Anisotropic growth of multigrain in equiaxial solidification simulated with the phase field method

The phase field method has been mainly used to simulate the growth of a single crystal in the past. But polycrystalline materials predominate in engineering. In this work, a phase field model for multigrain solidification is developed, which takes into account the random crystallographic orientations of crystallites and preserves the rotational invariance of the free energy. The morphological evolution of equiaxial multigrain solidification is predicted and the effect of composition on transformation kinetics is studied. The numerical results indicate that due to the soft impingement of grains the Avrami exponent varies with the initial melt composition and the solidification fraction.     

退火态Zr55A110Ni5Cu30块体非晶合金在轧制过程中的自由体积演化

将Zr55All0Ni5Cu30块体非晶合金在715K等温退火30min，引入少量纳米晶，然后于室温以不同的应变速率进行轧制，用差示扫描量热仪考察不同应变量样品的热稳定性和自由体积演化．结果表明：即使轧制到95％的最大应变量，样品的热稳定性也几乎没有发生改变．在各种应变速率下，随着应变量的增加，自由体积含量持续上升．但随着应变速率的增加，相同应变量下自由体积的含量先上升后降低，该规律与单一非晶态结构合金在塑性变形过程中自由体积的变化情况截然不同．     

Formation mechanism of anomalous eutectics in highly undercooled Ag--39.9 at%Cu alloy

<正>This paper investigates the solidification behaviour of the Ag—Cu eutectic alloy melt undercooled up to 100 K.It is revealed that lamellar eutectics grow in a dendritic form in the Ag-Cu eutectic melt with undercooling equal to or greater than 76 K.As undercooling increases,the remelted fraction of the primary eutectics during recalescence rises. The severe remelting and the subsequent ripening of the primary eutectic dendrites lead to the formation of anomalous eutectics.     

Phase-field study for the splitting behaviour of precipitate under applied stress

This paper employs the phase-field method to study the splitting behaviour of a single coherent particle under an applied uniaxiai stress. It finds that the splitting behaviour is greatly influenced by the initial shape of precipitates, the bulk free energy condition, the degree of supersaturation and the ratio of the interfacial energy to the elastic strain energy, etc. The simulated results demonstrate that the aspect ratio of the particle determines whether the splitting can occur and how many split plates can be obtained. The splitting of particle is sensitive to the interracial energy, i.e. the splitting becomes more and more difficult with increasing the ratio of the interracial energy to the elastic strain energy. And increasing the magnitude of the applied stress is favourable to the splitting. The splitting process is also explained from the point of view of the corresponding diffusion potential.     

非平衡凝固与固态相变中有关形核和长大的竞争研究

非平衡凝固和固态相变作为同热力学和动力学紧密相关的复杂理论体系,在物理冶金和凝聚态物理研究领域占据重要的地位。上述非平衡过程包含形核、生长和碰撞三个微观过程,其发生和发展均由一些竞争过程伴随、辅助甚至驱动。正确理解这些竞争过程决定了能否准确描述整个非平衡过程;材料制备中,其力学和物理/化学性能的提高也归根于这些竞争的发生与发展。因此,很有必要对非平衡凝固和固态相变中的竞争现象进行研究。本综述第一部分从形核/生长类相变角度对非平衡凝固和固态相变进行了概述,并强调了伴随其发生的竞争现象。第二部分立足于经典形核理论,对有关形核热力学、动力学以及形核方式的竞争现象进行了描述。第三部分对凝固中有关热扩散和溶质扩散以及固态相变中有关界面控制和扩散控制的生长方式竞争进行了描述。第四部分对本综述进行了总结性分析和概括,并对本领域的未来发展方向进行了预测。     

What happens to the initial planar instability when the thermal gradient is increased during directional solidification？

The positive thermal gradient is one of the most important parameters during directional solidification. The increase of the thermal gradient usually stabilizes the planar interface in the steady state analysis. However, in the initial transient range of planar instability, the thermal gradient presents complicated effects. Time-dependent analysis shows that the increase of the thermal gradient can enhance both the stabilizing effects and the destabilizing effects on a planar interface. The incubation time first decreases and then increases with the increase of the thermal gradient. Moreover, the initial average wavelength always increases with the thermal gradient increasing, contrary to the effect of the thermal gradient on the steady cellular/dendritic spacing. This reveals the types of spacing adjustment after planar instability.     

冲击磨损下团球状共晶体奥-贝钢磨损表面层组织的变化

采用ＭＬＤ－１０型磨损试验机、扫描电镜和透射电镜等研究了冲击磨损条件下团球状共晶体奥－贝钢磨损表面组织的变化。结果表明：奥－贝钢的冲击磨损表层中存在非晶态组织和纳米级晶粒;磨损亚表层中存在大量位错马氏体和形变贝工体组织;奥－贝钢从亚表层到磨损表面方向的硬度逐渐增加,具有梯度分布特征,能够有效抵抗磨料的冲击和凿削作用,同时磨损亚表层中的位错、形变马氏体和贝氏体组织具有很好的强韧性匹配,可抑制高冲击下     

二元合金高速定向凝固过程的相场法数值模拟

利用相场法对二元合金高速定向凝固过程进行了模拟，为简化计算，采用了温度冻结近似.模拟了定向凝固时平界面失稳、界面形态演化、溶质浓度分布以及高速绝对稳定性条件下胞晶向平面晶的转变等，得到了不同抽拉速度下胞晶间距、胞晶尖端温度以及有效溶质分配系数等参数，显示了高速生长条件下的溶质截留效应. 关键词： 定向凝固 相场法 高速绝对稳定性 溶质截留     

Zr60Al15Ni25大块非晶合金晶化动力学研究

通过等温示差扫描量热法研究了Zr60Al15Ni25大块非晶合金的晶化动力学.实验结果表明,晶化过程的孕育期很短,即使在743 K这样低的温度下也不过0.52 min,而放热峰宽(反应整个晶化过程进行的时间)却随退火温度的降低明显增大,这说明了结晶过程是一个晶核长大控制的过程.Avrami指数表明在不同的退火温度,该合金的晶化机制发生了变化.晶化机制的变化是由于合金原子在不同温度下的扩散能力相差很大所致.     

稀土元素La对Ni75B17Si8非晶合金形成能力和热稳定性的影响

本文发现微量稀土元素在某些合金中可不呈均匀固溶态分布,而与母合金某些原子反应生成高熔点化合物,或吸附于合金中异质核心表面改变其异质形核的活性。当稀土元素以这种状态存在时,不但失去稳定非晶相的作用,而且促进异质形核,降低合金的非晶形成能力和热稳定性。 关键词：