Effect of heat treatment on the structure and properties of magnesium alloy MA20 subjected to severe plastic deformation

Introduction. One of the most promising fields for the application of magnesium alloys is medicine. Their key advantages are bioresorbability and a low elastic modulus, comparable to that of human cortical bone (up to 30 GPa). Biocompatible Mg-Zn-Zr-Ce (MA20) system alloys are among the most promising for medical applications. Due to their relatively low mechanical properties, the development of severe plastic deformation (SPD) techniques for forming an ultrafine-grained (UFG) state in bulk billets of the Mg-Zn-Zr-Ce alloy to achieve optimal functional properties requires further research. Analyzing the conditions for forming a high-strength UFG state necessitates considering various strengthening mechanisms, including well-known ones related to the effect of UFG structures. Identifying the deformation and strain hardening mechanisms in magnesium alloys subjected to SPD is also highly relevant. The purpose of this work is to establish the mechanisms of strain hardening and to investigate the influence of heat treatment on the structure and properties of the MA20 magnesium alloy after combined SPD. Research methods. The study object was the MA20 alloy in a UFG state (wt. %: Mg – 98.0; Zn – 1.3; Ce – 0.1; Zr – 0.1; O – 0.5). The UFG state was achieved via a combined SPD process involving ABC-pressing followed by multi-pass rolling in grooved rolls. To study the effect of annealing on the microstructure and mechanical tensile properties, samples were annealed in air at temperatures of 200, 250, 300, and 500 °C for 24 hours. The microstructure and phase composition of the samples were investigated using optical and transmission electron microscopy. Results and discussion. It was established that applying a combined SPD method (ABC-pressing and multi-pass rolling) to the MA20 alloy results in the formation of an ultrafine-grained structure with an average grain size of about 1 μm. This leads to a significant increase in yield strength (σ0.2) to 250 MPa and ultimate tensile strength (σu) to 270 MPa, while simultaneously reducing ductility to 3%. Annealing at 200 °C was found to preserve the UFG state in the MA20 alloy and to lead to a 100% increase in ductility, with an 8% decrease in σ0.2 and a 4% decrease in σu compared to the initial UFG state (non-annealed). Conclusions. It was revealed that the grain boundary (σgrain = 202 MPa) and dislocation (σdis = 69 MPa) strengthening contributions provide the most significant increase in the strength of the UFG MA20 magnesium alloy. For the magnesium alloy in the UFG and fine-grained (FG) states, a critical grain size interval of (1–7) μm was identified, corresponding to a sharp increase in the intensity of change for the calculated contributions of dislocation (dσdis/ dd), grain boundary (dσgrain/ dd), overall strengthening (dσtotal/dd), and dislocation density (dρ/dd). For the coarse-grained (CG) state of the alloy in the grain size range (7–40) μm, these parameters stabilize.