铜基合金非平衡组织塑性变形机制控制因素研究

It is reported by the group that a delamination in grain size, not continuous, occurred along the direction from surface to matrix for Cu-Ti alloys subjected to surface severe plasticity deformation (SPD). The difference in microstructure and hardness is obvious between two sides of the delamination interface. A preliminary view on the phenomenon was put forward that it is possibly related to stacking fault energy (SFE), critical grain size for deformation and strain rate, and etc.. The project will focus on the new phenomenon by investigating Cu-Ti, Cu-Ni and Cu-Zn alloys, which are different in solute element and content. The SFE of the copper alloys are various depending upon the solute elements selected before and the states. A main aim is to make clear the universality of the delamination present in Cu-Ti alloy surface subjected to the SPD by comparing deformation layer characteristics, microstructure evolution process and deformation mechanism for the copper alloys in various states. Effect of the solute elements on the SFE will be investigated by experiment measurements and theoretical calculations so as to establish relationship between the SFE and deformation mechanism for the copper alloys. By analysizing the interface microstructures of the delamination, some parameters, that the deformation mechanisms depend upon, such as the critical grain size, the strain rate and etc., will be obtained so that the delamination could be interpreted reasonably and scientifically. Formation free energy of the solid solution will be calculated with the help of the Miedema semi-empirical thermodynamics model to investigate thermodynamics and kinetics processes of phase transformation for the unstable microstructures on high strain rate condition. In summary, results of the project will enrich and develop fundmental theory of grain refinement induced by the SPD and provide a new approch for fabricating gradient materials.

本课题组研究发现，Cu-Ti合金表面经剧烈塑性变形后，沿表面至基体方向晶粒尺寸不是呈连续分布，而是出现了分层现象。分层界面两侧显微组织和硬度有明显区别。初步认为，这可能与层错能、变形临界晶粒尺寸和应变速率等因素有关。本项目拟针对这一新实验现象，选择对层错能影响程度不同的元素和含量的Cu合金（Cu-Ti、Cu-Ni和Cu-Zn）为研究对象，比较不同处理状态下变形层特征、组织演化过程和形变机制，弄清楚表面细晶分层的规律性；通过实验和理论计算，研究溶质元素对层错能的影响，建立层错能和形变机制之间的定量关系；分析分层界面组织结构，得到剧烈塑性变形过程中形变机制发生转变所依赖临界晶粒尺寸和应变速率参数；探究非平衡组织在高应变速率下发生相变的热力学/动力学过程。这将进一步丰富剧烈塑性变形诱导材料晶粒细化的基础理论，并为梯度材料制备提供新途径。

前期研究发现，Cu-4wt.%Ti合金经表面机械研磨处理后，沿表面至基体晶粒尺寸不是连续分布，而是出现了明显的分层现象。针对这一现象，本项目通过在铜中添加不同合金元素来获得中等、中低和低层错能金属，研究层错能、应变速率和晶粒尺寸对塑性变形机制的影响。分别采用热力学法和X射线衍射峰位移法计算和测定了铜合金的层错能和层错几率，结果表明：Ti和Zn元素的加入均降低了铜合金的层错层，Ni元素的加入几乎不影响铜合金层错能。比较不同铜合金组织演变和形变机制发现：中等层错能金属Cu-7wt.%Ni合金，组织演变过程包括位错胞将粗晶细分，位错墙分裂为微带，微带进一步细分成小的方块和多角度晶粒的形成；中低层错能金属Cu-2wt.%Zn合金，组织演变过程包括少量孪晶的出现，微带的形成以及微带细分为方块和多角度晶粒的形成；低层错能金属Cu-2wt.%Ti、Cu-4wt.%Ti和Cu-20wt.%Zn合金，组织演变过程包括面排位错和孪生的形成；孪晶交割将晶粒细分为方块；位错墙的形成；微带的形成；微带进一步细分为方块以及多角晶粒的形成。结果表明，层错能是影响面心立方结构铜合金变形机制的主要因素。此外，探究了非平衡组织在剧烈塑性变形/相变同时发生的热力学/动力学过程。选用典型的时效硬化铜钛合金作为研究对象，研究了时效处理对铜钛合金剧烈塑性变形过程的影响。早期时效和峰时效对铜钛合金组织演变过程影响不大。过时效阶段，由于析出相β-Cu4Ti在显微组织中占主导，两相变形不均匀。析出相细化后，在最表层发生溶解。析出相溶解优先发生在析出相与基体的界面处。这将进一步丰富塑性变形诱导材料晶粒细化的理论基础，并为梯度材料制备提供新途径。