应变速率诱导金刚石纳米线及FCC金属纳米线的相变机制研究

Strain rate is an important factor which can affect the mechanical properties of materials. With the improvement of experimental technique, the effect of ultrahigh strain rate on mechanical properties and deformation mechanisms of materials is attracting considerable interest. Based on our preliminary work, in this project we will further study the strain-rate-induced phase transformation mechanisms of cubic diamond nanowires and fcc metal nanowires using molecular dynamics simulation method. In the present project, firstly, the effect of temperature, strain rate, the axial direction of the nanowires, direction of free surface and the state of stress effect on the strain-rate-induced phase transformation mechanisms will be carefully studied. Secondly, the energy barriers of the phase transformations will be calculated, the nucleation and growth mechanisms of the new phase will also be carefully studied. At the end of this project, we can achieve the following three aims: (1) make it clear that which factor can affect the strain-rate-induced phase transformation mechanisms of cubic diamond nanowires and fcc metal nanowires; (2) make the micromechanismes of the strain-rate-induced phase transformation clear; (3) propose a theory of the strain-rate-induced phase transformation. Finally, this project will show a vivid story about strain-rate-induced phase transformation mechanisms of cubic diamond nanowires and fcc metal nanowires, and will help us deep understand the plastic deformation of the two kinds of nanowires.

应变速率是影响材料力学性能的一个重要因素，随着实验技术的进步及人们探索未知领域的延伸，超高应变速率对材料的力学性能及变形机制的影响引起了科技工作者的广泛兴趣。本项目拟在我们前期研究工作基础上,通过对金刚石纳米线和FCC金属纳米线进行模拟计算，进一步深入研究这两种纳米线的应变速率诱导相变机制。主要研究温度、应变速率、纳米线的轴向晶向、自由表面晶向、横截面形状以及应力状态等因素对这两种材料应变速率诱导相变机制的影响；并进一步计算相变势垒，研究相变的形核及长大的微观机制，从理论上解释发生应变速率诱导相变的深层次原因。通过本课题的研究，预期可以实现如下目标：（1）明确影响应变速率诱导相变发生的各种因素；（2）明确应变速率诱导相变的微观机制；（3）构建相变理论，解释应变速率诱导相变发生的深层次原因。通过本项目的研究可使人们进一步深入认识金刚石及FCC金属纳米线的变形行为，为应用研究提供基础理论支撑。

应变速率是影响材料力学性能及变形机制的一个重要因素。 随着实验技术的发展，超高应变条件下材料的力学性能及变形机制引起了科技工作者的广泛兴趣。本项目通过分子动力学模拟及理论分析，首先研究了应变速率诱导金刚石-石墨相变，给出了该种相变发生的机制，计算了相变势垒，并进一步明确了影响该种相变发生的各种因素（拉伸晶向、自由表面方向、温度以及纳米线尺寸）。其次研究了Cu、Ni、Au和Ag四种FCC金属纳米线沿[100]晶向单轴拉伸条件下的变形机制，发现FCC金属纳米线在低温高应变速率拉伸条件下，应变超过弹性极限后纳米线沿垂直于加载方向的两侧面分别突然收缩和扩张，从而实现FCC-BCC相变。进一步研究发现该种相变也广泛存在于FCC金属体材料和纳米板以及金属间化合物Ni3Al沿[100]晶向的拉伸过程中。并进一步从晶体结构弹性稳定性角度深入研究了该种相变，揭示了相变发生的原因。最后我们还发现这种相变是可逆的，BCC铁沿[100]晶向压缩时也可发生从BCC到FCC的相变。通过本项目的研究，揭示了沿FCC金属[100]晶向拉伸也可以实现从FCC到BCC的Bain相变，加深了人们对Bain相变发生条件的认识，以及对金刚石-石墨相变的认识。