单晶铁沿[101]晶向冲击过程中面心立方相的形成机制

铁的冲击相变过程是科研工作者们关注的热点领域之一.铁沿[100]晶向冲击时会发生体心立方相到密排六方相的转变;而沿[101]晶向冲击时,相变产物除了密排六方相之外还出现一定量的面心立方相.人们已经明确了体心立方到密排六方相的转变机制,然而对于面心立方相的形成机制问题至今还在探索.本文通过分子动力学方法模拟了体心立方单晶铁沿[101]晶向的冲击过程,模拟结果显示体心立方相将转变为高压密排结构(密排六方相和面心立方相);并分析了面心立方相的形成机制:在冲击过程中,单晶铁沿[101]和101]晶向突然收缩,同时沿[010]晶向突然扩张,从而导致体心立方到面心立方相的转变.此外,本文进一步研究了不同应力状态下单晶铁的相变机制,发现沿[101]晶向单轴压缩以及沿[101]和[101]晶向双轴压缩时铁将发生体心立方到面心立方相的转变;而沿[101]和[010]晶向双轴以及三轴压缩时将会发生体心立方到密排六方相的转变.最后进一步计算了三个相的吉布斯自由能随压力的变化,并对冲击模拟结果进行了能量分析,给出了沿[101]晶向冲击条件下高压密排相产生的原因.

Formation mechanism of face-centered cubic phase in impact process of single crystal iron along[101]direction

The impact phase transformation of body-centered-cubic iron is one of the hotspots in current research.Many studies demonstrated that when iron is impacted along the [100] direction, body-centered-cubic phase will transform into hexagonal close-packed phase;while when it is impacted along the [101] direction, a certain amount of face-centered-cubic phase will also appear besides hexagonal close-packed phase. The transformation from body-centered-cubic to hexagonal close-packed phase has been clarified, however, the transformation from body-centered-cubic to face-centered-cubic phase still needs further exploring. In the present work, molecular dynamics simulation is used to study the phase transformation of body-centered-cubic iron impacted along the[101] direction. The results show that the body-centered-cubic phase will transform into a close-packed structure including hexagonal close-packed phase and face-centered-cubic phase). The formation mechanism of facecentered-cubic phase is as follow. In the loading process, single crystal iron suddenly contracts along the [101]and [101]cubic phase to face-centered-cubic phase. The formation mechanism of hexagonal close-packed phase can be divided into two stages: first,(101) plane is compressed into close-packed plane, then hexagonal close-packed phase is obtained by the relative sliding of adjacent close-packed planes. To further investigate the formation mechanism of the close packed structure, the effect of stress state on the phase transformation of body-centeredcubic iron is further studied. Under one-dimensional(along the [101] direction) or two-dimensional loading(along [101] and [101]loading process the lattice constants along the three dimensions are monitored. When the transformation from body-centered-cubic phase to face-centered-cubic phase finishes, the ratio of lattice constants along three directions is 1∶1∶1.31 under one-dimensional loading;while the ratio of lattice constants is 1∶1∶1 under twodimensional loading. Obviously, the body-centered-cubic phase transforms into distorted face-centered-cubic phase under one-dimensional loading. Under two-dimensional(along the [101] and [010] direction) and threedimensional loading(along the [101], [010] and [101]hexagonal close-packed phase. Gibbs free energy value for each of BCC, HCP and FCC phase is calculated. The calculation results show that the BCC phase is stable under low pressure, while the HCP and FCC phase are stable under high pressure. Finally, based on Gibbs free energy and the effect of stress state on the phase transformation, the phase transformation mechanism of body-centered-cubic iron under [101] impaction is investigated, and a reasonable explanation for the phase transformation is obtained.