Microstructure, Hardness Evolution, and Thermal Stability Mechanism of Mechanical Alloyed Cu-Nb Alloy during Heat Treatment

Ruoshan Lei,Mingpu Wang,Guangrun Chen,Huanping Wang,Shiqing Xu

doi:10.3390/met6090194

Abstract

The microstructure, hardness evolution, and thermal stability of mechanically alloyed (MA-ed) nanocrystalline Cu–10 wt %Nb solid solution during heat treatment were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) observations, and microhardness measurement. It is found that the pronounced precipitation of Nb from the Cu-Nb supersaturated solid solution occurs at temperatures up to 700 °C, and the annealed alloy shows a bi-nanostructure with Nb nanoparticles dispersed in the nanocrystalline Cu matrix. The bi-nanostructure remains stable with Cu crystalline grain size below 100 nm and Nb particle size around 10 nm even after annealing at 900 °C for 3 h. The microhardness of the annealed sample shows a small increase after annealing at 400 °C, and then it shows a slow decreasing trend with further increasing temperatures. With the help of the kinetics analyses, it is found that the coarsening of the stable Nb nanoparticles is controlled by volume diffusion. The enhanced stability of the nanocrystalline Cu microstructure is mainly attributed to the solute drag and precipitate pinning effects.

Highlights

Nanocrystalline materials have drawn considerable attention due to their excellent mechanical and functional properties [1,2,3,4]
Over the past few years, many nanocrystalline materials synthesized by MA have been found to possess a certain thermal stability, it is often thought that the nanocrystalline materials may offer a significant driving force for the grain growth [9,10,11,12]
It can be seen that Cu diffraction peaks have undergone a height increase and a breadth decrease with the increase of annealing temperatures, which can be attributed to the growth of Cu grains and the reduction of internal strain

Summary

Introduction

Nanocrystalline materials have drawn considerable attention due to their excellent mechanical and functional properties [1,2,3,4]. A high thermal stability to minimize the grain growth is required for nanocrystalline materials. There are some techniques to synthesize nanocrystalline materials, one of which is mechanical alloying [5,6,7,8]. %Ti powders retained an almost unchanged average grain size of about 24 nm after annealing at 1100 ◦ C for one week. They emphasized the stabilization effect of alloying element, and proposed that the nanostructured W-Ti alloy had a lower free energy

Methods

Results

Discussion

Conclusion