CREEP BEHAVIOR OF AS4I MAGNESIUM ALLOY NANO-COMPOSITES

Abstract

Nowadays the lightest metal that is used for structural applications is magnesium and its alloys. Magnesium alloys have a wide range of applications in automotive sector and aerospace sector because of their light weight, good specific strength, good machinability, excellent castability and also very good dimensional stability. Magnesium alloys suffer from some drawbacks like high tendency for oxidation, poor cold workability, and high thermal expansion coefficient. High temperature properties of magnesium alloys are not so good because otheir low melting temperature. At high temperature, deformation mainly occurs by slip which occurs on basal and non basal planes and by sliding of grains. In this work, it is endeavored to improve the creep resistance of AS41 alloy for usabiHty above 373 K by introducing the nano alumina particles into the matrix. Nano alumina reinforced AS41 magnesium alloy matrix composites with 2 wt% and 5 wt % Al2O3 were fabricated by combined stir casting and ultrasonic processing in order to distribute the nano alumina particles uniformly in the melt. Indentation creep tests were performed on the nano composites and the as cast AS4 I alloy at four different temperatures of 448 K. 473 K, 498 K. and 523 K and three different stresses of 109.2 MPa, 124.8 MPa and 140.4 MPa. The creep curves obtained in these tests show that creep resistance increased with the addition of nano- A1203 into the magnesium alloy, and nano-composites with 5 wt% Al2O3 exhibited highest creep resistance. The values of obtained stress exponent range from 3 to 3.5, 3.55 to 6, and 4 to 6.5 for the as-cast AS4I alloy, AS41/2wt% Al2O3, and AS41/5wt% A1203 composites, respectively. Thus, dislocation creep is the dominant creep mechanism. The calculated activation energies for as-cast AS41, AS41/2wt% A1203 and AS41/5wt% A1203 are approximately 66. 17KJ/mol. 83 .77KJ/mol, and 88.3 KJ/mol, respectively. The activation energy values for the creep of composites are close to that of the pipe diffusion (92 kJ/mol) of magnesium. The thesis includes five chapters in which Chapter 1 gives the brief introduction of the Mg alloys and Mg alloy base composites Chapter 2 gives the general description of the theoretical background of creep mechanisms in metallic materials, and discussion about the creep of magnesium alloys, Mg based composites. Chapter 3 gives the motivation and objective for this investigation Chapter 4 gives the description about the experimental methodology employed for this dissertation work. Chapter 5 discusses about the resultsobtained for various experiments conducted. Chapter 6 gives the conclusions of the overall thesis work. Suggestions for ftiture work and references are given at the end of the thesis