Ni-Si过共晶合金的定向凝固特性及性能研究

发布时间：2018-05-09 05:23

  本文选题：定向凝固 + Ni-Si过共晶　； 参考：《西安建筑科技大学》2017年硕士论文

【摘要】：Ni-Si过共晶复合材料拥有异常的R效应(即随其应用温度的提升,强度不是连续下降,而是先增大再下降)、耐高温氧化腐蚀以及金属和陶瓷的双重特性等特点。为了探讨某一材料在较大组元成分区域内的凝固规律,本文研究了不同凝固速率下的Ni-Si过共晶组织的凝固特性。本文利用真空感应熔炼法制得了Ni-12wt%Si过共晶的母合金,并选用改进后的Bridgman定向凝固方法制得了不同凝固速率下的Ni-Si过共晶自生复合材料。利用金相显微镜、X射线衍射仪和能谱仪等对Ni-Si过共晶材料在不同凝固速率时的组织形貌、相的析出性能以及各相的成分组成进行了讨论;研究了Ni-Si过共晶复合材料的晶体生长机制;退火温度对亚稳相组织的影响以及Ni-Si过共晶组织间的位向关系;并检测计算了Ni-Si过共晶合金的显微硬度、弹性性质以及室温断裂韧性等力学性能。研究发现,Ni-Si过共晶定向凝固组织中的晶粒随着凝固速率的增大越来越细小,层片间距也随之呈缩短趋势。未定向凝固时的组织因为热流方向呈各向同性而形成树枝晶组织,在定向凝固初始阶段时择优取向表现较弱,组织相对粗大;随着热流逐渐趋于稳定,晶体沿最优生长方向生长,组织相对细小且生长稳定;淬火界面始终以平界面方式生长。Ni-Si过共晶组织包括基体α-Ni相、亚稳Ni31Si12相和Ni-Ni3Si过共晶相三相。其中基体相呈非小平面相,亚稳相以及过共晶相呈小平面相,Ni3Si相的组织以板条状进行生长,而Ni31Si12相主要是以不规则的多边形形貌生长。亚稳相Ni31Si12的存在使得Ni-Si过共晶合金的硬度增加,脆性增加,经过不同温度退火,对比后得出,在1100℃退火,保温30min后获得的显微组织中亚稳相Ni31Si12的含量显著减少。不同凝固速率下的Ni-Si过共晶材料的位向关系是不同的。有实验所得结果可知,任一凝固速率下的Ni-Si过共晶材料的平均硬度值都比纯的Ni3Si的硬度值大。通过不同种共晶方式得到的Ni-Ni3Si材料的硬度值由小到大依次为亚共晶、共晶、过共晶。已知单质Ni的弹性模量为180.0GPa,使用第一性原理计算获得Ni3Si的弹性模量为345.0GPa,理论上经定向凝固后得到的组织的弹性变形量与纯的Ni3Si相比会适量增加,当R=25μm/s时增加最多,脆性改善最明显。与电弧熔炼后的试样比较经Bridgman定向凝固后材料的室温断裂韧性在R=6μm/s和9μm/s时有所增加,在R=25μm/s和40μm/s时,由于凝固速率大于一临界速率,试样在断裂时的热激活能被抑制,位错会塞积在裂纹的尖端,使其不容易出现钝化现象,只能够沿着裂尖位置的最大主应力的平面方向来展开,使材料的脆化倾向变大。
[Abstract]:Ni-Si hypereutectic composites have abnormal R effect (I. e., with the increase of application temperature, the strength does not decrease continuously, but increases first and then decreases, resistance to oxidation and corrosion at high temperature, and dual properties of metal and ceramics, etc. The solidification characteristics of Ni-Si hypereutectic structure at different solidification rates were studied in order to study the solidification law of a material in the region of larger composition. In this paper, the Ni-12wt%Si hypereutectic master alloy was prepared by vacuum induction melting, and the Ni-Si hypereutectic in situ composites with different solidification rates were prepared by using the modified Bridgman directional solidification method. The microstructure, precipitation properties and composition of Ni-Si hypereutectic materials at different solidification rates were discussed by means of X-ray diffractometer (XRD) and energy spectrometer (EDS). The crystal growth mechanism of Ni-Si hypereutectic composites, the effect of annealing temperature on metastable phase structure and the orientation relationship among Ni-Si hypereutectic structures were studied. The microhardness of Ni-Si hypereutectic alloy was measured and calculated. Mechanical properties such as elastic properties and fracture toughness at room temperature. It is found that the grains in the hypereutectic directionally solidified microstructure become smaller and smaller with the increase of solidification rate and the interlaminar spacing decreases with the increase of solidification rate. In the initial stage of directional solidification, the preferred orientation of the dendritic structure is weak and the microstructure is relatively coarse, which tends to stabilize with the heat flux. The crystal grows along the optimal growth direction, the microstructure is relatively small and stable, and the quenched interface is always flat interface growth. The hypereutectic structure consists of matrix 伪 -Ni phase, metastable Ni31Si12 phase and Ni-Ni3Si hypereutectic phase three-phase. The matrix phase is non-Xiaoping, the metastable phase and hypereutectic phase grow in the shape of strip, while the Ni31Si12 phase grows mainly in irregular polygon. The hardness and brittleness of Ni-Si hypereutectic alloy were increased by the existence of metastable phase Ni31Si12. After annealing at different temperatures, it was found that the content of Ni31Si12 in metastable phase of the microstructure after annealing at 1100 鈩，

本文编号：1864765