ZrCu基金属玻璃复合材料的马氏体相变增韧与断裂韧性间的关联机制研究

Bulk metallic glasses (BMG) have shown a unique combination of mechanical, chemical and physical properties compared to traditional crystalline metals. However, lack of room temperature plasticity has been the main obstacle for BMGs to be applied in practical applications. Recently bulk metallic glass composites (BMGMCs) containing B2-ZrCu phase have attracted intensive interest due to transformation-induced plasticity which could impart an appreciable work-hardening capability (the alloy becomes stronger as it deforms). Tracing the history of new material development, it is not coincident. Back in the 1980’s, stress-induced martensitic transformation has effectively toughened brittle ceramics-Zirconia reinforced alumina. For three-point bending test, the stress induced martensitic transformation could induce volume expansion between ceramic matrix and shape memory reinforcement ZrO2. The internal volume expansion will subsequently affect crack tip stress field of Zirconia toughened alumina. Martensitic transformation could therefore further affect micro-crack nucleation and propagation. Amorphous alloys and ceramics have fundamentally different microstructure. Amorphous alloys are metallic-bonded with no long range order. Free volume within their randomly arranged microstructure plays a key part to their fracture behaviour; while ceramics are mainly composed of ionic or covalent bonding with regularly arranged microstructure. It is both scientifically intriguing and of practical importance to understand how martensitic transformation and the associated internal volume change, stress field change will affect the fracture toughness behaviour (resistance to cracking) of shape memory BMGMCs. Though there has been intensive research on fracture behaviour of shape memory bulk metallic glass composites, the majority of them were focusing on uniaxial compression or tension testing. The study of martensitic transformation effect on crack tip stress field of three-point bending test is scarce..To establish the connection between martensitic transformation and its associated stress/ energy field change near crack tip of shape memory BMGMCs, the scope of the proposed study will be:1) To fine control the nucleation and growth of the shape-memory phase (B2-ZrCu) by micro-alloying and processing parameter (heating current/time, cooling speed) adjustment so as to achieve homogeneously distributed reinforcement phase within a continuous glass matrix 2) To understand the fracture toughness behaviour of shape memory bulk metallic glass composites and how particle size, distribution, volume of B2-ZrCu will affect fracture behavior of the corresponding material 3) To understand how martensitic transformation induced stress/energy filed change will affect crack tip behaviour of shape memory bulk metallic glass composites, therefore could possibly toughen ZrCu-based bulk metallic glass composites. The fracture mechanism between martensitic transformation and fracture toughness in shape memory ZrCu-based BMGMCs will thus be systematically investigated.

断裂韧性是反映材料抵抗裂纹失稳扩展能力的性能指标，对构件的强度设计具有十分重要的意义。金属玻璃作为一种新型材料，缺乏宏观塑性是其主要应用瓶颈。近年来研究发现马氏体转变能够有效的提高ZrCu基金属玻璃复合材料的室温塑性和加工硬化性能。马氏体转变增韧机制在钢铁和氧化锆陶瓷增韧中也有深入系统的研究。然而目前绝大部分报道的断裂机制都是基于单轴压缩或拉伸测试。本课题通过马氏体增韧机制对金属玻璃复合材料裂纹扩展和断裂韧性的研究，了解马氏体形变产生的体积变化、相变诱发的应力场变化等因素对裂纹尖端塑性变形区、预裂纹萌生与扩展的影响。通过系统研究马氏体增强相的大小（微米到纳米级别）、分布、体积百分比在三点弯曲状态下对ZrCu基金属玻璃复合材料断裂韧性的影响，探讨金属玻璃基复合材料中马氏体相变与断裂韧性的关联机制，为设计实现高韧性的材料提供有利的科学依据和实践基础。

马氏体增强相金属玻璃复合材料是改善金属玻璃脆性的有效方法之一。但是CuZr颗粒的大小、均匀分布的调控一直是一个难题，而CuZr颗粒的尺寸、大小以及分布对CuZr基金属玻璃复合材料的力学行为、抵抗破坏的能力有本质而显著的影响。为了进一步了解CuZr颗粒的尺寸大小、体积分布与冷却速度的关系，本课题首先就Cu51.5Zr48.5这个成分在同一凝固状态下不同位置的显微组织结构与冷却速度做了关联性研究。研究发现在铜模的表面与芯部，材料的冷却速度差异性可以从高达5000 K/s降低到约为200 K/s。但是由于CuZr晶体生长速度速度过快(23 mm/s), 这种冷却速度的强烈差异性并没有对CuZr在金属玻璃中的形核与长大起到相应的影响与作用。其次，本课题通过系统研究Cu51.5Zr48.5单轴向压缩状态下的力学行为，首次探讨了CuZr马氏体颗粒相变过程中，由体心立方B2结构转化为单斜体B19′造成的形状以及体积效应对材料压缩塑性应变以及断裂能的增强作用。并且首次在金属玻璃基复合材料中提出了微裂纹增韧与微裂纹偏转机制对断裂抵抗力的积极作用。微裂纹增韧以及偏转机制在相变陶瓷，生物材料如骨头、贝壳等中都有详细报道，有可能为金属玻璃基复合材料在对抗热振领域应用提供基础。