The influence of thermal residual stresses and thermal generated dislocation on the mechanical response of particulate-reinforced metal matrix nanocomposites

Due to thermal expansion mismatch between reinforcing particles and matrix, thermal induced dislocations are generated in metal matrix nanocomposites (MMNCs) during cooling down from the processing temperature. These dislocations have been identified as an important strengthening mechanism in particulate-reinforced MMNCs. In this study, the development of thermal residual stresses and thermal induced dislocations in MMNCs are predicted using discrete dislocation simulation, assuming the whole material is under uniform temperature change. Shear deformation is applied after the composites are cooled down to room temperature and the influence of thermal residual stresses and thermal generated dislocation on the overall response of particulate-reinforced MMNCs are investigated. The results show that the thermal residual stresses are high enough to generate dislocations and the dislocation density is higher in the interfacial region than the rest of the matrix. The predicted mechanical behavior of the MMNCs matches the experimental results better when thermal residual stresses are included in the simulations.     

Hydrogen embrittlement behavior in interstitial Mn–N austenitic stainless steel

The susceptibility to hydrogen embrittlement behavior was investigated in an interstitial Mn–N austenitic steel HR183 and stainless steel 316L. Hydrogen was introduced by cathodic hydrogen charging at 363 K. HR183 has stronger austenite stability than 316L despite its lower nickel content, the addition of manganese and nitrogen inhibited martensitic transformation during the slow strain rate tensile deformation. Due to the diffusion of hydrogen being delayed by the interstitial solution of nitrogen atoms and the uniform dislocation slips, hydrogen permeates more slowly in HR183 than 316L, contributing to an 84.79 μm thinner brittle fracture layer in HR183 steel. Hydrogen charging caused elongation losses in both 316L and HR183 steels associated with the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanism. However, the hydrogen embrittlement susceptibility of HR183 is 3.4 times lower than that of 316L according to the difference in elongation loss between the two steel after hydrogen charging. Deformation twins trapped a lot amount of hydrogen leading to brittle intergranular fracture in 316L. The multiple directions of slip in HR183 steel suppressed the strain localization inside grains and delayed the adverse effects conducted by HELP and HEDE mechanism, eventually inhibiting server hydrogen embrittlement in the HR183 steel. This study is assisting in the development of low-cost stainless steel with excellent hydrogen embrittlement resistance that can be used in harsh hydrogen-containing environments.     

基于分布位错法研究多条微裂纹对偏折主裂纹的影响

目的 采用理论方法求解多条微裂纹对偏折主裂纹的影响,重点分析偏折主裂纹尖端的力学行为及微裂纹对主裂纹扩展角度和闭合区域的影响等问题,为实际的工程应用提供理论依据。方法 运用叠加原理将主问题分解成2个子问题,通过材料力学方法求解子问题一;基于分布位错方法求解子问题二。进一步建立关于位错密度的奇异积分方程,利用Gauss-Chebyshev数值求积分法解决位错密度方程的奇异性问题,并通过计算机编写程序,最终得到相关力学参量的数值解。结果 得到了偏折主裂纹附近的应力场以及微裂纹长度、微裂纹个数对偏折主裂纹尖端应力强度因子的影响等相关力学参量。分析了主裂纹不同偏折角度时的闭合区域,以及微裂纹的方位角、微裂纹个数等对偏折主裂纹扩展角度的影响。结论 裂纹面对拉应力有屏蔽作用,导致拉应力在裂纹面附近应力松弛,而裂纹尖端对拉应力有放大作用,随着应力增加将导致裂纹的扩展。一条微裂纹位于主裂纹尖端约–30°<θ<50°时,将使主裂纹尖端应力强度因子增加,促进主裂纹的扩展,而微裂纹位于50°<θ<90°或–90°<θ<–30°时,将使主裂纹尖端应力强度因子减小,抑制主裂纹的扩展。主裂纹尖端应力强度因子随微裂纹长度的增加而变大,随微裂纹与主裂纹间距离的增加而减小。     

