under dynamic loadings is controlled chiefly by the twin mechanism,whereas under quasi-static loadings that is also governed by the dislocation slip mechanism as well.In addition,the proportion of crystal boundaries of lamellar crystal is larger than that of equiaxed crystal and crystal boundary slipping plays an important role in the deformation process of NL TiAl.Therefore,the density of twinnings and stacking faults in NL TiAl is lower compared with NG TiAl and equiaxed crystal in DP TiAl. The deformation of DP TiAl is concentrated in the equiaxed crystal and there are no obvious deformation characteristics in the lamellar crystal colony in DP TiAl under dynamic loadings.As a result of the twin mechanism,the strength of TiAl under dynamic loadings is obviously higher than that under quasi-static loadings,the former being not sensitive to the strain rates within the range in this study,and the work-hardening parts in the strain-stress curves are basically linear,with an exception that the work-hardening parts of NL TiA1 deviate somewhat from linearity due to the slipping of the crystal boundaries of lamellar crystals.Moreover,the twin mechanism is found capable of increasing the ductility of TiAl.Consequently,below BDTT,the ductility under dynamic loadings is better than that under quasi-static loadings.Upon the basis of Zerrilli-Armstrong model and the deformation mechanism of TiAl,a BCC type of Z-A model is employed to describe the flow behavior of the three TiAls,and a method is proposed for determining the parameters involved in the model.The fitted curves find comparatively good agreement with the test results.To sum up,from the point of view of macro-mechanic behavior and deformation mechanism,it is concluded that TiAl intermetallics will probably become a structural material that is capable of suffering both high temperature and impact loadings.