Phase field simulation of coherent precipitation of Ni4Ti3 particles during stress-assisted aging of a porous NiTi alloy

Morphological evolution and growth behavior of Ni4Ti3 precipitates during stress-assisted aging of a porous NiTi alloy are investigated by means of phase field method through introducing nano-scale volume elements around the micro-sized pores. The model naturally takes into account the stress redistribution arising from the structural and elastic inhomogeneity across the matrix of the porous NiTi alloy aged under compressive stress. Simulation results show that the orientation and distribution of Ni4Ti3 precipitates in different volume elements are evidently distinct. A gradient distribution in terms of the area fraction of Ni4Ti3 precipitates across the selected volume elements can be seen. The area fraction and average size of Ni4Ti3 precipitates are dependent on position of the volume element, i.e. distance to the pore. Differences in the orientation, distribution and average size of Ni4Ti3 precipitates among different selected volume elements are attributed to the disparities of stress states in the volume elements.