Abnormal grain growth behavior in gradient nanostructured titanium investigated by coupled quasi-in-situ EBSD experiments and phase-field simulations

Abstract

Gradient nanostructured (GNS) metals exhibit superior mechanical properties compared with their counterparts containing a homogeneous microstructure. However, GNS materials usually suffer from the abnormal grain growth (AGG) when subjected to elevated temperatures, resulting in the instability of the gradient nanostructure and the degradation of mechanical properties. Investigating AGG and thermal stability in GNS metals is crucial for improving their high-temperature performance, but it poses significant challenges due to the inherent complexity in the GNS microstructure. In this paper, quasi-in-situ electron backscatter diffraction (EBSD) experiments and multi-order-parameter phase-field (MOP-PF) simulations are combined to perform a comprehensive study on the AGG mechanism of GNS-Ti. Both experimental and simulation results show that AGG occurs in the deformation twin enriched layer (280 μm depth) at 700/°C, but not at 550/°C. Such difference is attributed to the larger stored energy difference between distinct microstructural layers and the faster grain boundary mobility at the higher temperature of 700/°C. Moreover, we reveal a dual role of deformation twins in the thermal stability of GNS-Ti. The reduced interface energy and mobility of twin boundaries contribute to an improved thermal stability of the corresponding microstructure layer of GNS-Ti. However, on the other hand, the associated change in the stored energy heterogeneity among microstructural layers may promote AGG. Based on these findings, potential microstructure strategies for enhancing the thermal stability of GNS-Ti and similar alloys are provided. It is anticipated that the presently developed approach should be suitable for understanding the thermal stability mechanisms in different GNS metals.