熔体冷却速率与块体非晶合金塑性的关系研究

Metallic glasses often show worse plasticity during deformation in room temperature, which considerably confines its practical application. Therefore, it will have important scientific significance and engineering applications value to investigate the plastic deformation mechanism of metallic glasses. The existing results showed that the plasticity of bulk metallic glasses can be improved by increasing ordering or disordering degree. That is to say, the bulk metallic glasses with the same composition would experience a nonmonotonic change in plasticity while the microstructure goes from disorder to order, i.e., a transition of "large-small-large" in the plasticity. However, there are still not enough data to verify that such inference is right or not. Therefore, bulk metallic glasses will be prepared by controlling the cooling rates of melt and then be deformed in this project. Accordingly, a relationship between cooling rate and the plasticity of bulk metallic glasses with different composition will be analyzed. In addition, so far the deformation mechanism of metallic glasses is still not clear, especially for the metallic glasses with homogeneous structure. Therefore, the effect of cooling rate of melt and composition on the microstructure of bulk metallic glasses will be investigated by means of transmission electron microscope (TEM), X-ray diffractometer (XRD) and differential scanning calorimeter (DSC) in this project. As a result, a relationship between cooling rate of melt, composition, microstructure and plasticity will be established, and it will be attempted to understand the plastic deformation mechanism of metallic glasses at atomic length scales.

非晶合金在室温变形过程中通常表现出较差的塑性，使其实际应用受到一定程度的影响，因此研究非晶合金的塑性变形机制具有重要的科学意义和工程应用价值。已有研究表明，增加原子结构的有序度或无序度都可以有效改善块体非晶合金的塑性。这意味着，相同成分块体非晶合金的塑性随着微观结构的变化并不是单调增加或减小，而是经历一个由高到低再升高的转变。但该推论是否成立，仍缺乏充足的实验依据。为此，本项目将通过控制熔体冷却速率制备具有不同微观结构的块体非晶合金，并对其进行变形，分析不同成分块体非晶合金塑性与冷却速率之间的关系。另外，人们目前对非晶合金塑性变形机制的认识还不是很清楚，尤其是具有单一结构的非晶合金。为此，本项目将主要采用透射电镜、X射线衍射和热分析，系统地研究熔体冷却速率与合金成分对微观结构的影响，建立熔体冷却速率、合金成分、微观结构与塑性之间的内在关系，试图从原子尺度上认识非晶合金的塑性变形机制。

块体非晶合金在远低于玻璃转变温度时通常表现出明显的脆性，严重的限制了其作为工程材料的应用。为了进一步探究块体非晶合金的塑性变形机理，本项目以Zr-Al-Ni-Cu块体非晶合金作为研究对象，利用差示扫描量热法（DSC）、X射线衍射（XRD）、高分辨电镜（HRTEM）、扫描电镜（SEM）、拉伸试验机等手段研究了熔体冷却速率与块体非晶合金塑性之间的关系；分析了自由体积含量和结构无序度与单一结构块体非晶合金塑性的关系；建立了母合金铸锭显微结构与对应的块体非晶合金结构无序度之间的关系；研究了玻璃形成能力与韧脆性转变之间的关系。结果表明，随着熔体冷却速率的降低，Zr55Al10Ni5Cu30块体非晶合金的弯曲塑性随着熔体冷却速率的降低出现了由高到低再升高的转变，而自由体积含量和纳米晶体积分数的变化是导致这一现象的主要因素；自由体积和结构无序度都是影响单一结构Zr55Al10Ni5Cu30块体非晶合金塑性的主要因素，但自由体积与塑性的关系在很大程度上取决于结构无序的程度；随着重熔次数的增加，母合金铸锭的凝固组织明显细化，相应的Zr55Al10Ni5Cu30块体非晶合金中有序团簇尺寸显著减小，原子无序度增加；对于较低玻璃形成能力的Zr65Al7.5Ni10Cu17.5块体非晶合金，其弯曲塑性随着熔体冷却速率的降低出现了单调下降，并未出现韧脆性转变。这些结果进一步完善了块体非晶合金的塑性变形机理，为改善块体非晶合金的塑性提供了理论依据。