Thermal stability of extruded Mg-Y-Nd alloy structure

Abstract

Introduction. Today, bioresorbable magnesium alloys possessing the required physical, mechanical, corrosion, and biological properties, are promising materials for orthopedic and cardiovascular surgery. The addition of rare earth elements such as yttrium, neodymium, and cerium to magnesium alloys improves its properties. Compared to widely used titanium alloys, magnesium alloys have a number of advantages. Bioresorbable materials slowly dissolve in the body, and recurrent operation to remove the implant is not needed. Biocompatible magnesium alloys have a fairly low elastic modulus (10 to 40 GPa), approaching to that of cortical bone, that reduces the contact stress in the bone-implant system. At the same time, strength properties of magnesium alloys alloyed with rare earth elements do not always meet the requirements for medical applications. Severe plastic deformation, for example, equal channel angular pressing, torsion under quasi-hydrostatic pressure, uniaxial forging, extrusion, is therefore very promising technique to gain the high level of mechanical properties of metals and alloys. Severe plastic deformation of magnesium alloys improves its structural strength by 2.5 times due to the generation of an ultrafine-grained and/or fine-grained structure. The issues related to the study of heat resistance, structure and phase composition of magnesium alloys with appropriate strength are relevant. Purpose of the work is to determine the influence of thermal effects on the microstructure of the extruded Mg-Y-Nd alloy. Methodology. The extruded Mg-2.9Y-1.3Nd alloy (95.0 wt. % Mg, 2.9 wt. % Y, 1.3 wt. % Nd,  0.2 wt. % Fe,  0 wt. % Al) is investigated in this paper. The thermal stability of the alloy microstructure is studied after annealing at 100, 300, 350, 450 and 525 °С in argon for one hour. The microstructure and phase composition are investigated using optical, transmission and scanning electron microscopes and analyzed on an X-ray diffractometer. Results and discussion. The extruded Mg-2.9Y-1.3Nd alloy has the bimodal fine-grained microstructure. It is found that along with the stable α-Mg phase, the alloy structure consists of Mg24Y5 intermetallic particles and -, -, and 1-phase precipitates. Annealing in the temperature range of 100–450 °С for one hour has no effect on the structure of the Mg-2.9Y-1.3Nd alloy, but promotes the growth in the linear dimensions of -, -, and 1-phases precipitates. In the temperature range of 300–450 °С, the morphology of -, ,- and 1-phases changes, while the average grain size of the major -phase remains unchanged. Annealing at 525 °С leads to a notable transformation of the bimodal microstructure of the alloy, which is associated with the intensive growth in the grain size of the -phase, Mg24Y5 particles, and -, -, and 1-phases precipitates. Annealing in the temperature range of 100–450 °C leads to an increase in the linear dimensions of Mg24Y5 particles, -, -, and 1-phases precipitates and bimodal microstructure of the Mg-2.9Y-1.3Nd alloy remains unchanged.