Hydrogen-Induced Transformation Superplasticity of Titanium and Ti-6Al-4V

对近年来发现的可逆化学循环引起的相变失配塑性和超塑性现象进行了实验和理论研究。作为一种普遍的变形机制，变形失配塑性和超塑性是由于在同素异性转变过程中产生的外部应力和内部失配应力的偏置而产生的。这些机制在金属中是众所周知的，由于温度在其同素异形体温度周围循环而发生相变。最近，研究表明，通过在钛中反复加氢和脱氢，在恒温下循环化学成分也可以诱导出相同的机制。这种新的变形现象在纯钛和钛合金中进行单向蠕变实验，其中施加的应力和氢循环特性系统地变化。基于控制错配应变发展和氢在金属中扩散的微观力学的基本机制、连续力学、封闭模型和有限元，开发了数值方法来定量、预测描述这一现象。实验旨在支持这些模型，这些模型将描述变形过程中的瞬时应变率和平均应变率作为实验参数的函数。这是第一次系统地研究简单金属（钛）及其合金（Ti-6Al-4V）中氢诱导相变错配塑性的新现象，并将其作为所有相关化学和机械参数的函数。该程序将证明，超塑性可以在化学循环（即，在重复循环和平均应变率与施加应力之间的线性比例超过100%的应变积累）下诱导。基于内应力产生和偏松弛机制，将开发模型，为这种新的变形机制提供理论和预测理解。这些知识将促进对非平衡条件下的塑性和超塑性的基本科学理解，并最终允许基于氢诱导相变错配超塑性的超塑性工业工艺的发展。＊＊＊

9987593DunandThe recently discovered phenomena of transformation-mismatch plasticity and superplasticity induced by reversible chemical cycling are investigated experimentally and theoretically in titanium. As a general deformation mechanism, transformation-mismatch plasticity and superplasticity are known to occur as a result of biasing by an external stress of internal mismatch stresses produced during an allotropic transformation. These mechanisms are well known in metals subjected to a phase change by temperature cycling around their allotropic temperature. Recently, it was shown that the same mechanism could be induced by cycling the chemical composition at constant temperature, upon repeated addition and removal of hydrogen in titanium. This novel deformation phenomenon is examined in pure titanium and in a titanium alloy by performing unidirectional creep experiments, where the applied stress and the hydrogen cycling characteristics are systematically varied. Based on the fundamental mechanisms controlling the micromechanics of mismatch strain development and the diffusion of hydrogen in metals, continuum-mechanics, closed-form models and finite-element, numerical methods are developed to allow a quantitative, predictive description of the phenomenon. Experiments are targeted to support these models, which will describe the instantaneous and average strain-rate during deformation as a function of experimental parameters.%%%This is the first systematic investigation of the novel phenomenon of hydrogen-induced transformation-mismatch plasticity in a simple metal (titanium) and one of its alloys (Ti-6Al-4V) as a function of all relevant chemical and mechanical parameters. The program will demonstrate that superplasticity can be induced under chemical cycling (i.e., accumulation of strains in excess of 100% upon repeated cycling and linear proportionality between average strain rate and applied stress). Models will be developed that provide a theoretical and predictive understanding of this new deformation mechanism, based on the mechanics of internal stress creation and biased relaxation. This knowledge will advance the basic scientific understanding of plasticity and superplasticity under non-equilibrium conditions, and will eventually allow the development of superplastic industrial processes based on hydrogen-induced transformation-mismatch superplasticity. ***