Macroscale stress induced stabilization of coherent precipitates

Abstract

In systems with 'moderate' misfit, on nucleation and in the initial stages of the growth, precipitate crystals may be coherent with the matrix. This reduces the interfacial chemical energy; but a higher cost is paid in terms of the energy stored in the long range strain fields. On the growth beyond a critical size (r*), the energy of the system is minimized via the formation of interfacial misfit dislocation loops. It has been shown that the coherent state is stable in two separate size regimes of the precipitate in finite bodies (i.e. shows a re-entrant interface character transition) and further the coherent state can be fully stabilized below a critical size of the domain (R*). The presence of stress fields in the material, either due to internal sources or due to externally applied loads, is expected to alter the magnitude of the r*. In the current work, finite element simulations are used to study the effect of elastic stress fields on r*. The Cu-2wt.%γFe alloy is used as a model system towards this end. A map is generated demarcating the regions of stability of the coherent and semi-coherent states; with normalized r*, domain size (R) and elastic stress (σyy) as axes. We conclude the following. (i) Stresses of the order of GPa leads to a change in r*. (ii) The re-entrant interface character transition to the coherent state is suppressed by tensile stress. (iii) The coherent state is stabilized due to compressive stress. (iv) Compressive stress expands the phase field of the coherent state, while tensile stress has the opposite effect. (v) In 'very small' domains the coherent state can be stabilized in spite of being subjected to tensile stress.