基于准连续方法的晶体缺陷对材料变形行为影响研究

发布时间：2018-01-29 17:29

本文关键词： 准连续多尺度方法孔洞晶界纳米压痕　出处：《合肥工业大学》2015年硕士论文　论文类型：学位论文

【摘要】：先进塑性成形技术在很多方面发挥着越来越重要的作用,如促进国民经济发展,推动国防现代化和提高国家竞争力等。材料的初始塑性变形对后续塑性变形起着重要的作用,目前通过无缺陷材料的纳米压痕过程对初始塑性变形的研究已经有了一定的深度,然而材料并不是完美无缺陷的,因此,进一步研究孔洞、晶界等缺陷对材料初始塑性变形行为的影响具有极其重要的意义。本文通过QC方法分别建立含有孔洞以及晶界的多尺度计算模型,探究孔洞以及晶界对材料初始塑性变形的影响。本文采用准连续多尺度方法建立了有、无孔洞的多尺度纳米压痕模型,发现了晶体中存在孔洞时位错发射的迟滞现象,并对位错发射迟滞现象的原因进行了计算分析。讨论了多种参数如孔洞形貌、大小和位置对纳米压痕过程位错发射的影响。结果表明,孔洞形貌对位错发射的迟滞效应无影响,对于特定大小的孔洞,孔洞对位错发射迟滞现象的影响存在一个临界距离,当孔洞与压头的距离大于此临界距离时,不存在位错发射的迟滞现象；临界距离随着孔洞的增大而增大。为揭示孔洞缺陷对材料弹性模量和屈服强度的影响,本文采用准连续多尺度方法模拟研究了压头正下方矩形以及椭圆孔洞几何参数、孔洞深度对材料弹性模量及屈服强度的影响。结果表明,当孔洞与压头的距离较远时,孔洞对其弹性模量及屈服强度影响很小；当孔洞与压头的距离较近时,薄膜材料的弹性模量及其屈服强度随着孔洞水平尺寸a的增大而降低,然而其随着孔洞竖直尺寸b的增大而升高,当b达到一定值后,孔洞对薄膜材料弹性模量及屈服强度的影响甚微。并通过变化椭圆孔洞长短轴比值b/a,得到了椭圆孔洞长短轴比值与弹性模量及屈服强度的关系,随着b/a数值的增大其弹性模量及屈服强度增大,当b/a=4时,孔洞对薄膜材料纳米压痕过程中的弹性模量以及屈服强度影响已经很小。最后,建立了含有晶界的双晶材料多尺度模型,探究了晶体取向及上层膜厚度对初始位错发射情况的影响。结果表明,对于不同晶体取向单晶材料的纳米压痕过程,初始塑性变形的临界载荷显著不同,这是由于该晶体取向下位错发射的容易决定的；对于由上述晶体取向构成的双晶材料,上层膜厚度显著影响着双晶材料的初始塑性变形行为,材料的屈服强度有随着上层膜厚度的增加而增大的趋势,对于不同取向的上层膜发生上述趋势的机理不同。
[Abstract]:Advanced plastic forming technology plays a more and more important role in many aspects, such as promoting the development of national economy. The initial plastic deformation of materials plays an important role in subsequent plastic deformation. At present, the study of initial plastic deformation has a certain depth through the nano-indentation process of non-defect materials. However, the materials are not perfect, so further study of pores. The influence of grain boundary and other defects on the initial plastic deformation behavior of materials is very important. In this paper, a multi-scale calculation model with holes and grain boundaries is established by QC method. In order to explore the effect of porosity and grain boundary on the initial plastic deformation of materials, a multi-scale nano-indentation model with and without pores is established by using quasi-continuous multi-scale method. The phenomenon of dislocation emission hysteresis in the presence of holes in crystals was found, and the causes of dislocation emission hysteresis were calculated and analyzed, and various parameters such as pore morphology were discussed. The results show that the pore morphology has no effect on the hysteresis effect of dislocation emission, but it has no effect on the specific size of the pore. When the distance between the hole and the pressure head is larger than this critical distance, there is no dislocation emission hysteresis. The critical distance increases with the increase of the porosity. In order to reveal the influence of the pore defects on the elastic modulus and yield strength of the material. In this paper, quasi-continuous multi-scale method is used to simulate and study the effects of geometric parameters of rectangular and elliptical holes under the pressure head and the depth of the holes on the elastic modulus and yield strength of the materials. When the distance between the hole and the pressure head is long, the influence of the hole on the elastic modulus and yield strength is very small. The elastic modulus and yield strength of the thin film decrease with the increase of the horizontal pore size a, but increase with the increase of the vertical size b of the pore when the distance between the hole and the pressure head is close. When b reaches a certain value, there is little effect on the elastic modulus and yield strength of the thin film, and the ratio b / a of the long and short axis of the elliptical hole is changed by changing the ratio of the long and short axis of the elliptical hole. The relationship between the ratio of long and short axis of elliptical holes and elastic modulus and yield strength is obtained. The elastic modulus and yield strength increase with the increase of b / a value, when b / a = 4:00. The effect of pore on the elastic modulus and yield strength of nanocrystalline indentation is very small. Finally, a multi-scale model of double-crystal material with grain boundary is established. The effect of crystal orientation and film thickness on initial dislocation emission is investigated. The results show that the critical load of initial plastic deformation is significantly different for the nanocrystalline indentation process of single crystal materials with different crystal orientations. This is because the dislocation emission under the crystal orientation is easy to determine; For the bicrystalline material which is composed of the above crystal orientation, the thickness of the upper layer obviously affects the initial plastic deformation behavior of the double crystal material, and the yield strength of the material increases with the increase of the thickness of the upper film. The mechanism of the above trend is different for the films with different orientations.
【学位授予单位】：合肥工业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：O77;TB303

【相似文献】