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

本文从实验和理论上研究了最近发现的钛合金可逆化学循环引起的相变失配塑性和超塑性现象。相变失配塑性和超塑性是一种普遍的变形机制，它是由于同素各向异性相变过程中产生的内部失配应力的外应力偏置而产生的。这些机制在金属中是众所周知的，这些金属通过温度在其同素各向同性温度周围循环而发生相变。最近，有研究表明，在恒温下循环化学成分，重复添加和去除钛中的氢，也可以产生同样的机理。通过单向蠕变实验研究了纯钛和钛合金中的这种新的变形现象，在单向蠕变实验中，施加的应力和氢循环特性发生了系统的变化。基于控制失配应变发展和氢在金属中扩散的微观力学的基本机制，连续介质力学、闭合模型和有限元，发展了数值方法来定量地、预测地描述这一现象。实验旨在支持这些模型，该模型将描述变形过程中的瞬时和平均应变率作为实验参数的函数。这是首次系统地研究了简单金属(钛)及其合金(Ti-6Al-4V)的氢致相变-失配塑性这一新现象作为所有相关化学和力学参数的函数。该程序将演示在化学循环(即重复循环时累积的应变超过100%以及平均应变率与施加的应力之间的线性比例)下可以诱导超塑性。将根据内应力产生和偏向松弛的力学原理，开发出模型，为这种新的变形机制提供理论上和预测上的理解。这一认识将促进对非平衡条件下塑性和超塑性的基本科学认识，并最终发展基于氢诱导相变-失配超塑性的超塑性工业过程。***

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. ***