膨胀螺栓在混凝土中的拉力试验

研究了安装在混凝土中的膨胀螺栓的承载力、位移等特性 ,建立了一套规范的膨胀螺栓承载力 (拉力 )检测方法     

南水北调西线工程区及邻域的活动构造

南水北调西线一期工程在构造上位于青藏高原东部的巴颜喀拉地块内部,块体的四周边界以巨型走滑断裂和逆断裂为特征,构造运动非常强烈,是块体运动引发应力集中和释放能量的主要场所。但块体内部构造活动性较弱,仅发现甘德—阿坝断裂系北支断裂在阿坝盆地段的部分断裂、色达—洛若断裂和康勒断裂等晚更新世以来有过活动,且规模和运动强度远小于地块边界断裂。晚第四纪以来地块边界断裂的位移速率大多在7 mm/a以上,最大达18 mm/a,最小为1～2 mm/a,而输水隧洞附近最主要的活断层顺河断裂的位移速率仅为0.18 mm/a,相对地块边界断裂小1～2个数量级,可以认为工程场地处于构造相对稳定的地区。给出了阿坝盆地北缘断裂、顺河断裂等活断裂在工程使用期内最大设防的倾滑位移量,分别为0.67和2.62 m。     

正火中碳钢在高周疲劳下循环硬化的研究

金属材料在低周疲劳中可测出材料的循环硬（软）化现象，但无法得到高周疲劳时材料循环曲线．本文利用测定疲劳后材料力学性能来反映高周疲劳产生的材料硬化现象，而且分析了不同载荷和不同加载顺序对材料硬化程度的影响，与试验结果相符合．     

钛单晶纳米柱拉压不对称性的分子动力学模拟

本文基于分子动力学模拟,通过研究钛单晶纳米柱在拉伸和压缩下的力学响应特征及晶体结构演化行为,揭示其塑性变形机制。结果表明沿[0001]晶向拉伸条件下主要塑性变形机制为伴生的{101 ?2}孪晶和基面层错;而沿[0001]晶向压缩条件下,基面位错作为优先形核的缺陷参与到塑性变形过程,随后锥面位错出现并协调了轴向和横向变形,压缩条件下无孪晶产生。拉伸模拟过程中观察到一种有别于传统孪生的晶体再取向现象,其孪晶与基体间呈现基面/柱面对应关系。     

回复与再结晶退火对TWIP钢组织及性能的影响

用室温拉伸试验机、布氏硬度计、光学显微镜和透射电镜分析了冷轧态的TWIP钢及其在100~750℃退火后的力学性能和显微组织结构随温度的变化规律。试验结果表明,TWIP钢的强度随退火温度的提高呈现出非单调性变化,硬度随温度提高先升高而后降低,伸长率从500℃开始显著升高;TWIP钢回复退火使强度有一定的提高。     

AgInCd合金压缩蠕变性能研究

用RDL-50型拉伸蠕变试验机进行改装后的实验装置研究了铸态AgInCd合金在温度300~400℃及应力范围12~24 MPa内的压缩蠕变行为,分析了稳态速率与温度和应力的关系,计算了应力指数(n)和蠕变激活能(Q_a),并结合蠕变后样品在透射电子显微镜下的微观形貌及位错组态,探讨了合金的压缩蠕变机制。结果表明:随温度和应力水平的升高,合金的稳态蠕变速率增加。相比较指数关系,蠕变速率与应力之间更符合幂函数关系。300、350和400℃条件下,合金的蠕变应力指数n分别为3.31、4.09和5.77;12、18和24 MPa条件下,合金的蠕变激活能Q_a分别为68.1、103.7和131.6 kJ/mol。微观形貌以层错为主,孪生为300℃的主要蠕变机制,位错攀移生成位错墙为400℃的主要蠕变机制。