Effect of the external surface layer on the phase transition (PT) between the low pressure phase and high pressure phase (HPP) in a NiAl bicrystal is investigated. Using a phase field model, the external surface layer is included, within which the elastic properties and surface energy are properly distributed. After resolving a stationary layer, the coupled phase field and elasticity equations are solved to capture the HPP evolution. Residual stress concentrator is included as a shear band representing an inelastic shear strain. Due to the small grain size, the surface layer can influence the stress distribution and consequently, the critical inelastic shear strain \(\gamma _{cr}\) for the HPP growth. Above a certain applied pressure, the surface layer width \({\Delta }_{{\xi }}\) shows no effect on \(\gamma _{cr}\), e.g., \(P=10\) GPa for the grain size of L \(=\) 20 nm. For lower pressures, \(\gamma _{cr}\) increases as pressure reduces. Due to the interplay of size addition by the surface layer and size reduction by the transformation strain, \(\gamma _{cr}\) reduces versus \({\Delta }_{{\xi }}\) and then increases for larger \({\Delta }_{{\xi }}\). For smaller grain sizes, the surface layer effect is promoted as it is imposed to a larger transformation work. The lowest \(\gamma _{cr}\) is obtained for \(P=19\) GPa, in good agreement with the theoretical pressure of 20 GPa. Combining the external shear on pressure adds an extra shear term to the transformation work, which allows for the relaxation of the shear band and results in a nonlinear reduction of the PT pressure versus applied shear.

研究了外表面层对 NiAl 双晶低压相和高压相 (HPP) 之间相变 (PT) 的影响。使用相场模型，包括外表面层，其中弹性特性和表面能适当分布。求解静止层后，求解耦合相场和弹性方程以捕获 HPP 演化。残余应力集中器作为代表非弹性剪切应变的剪切带包含在内。由于晶粒尺寸较小，表面层会影响应力分布，从而影响 HPP 生长的临界非弹性剪切应变 \(\gamma _{cr}\)。在一定的施加压力之上，表层宽度 \({\Delta }_{{\xi }}\) 对 \(\gamma _{cr}\) 没有影响，例如 \(P=10\) GPa晶粒尺寸 L \(=\) 20 nm。对于较低压力，\(\gamma _{cr}\) 随着压力降低而增加。由于表面层尺寸增加和转变应变尺寸减小的相互作用，\(\gamma _{cr}\) 相对于 \({\Delta }_{{\xi }}\) 减小，然后增大较大的\({\Delta }_{{\xi }}\)。对于较小的晶粒尺寸，当施加较大的转变功时，会促进表面层效应。最低的\(\gamma _{cr}\)是在\(P=19\) GPa下获得的，与20 GPa的理论压力非常吻合。将外部剪切与压力相结合会为变换工作添加额外的剪切项，从而使剪切带松弛，并导致 PT 压力相对于施加剪切的非线性减小。