具有“深共晶”特征的合金体系有利于形成金属玻璃。对于三元合金系，虽然已经发现了众多可形成块体金属玻璃(BMG)的体系，但由于缺乏完整可靠的相图，BMG形成成分与共晶反应之间的联系还未被揭示。澄清两者间的关系对于理解BMG的形成和发展具有高玻璃形成能力(GFA)的合金具有重要意义。
本工作选取了相图研究较详细的Cu-Ti-Zr三元合金系，对Cu-Ti二元系侧附近成分区域内GFA对成分变化的依赖关系进行了系统的研究。并采用Ni元素部分替代Cu元素进一步提高三元合金的GFA，并尝试揭示三元和四元BMG形成合金熔体凝固所经历的共晶反应。主要结论如下：
1. 在Cu-Ti-Zr三元系中Cu-Ti二元系侧发现了具有块体玻璃形成能力的成分区域(55-57 at.% Cu, 30-31 at.% Ti, 13-14 at.% Zr)，区域内的合金铜模浇铸条件下的临界直径为2 mm。
2. 通过对电弧熔炼母合金凝固组织的研究，确定在Cu-Ti-Zr三元系中具有块体玻璃形成能力的成分区域内，与玻璃形成相竞争的相为γ-CuTi、Cu51Zr14和Cu2TiZr三相，与玻璃形成相关的共晶反应为(Lγ-CuTi+Cu51Zr14+Cu2TiZr)三元共晶反应。
3. 以Cu56Ti31Zr13三元合金成分为基础，用Ni元素部分替代Cu元素，采用“3D法”将Cu-Ni-Ti-Zr四元系处理成(Cu,Ni)-Ti-Zr伪三元系，在三维成分空间内定位出GFA最强的合金为Cu50.4Ni5.6Ti31Zr13，其铜模浇铸条件下形成完全非晶态样品的直径可达6 mm。
4. Ni元素在提高玻璃形成能力方面的作用主要体现在两个方面：一方面Ni元素加入后使Cu50.4Ni5.6Ti31Zr13合金的液相线温度较起始三元合金降低了26 K，起到了稳定液体的作用；另一方面，Ni元素的加入抑制了Cu51Zr14的析出和γ-CuTi相的长大，使与玻璃形成相关的共晶反应变为(Lγ-CuTi+ Cu2TiZr)伪二元共晶反应。; A “deep eutectic” feature of an alloy system is usually a clue of easy glass formation. It’s easy to clarify the relationship between glass formation and eutectic reaction in a binary alloy system. Although bulk metallic glass formers were found in many ternary alloy systems, the relationship is not well established due to the lack of exact and detailed phase diagramms. It’s of interst to clarify the correlation between the BMG-forming and eutectic reaction, to understand the origin of high glass-forming ability.
In the present work, Cu-Ti-Zr system was selected as our target because of its more detailed phase diagramm information. The composition dependence of the GFA in the Cu-Ti-Zr system near the Cu-Ti subsystem was revisited. Ni was selected to substitute for Cu to improve the GFA of ternary alloy using the “3D pinpointing approach”. The microstructure of arc-melted alloys with optimal BMG-forming composition in the ternary and quaternary systems was characterized and the detailed information regarding phase selection and morphology is used to establish the correlation between the BMG-forming composition and the eutectic reaction that the liquid undergoes during cooling with and without Ni substitution. The following conclusions are drawn:
1. In the Cu-Zr-Ti ternary system, a new composition zone with capability to form bulk metallic glasses was discovered, located at the 55-57 at.% Cu, 30-31 at.% Ti and 13-14 at.% Zr, near Cu-Ti binary subsystem. Within this composition region, BMG rods of 2 mm in diameter can be fabricated using copper mould casting.
2. Undergoing a systematic investigation with “3D pinpointing approach”, the optimized BMG-forming composition in the (Cu,Ni)-Ti-Zr pseudo ternary system is located at Cu50.4Ni5.6Ti31Zr13. The critical diameter of BMG formation is elevated up to 6 mm.
3 The easy glass-forming feature in the ternary composition zone is expected to correlate with (Lγ-CuTi+Cu2TiZr+Cu51Zr14) eutectic reaction. Combined with the previous work, it is revealed that there exist three BMG-forming composition zones in the Cu-Zr-Ti ternary system at least, related to their respective eutectic reactions.
4. The effect that the GFA of the ternary base alloy was improved by substitution of Ni for Cu is attributed to retarding the crystallization of Cu51Zr14 intermetallics and the growth of γ-CuTi phase, which lead to the change of relative eutectic reaction to be Lγ-CuTi+ Cu2TiZr pseudo binary eutectic reaction
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