Evolution of microstructure and hardness during artificial aging of an ultrafine-grained Al-Zn-Mg-Zr alloy processed by high pressure torsion

Abstract

An ultrafine-grained (UFG) Al-4.8%Zn-1.2%Mg-0.14%Zr (wt%) alloy was processed by high pressure torsion (HPT) technique and then aged at 120 and 170 °C for 2 h. The changes in the microstructure due to this artificial aging were studied by X-ray diffraction and transmission electron microscopy. It was found that the HPT-processed alloy has a small grain size of about 200 nm and a high dislocation density of about 8 × 1014 m−2. The majority of precipitates after HPT are Guinier–Preston (GP) zones with a size of ~ 2 nm, and only a few large particles were formed at the grain boundaries. Annealing at 120 and 170 °C for 2 h resulted in the formation of stable MgZn2 precipitates from a part of the GP zones. It was found that for the higher temperature the fraction of the MgZn2 phase was larger and the dislocation density in the Al matrix was lower. The changes in the precipitates and the dislocation density due to aging were correlated to the hardness evolution. It was found that the majority of hardness reduction during aging was caused by the annihilation of dislocations and some grain growth at 170 °C. The aging effect on the microstructure and the hardness of the HPT-processed specimen was compared to that observed for the UFG sample processed by equal-channel angular pressing. It was revealed that in the HPT sample less secondary phase particles formed in the grain boundaries, and the higher amount of precipitates in the grain interiors resulted in a higher hardness even after aging.

Ultralow-temperature superplasticity and its novel mechanism in ultrafine-grained Al alloys

“…annealing the HPT-processed sample for 2 h at 120°C, the average grain size was practically unchanged, and it increased only to about 300 nm at 170°C, showing a stable UFG structure in this sample [14]. An unusually high strain rate sensitivity, m, of 0.43 ± 0.02 was obtained by using nanoindentation creep test in the same temperature region, indicating that superplasticity can be expected at low temperatures.…”

The Microstructure and Strength of UFG 6060 Alloy after Superplastic Deformation at a Lower Homologous Temperature

“…In the meantime, the recent works have shown that the formation of UFG states in Al alloys with a regulated distribution of alloying elements to form secondary hardening phases as well as segregations or interlayers along grain boundaries can lead to a unique combination of properties, when the strength, plasticity and electrical conductivity of the material simultaneously increase [ 20 , 21 , 22 ]. In addition, it was shown that grain refinement to the nanoscale range allows for realizing the SP effect at low and even ultralow (below 0.5 T m ) temperatures [ 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Under such conditions, the thermally induced degradation of the UFG microstructure could be prevented, which would ensure maintaining high strength of Al alloys after SP forming.…”

The Effect of Heat Treatment and Pressing at 400 °c With Coconut Shell Charcoal Media on the Hardness, Microstructure, and Density of Al-Si Alloys

“…Gubicza et al [14] processed the ultrafine-grained (UFG) Al-4.8%Zn-1.2%Mg-0.14%Zr alloy by high-pressure torsion (HPT) technique and then aged at 120 and 170 °C for 2 hours. These microstructural changes due to artificial aging were studied by X-ray diffraction and transmission electron microscopy.…”