纯钛拉伸变形行为的尺寸效应研究

发布时间：2018-08-31 14:12

【摘要】：随着现代工业逐渐向轻量化和微型化发展,微成型、超精密机械加工等微细加工技术不断进步,使微小尺寸材料应用更加广泛。然而,在微小尺寸的金属材料中存在着一种尺寸效应现象,即其性能会受样品尺寸或晶粒尺寸的影响,使微小尺寸金属材料呈现出与常用尺寸金属材料不同的性能特点。本文以工业纯钛TA2为材料,通过退火热处理的方法,将厚度分别为50μm、100μm、200μm和800μm的纯钛薄板的晶粒尺寸、晶体取向和位错组态调控到基本一致。采用常规拉伸试验,测试了不同厚度退火态纯钛的拉伸性能,发现在本文所选用的几个厚度尺寸下,随着样品厚度的减小,其强度和延伸率均呈下降趋势。运用基于OM-DIC的原位拉伸技术对不同厚度纯钛的拉伸变形行为进行研究,宏观上,建立了不同厚度的纯钛在不同工程应变量下对应的应变分布规律:随着厚度的减小,产生应变局域化现象所对应的工程应变量减小,应变分布均匀性下降。产生应变局域化现象所对应的工程应变量越小,即材料的均匀延伸率越小。应变分布的均匀性越差,表明材料整体参与塑性变形的能力越差。纵向应变的分布情况与颈缩程度的大小一致,剪切应变的分布情况与材料断裂路径一致。运用基于SEM-DIC原位拉伸技术对不同厚度纯钛的拉伸变形行为进行研究,微观上,由不同厚度的纯钛在不同工程应变量下对应的晶粒应变分布情况,反应出不同厚度纯钛的晶粒变形规律:相同工程应变量下,随着厚度的减小,呈压应变分布的晶粒占比减少,即未参与变形晶粒占比减少,各晶粒的应变分布差异增大,即晶粒间的变形程度差异增大。表明厚度越小的纯钛,微观变形的协调性一致性越差。通过透射微观组织观察并结合宏观、微观变形规律,揭示了厚度尺寸对纯钛的拉伸性能产生影响的根本原因在于厚度方向的晶粒情况。其中,塑性方面,厚度小的纯钛,其厚度方向的晶粒数少,开始塑性变形的晶粒起的作用就相对较大,且晶粒数少限制了晶粒间的协调变形,使得各晶粒的变形程度差异大,进而宏观上的应变分布不均匀,更容易产生应变集中,此外,厚度减小到50μm时,孪生系统启动产生了孪晶,诱导应力应变进一步集中,因此厚度越小塑性越差;强度方面,厚度小的纯钛,其晶粒数少,晶粒的承载能力弱,且表面层晶粒占比大,位错可以自由的滑出表面,加之孪生变形诱导应力集中作用,这几个因素均削弱了材料的加工硬化作用,导致厚度越小,强度越低。
[Abstract]:With the gradual development of modern industry towards lightweight and miniaturization, micro-forming, ultra-precision machining and other micro-processing technologies have made micro-size materials more widely used. However, there is a phenomenon of size effect in micro-size metal materials, that is, their properties will be affected by sample size or grain size, so that they are small. In this paper, the grain size, crystal orientation and dislocation configuration of pure titanium sheets with thickness of 50, 100, 200, and 800 microns are adjusted to be basically the same by annealing heat treatment using industrial pure titanium TA2 as material. The tensile properties of annealed pure titanium with different thicknesses were investigated. It was found that the tensile strength and elongation decreased with the decrease of the thickness of the sample. The tensile deformation behavior of pure titanium with different thicknesses was studied by OM-DIC-based in-situ tensile technology. The strain distribution law under different engineering strain variables is as follows: with the decrease of thickness, the engineering strain corresponding to strain localization phenomenon decreases, and the uniformity of strain distribution decreases. The tensile deformation behavior of pure titanium with different thicknesses was studied by means of SEM-DIC in-situ tensile technique. Microscopically, pure titanium with different thicknesses was processed in different conditions. The grain strain distribution of pure titanium with different thickness is reflected by the grain strain distribution under the range strain. Under the same engineering strain, with the decrease of thickness, the proportion of grains with compressive strain distribution decreases, that is, the proportion of grains not involved in deformation decreases, and the difference of strain distribution among grains increases. The results show that the smaller the thickness of pure titanium, the worse the compatibility of micro-deformation is. By means of transmission microstructure observation and combining with macro-and micro-deformation laws, it is revealed that the fundamental reason why the thickness size affects the tensile properties of pure titanium lies in the grain condition in the thickness direction. When the number of grains is small, the grains which begin to deform plastically play a relatively large role, and the number of grains is small, which limits the coordinated deformation between grains, so that the deformation degree of grains varies greatly, and then the macroscopic strain distribution is uneven, which makes it easier to produce strain concentration. In addition, when the thickness is reduced to 50 micron, the twin system starts to produce twins and induces stress. In terms of strength, pure titanium with small thickness has less grain number, weak bearing capacity of grain, large proportion of grain in surface layer, and dislocation can slide freely out of the surface. In addition, twin deformation induces stress concentration, which weakens the work hardening effect and leads to thickness. The smaller, the lower the intensity.
【学位授予单位】：哈尔滨理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.23

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