Unveiling the abnormal grain growth behavior in diffusion-bonded joints of zirconium alloys enhanced by gradient nanostructures

Abstract

Gradient nanostructured (GNS) materials have shown benefits in the diffusion bonding process; however, the occurrence of abnormal grain growth (AGG) of GNS materials at high temperatures remains insufficiently explored, particularly in relation to joint microstructure and properties. Herein, gradient nanostructures were fabricated on Zr-4 alloy plates via ultrasonic impact treatment, and subsequently utilized in diffusion bonding at 740–800 °C. The fabricated GNS-Zr exhibited a gradient microstructure, transitioning from surface nanograins (minimum of ∼40 nm) to coarse-grained regions within the matrix. During diffusion bonding, the thermal instability of GNS-Zr triggered AGG, forming a novel layered bimodal structure in the bonded joints, characterized by rapidly grown nanograins (RGGs), abnormal large grains (AGs), and normally grown grains (NGs). The AGs reached up to eight times the size of the original matrix grains at 800 °C, allowing their orientations to dominate the overall texture. The formation of AGs was driven by the non-uniform distribution of stored strain energy in GNS-Zr, with most grain orientations inherited from the original matrix. Although AGs exhibited lower hardness and greater susceptibility to twinning, the layered bimodal structure enhanced the joint shear strength through heterogeneous deformation between AGs and surrounding finer grains. This interaction provided additional work-hardening to the smaller grains while also improving the effective strength of AGs. Consequently, AGG contributed to a significant enhancement in joint shear strength by 39.8 % at 780 °C and 21.8 % at 800 °C compared to joints bonded with as-received Zr-4. These findings highlight the essential role of AGG in determining joint properties and offer insight into the microstructural design strategies for diffusion-bonded joints.