Fatigue of Superplastic-Nanocrystals

为了研究超细晶材料在室温下表现出超塑性的疲劳机制，我们测试了(1)单晶试件、(2)“等通道转角挤压”(ECAE)法制备的超细晶铜和(3)等通道转角挤压(ECAE)法制备的Pb62%Sn2%Al合金的循环应力应变响应。研究了单晶和Tr晶铜、Fe-Cr合金和Fe-Ni-Cr合金的循环响应的基本方面。单晶实验表明，在一定的塑性应变范围内，疲劳变形会形成“持续滑移带”(PSB)。PSB的形成引起了包辛格能量参数的剧烈变化。在三晶试件的疲劳试验中，实验表明，相邻晶粒之间的残余应力是晶体材料中包辛格效应的来源之一。在循环变形中，…对超细晶铜进行了大量的疲劳实验，以表征疲劳纳米晶体的基本特征。经ECAE方法细化的铜材的应力幅值可达250 Mpa。包辛格效应也非常高。然而，超细晶铜的这种优良性能对热处理非常敏感：在200ﾟC下热处理，应力幅值和包辛格效应都明显降低。在单调拉伸试验中，即使在室温下，铅-锡和锌-铝样品也都表现出很大的伸长率。这两种材料的流动应力也表现出显著的应变率相关性。在循环变形试验中，尽管退火铜的循环硬化曲线明显分为硬化阶段和饱和阶段，但从实验开始，铅-锡和锌-铝合金的应力幅值几乎是饱和的。随着应变率的增加，锌铝合金的应力幅值和包辛格效应明显增大。而铅锡合金的应力幅值与应变率无关，也不存在包辛格效应。在循环变形的铅锡合金中得到的这一趋势与单调试验的结果完全不同。(退火铜的循环响应为-。与单调变形下的应变率和屈服应力无关。)在循环变形的铅锡合金中没有应变率相关性的原因可以归因于在单调变形中占主导地位的热激活过程的缺失：由于与单调变形相比，塑性应变的幅值很低，因此施加的塑性应变很可能被位错运动所调节。另一方面，锌铝合金的循环变形可能涉及位错攀移、晶界扩散等热激活过程。由于锌铝的晶粒度很小，因此塑性应变不能仅由位错运动来调节是可行的。较少

In order to investigate fatigue mechanisms of ultrafine grained materials showing superplasticity at room temperature, we have examined cyclic stress-strain response of (1) single crystal specimens, (2) ultrafine grained copper produced by "Equal Channel Angular Extrusion (ECAE)" method, and (3) Pb-62%Sn and Zn-22%AI alloy fine-grained by the ECAE method.Fundamental aspects of the cyclic response in crystalline materials were investigated in single- and tr-crystal specimens of copper, Fe-Cr alloy and Fe-Ni-Cr alloy. The single crystal experiments revealed that fatigue deformation gives rise to formations of "persistent slip bands" (PSBs) at a certain range of plastic strain amplitude. The formation of the PSBs induced a drastic changes in Bauschinger energy parameter. In the fatigue tests on the tricrystal specimens, it was experimentally shown that residual stresses between adjoining grains is one-of sources for the Bauschinger effect in the crystalline materials.in the cyclic deforma … More tion tests on ultrafine grained copper to characterize basic feature of fatigued nano-crystals, following results were obtained. The stress amplitude of the copper fined-grained by the ECAE method became up to 250 MPa. The Bauschinger effect was also significantly high. However, this kind of excellent property of the ultrafine grained copper was very sensitive to heat treatment : both the Stress amplitude and the Bauschinger effect were apparently reduced by an annealing at 200ﾟC.In the monotonic tensile tests, both the Pb-Sn and Zn-Al specimens showed large elongations even at room temperature. These two materials also showed significant strain-rate dependence of flow stress. In the cyclic deformation tests, the stress amplitudes in the Pb-Sn and Zn-Al alloys were almost saturated from the beginning of the experiments, although a cyclic hardening curve of the annealed copper was divided clearly into the hardening and saturation stages. The stress amplitude and the Bauschinger effect in the Zn-Al alloy apparently increased with increasing strain rate. On the other hand, the Pb-Sn alloy revealed no strain rate dependence of the stress amplitude and the Bauschinger effect. This tendency obtained in the cyclically deformed Pb-Sn alloy is completely different from the result in the monotonic- test. (The cyclic response of the annealed copper was -. independent of the strain rate as well as the yield stress under monotonic deformation.) The absence of the strain-rate dependence in the cyclically deformed Pb-Sn alloy can be attributed to the missing of thermally activated processes which are dominant in the monotonic deformation : the applied plastic strain was probably accommodated substantially by dislocation motion because the plastic strain amplitude is very low with compared to the monotonic deformation. On the other hand, the cyclic deformation in the Zn-Al alloy may involve the thermally activated processes such as dislocation climb and grain boundary diffusion. It is feasible that the plastic strain can not be accommodated only by the dislocation motion because the grain size of the Zn-Al was very small. Less