Cavitation and grain boundary sliding during creep of Mg-Y-Nd-Zn-Mn alloy

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30-80 MPa. Tensile creep tests were performed at 300℃up to the final fracture.Several tests at 50 MPa were interrupted after reaching the steady state creep;and another set of creep tests was interrupted after the onset of ternary creep.Fraction of cavitated dendritic boundaries was evaluated using optical microscopy.Measurement of grain boundary sliding by observation of the offset of marker lines was carried out on the surface of the crept specimens after the test interruption by scanning electron microscopy and by confocal laser scanning microscopy.The results show that the dominant creep mechanism in this alloy is dislocation creep with minor contribution of the grain boundary sliding.Creep failure took place by the nucleation,growth and coalescence of creep cavities on the boundaries predominantly oriented perpendicular to the applied stress.Increasing amount of cavitated boundaries with time of creep exposure supports the mechanism of continuous cavity nucleation and growth.

Tensile and fracture properties of an Mg-RE-Zn alloy at elevated temperatures

Magnesium alloy EZ10 （Mg-RE-Zn） was deformed in tension at temperatures from 20 up to 520℃. A rapid decrease of the yield and tensile strength with temperature was observed at temperatures higher than 300 ℃. On the other hand, ductility of sam-ples rapidly increased in the same temperature range. Light microscopy and scanning electron microscopy was used to reveal the rea-son for these behaviours. Intermetallic particles in grain boundaries are responsible for excellent mechanical properties at lower tem-peratures. Diffusional processes occurring at temperatures higher than 300 ℃ significantly influenced the deformation mechanism as well as the fracture character.

SCM420钢在600℃至熔点温度范围内的热塑性

通过在Ｇｌｅｅｂｌｅ—１５００热模拟试验机上进行热拉伸试验，研究了ＳＣＭ４２０齿轮钢在高温下的热塑性及变形断裂的行为与机制。结果表明，ＳＣＭ４２０钢在７５０～９００℃和在１３５０℃以上时出现沿晶脆性断裂，在９５０～１３００℃为穿晶断裂；在沿晶断裂过程中，晶界处的硫化物粒子作为应力集中核心促进了微孔聚合或晶界滑移，恶化了塑性；拉伸变形前的预相变过程促进了固溶温度较高的（ＭｎＦｅ）Ｓ粒子的大量析出，从而严重恶化了ＳＣＭ４２０钢的热塑性，使塑性凹槽向高温区扩展了约１５０℃左右。

碳化硅陶瓷磨损特性的研究

本文研究了高温结构陶瓷碳化硅的滑动干摩擦磨损性能,运用扫描电镜、透射电镜,X射线衍射技术、金相显微镜以及表面轮廓仪观察和分析了磨损前后摩擦副的表面形貌、截面特征、磨屑的形状及其相组成.本文试图揭示该材料的干滑动摩擦磨损规律并初步探讨其磨损机理.实验结果表明:在本实验条件下。SiC/SiC、SiC/Si_3N_4之间的主要磨损机理是脆性断裂磨损.

铝锌镁锆合金超塑变形后位错结构与空洞长大的研究

本文对铝锌镁锆合金超塑变形后位错结构及空洞长大规律进行了研究。实验证明该合金在晶界滑移中存在位错蠕变的协调机制。在超塑变形中出现了品界位错并参与了品界滑移,它们是由晶界吸附晶格位错而形成的,或由晶界结构本身在品界滑移中形成的。各种应变速率下测定了空洞长大速率的参数η,探讨了η与 m 值的关系。

铌锆合金高温力学性能及变形机制研究

测试了铌锆合金在1 000~1 800℃下的抗拉强度、屈服强度、延伸率和断面收缩率,并对单向拉伸后的断口形貌进行了观察。结果表明:随着温度升高,铌锆合金在高温下的抗拉强度和屈服强度的下降速度由快转慢,且二者逐渐接近;延伸率在1 000~1 200℃温度区间的变化不大,但在1 200℃以上随温度增加而快速增大;断面收缩率在试验温度范围内变化不大;铌锆合金在1 000~1 200℃温度区间内的主要变形机制为位错滑移,在1 400~1 800℃温度区间内的主要变形机制为晶界滑动;铌锆合金组织中有两种第二相粒子,其中氧化锆颗粒主要起弥散强化作用,而碳化锆颗粒主要起沉淀强化作用。