【作者】 昝祥；

【导师】 夏源明； 汪洋；

【作者基本信息】 中国科学技术大学 ， 固体力学， 2008， 博士

【摘要】 目前对可能成为新一代高温结构材料的TiAl金属间化合物的力学性能的研究主要为TiAl在不同温度下准静态力学行为的研究,而对TiAl的抗冲击性能的研究还仅仅处于起步阶段,且集中于TiAl在不同温度下动态压缩力学行为的研究当中。本课题来源于国家自然科学基金（项目批准号90505002）,旨在对TiAl金属间化合物的动态高温拉伸力学性能和变形机理进行初步的探索。本文对高温冲击拉伸试验技术中的加温技术进行了改造,研制了第四代基于大热容量温升极大值原理的快速接触加温技术,从而将最高试验温度由750℃提高至850℃。利用该高温冲击拉伸试验技术,在室温至840℃下对等轴（NG）、近片层（NL）和双态（DP）组织的Ti-46.5Al-2Nb-2Cr在320、800和1350s^-1应变率下的冲击拉伸力学行为进行了实验研究;同时在MTS 809材料试验机上对相应温度下应变率为0.001s^-1的准静态拉伸力学行为进行了实验研究。试验结果表明,三种不同微观组织的TiAl具有类似的温度和应变率相关性。BDTT均随应变率的增加而上升。动载和静载下强度（包括屈服应力和定应变下的流动应力,下同）的温度相关性曲线均被“特征温度”（NG TiAl约为650℃,其它两种组织的TiAl约为500℃）和BDTT划分为Ⅰ,Ⅱ和Ⅲ三个区域;其中在Ⅰ和Ⅲ区,强度随温度的上升而下降,在Ⅱ区,强度随温度的上升几乎无变化;且动载Ⅱ区的温度跨度要明显大于静载Ⅱ区的温度跨度,也即动载下强度随温度上升而基本不变的温度跨度要明显大于静载下的。总体上动载下材料的强度以及高温下的失稳应变ε_b高于静载下的强度和高温下的ε_b（ε_b指的是BDTT以下的）,但在本文的高应变率加载范围内(320～1350s^-1)动载下的强度和ε_b的应变率相关性并不明显。同时,动态加载下的硬化率均可看成与温度和应变率无关。由此可看出,TiAl不仅呈现“高速韧”,而且呈现了高温高速韧;也就是说材料动态下的高温综合力学性能优于准静态下的。必须指出,三种微观组织的TiAl随组织中片层晶粒含量的增加（NG→DP→NL）,准静态下的BDTT逐渐升高;动态下的真应力-真应变曲线中屈服点逐渐不明显,且硬化段也逐渐偏离线性。SEM断口分析表明,三种微观组织的TiAl的断裂方式具有类似的温度和应变率相关性。BDTT以下,无论静载还是动载下的材料均为脆性断裂,且随温度的升高,材料的断裂方式由穿晶解理断裂方式逐渐转变为沿晶断裂方式;而BDTT以上材料为塑性断裂,断口中出现大量韧窝。TEM显微分析表明,三种组织的TiAl的变形机理也具有类似的温度和应变率相关性。动载下试件中的位错密度明显小于静态下的位错密度,而形变孪晶的密度却明显高于静载下的密度;动载下与静载下类似,温度越高越容易形成孪晶,同时高温下形成的形变孪晶均会贯穿整个晶粒;但在本文的动态加载范围内(320～1350s^-1),形变孪晶的密度随应变率的增加无明显变化;且在动态加载下存在一个类似于上述强度的温度相关性曲线中的“Ⅱ区域”,在此“Ⅱ区域”的温度范围内形变孪晶的密度基本上不随温度的上升而变化。层错不仅在不同温度动载下的试件中的密度明显的高于静载下的密度,而且在本文的高应变率加载范围内(320～1350s^-1),应变率越高越容易出现层错。综上所述,动态加载下TiAl的变形主要是受孪生（层错）机制控制的,而准静态加载下,TiAl的变形还受位错滑移机制控制。还必须指出,由于片层晶粒的晶界面积要高于等轴晶粒的,NL组织的晶界间相互运动在整个材料的变形过程中占有一定的比例,导致片层晶内的孪晶/层错带的密度要明显小于NG TiAl以及DP TiAl中等轴晶粒内的;而DP组织中的变形主要集中在等轴晶粒内,而高应变加载下DP TiAl的片层晶群内几乎无明显的变形特征。孪生（层错）机制导致TiAl的动强度明显高于静强度,但在本文的高应变率范围内(320～1350s^-1)动强度与应变率的相关性不明显。孪生（层错）机制还使得TiAl的硬化段基本成线性（NL TiAl的硬化段受晶界运动影响有些偏离线性）且与温度和应变率无关。同时孪生（层错）机制还可提高材料的塑性,因此BDTT以下相同温度时,材料动载下的塑性也高于静载下的塑性。基于Zerrilli-Armstrong模型,结合TiAl变形机理的特征,本文采用了BCC形式的Z-A本构模型以描述TiAl的流动特性,并给出了确定本构模型所用参数的方法。拟合曲线与试验结果吻合较好,说明该模型能较好地表征三种不同微观组织的TiAl在350～840℃应变率为320～1350S^-1下材料的流动特性。总之,本文从宏观力学行为以及变形机理的角度,初步揭示了TiAl有可能成为一种抗冲击的高温结构材料。更多还原

【Abstract】 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.更多还原