Mechanical Properties and Behaviour

Abstract

The glassy or fine scale crystalline microstructures and extended or novel composition ranges of rapidly solidified materials might be expected to exhibit interesting behaviour under applied mechanical stress. Metallic glasses exhibit flow stress σ _y as high as E/50, where E is Young’s modulus (Table 5.1), approaching the levels ≈E/30 found for perfect dislocation-free single-crystal metallic whiskers. The associated high values of hardness and wear resistance have been put to good use, for example, in recording/replay or read/write heads for audio, video, computer or instrumental recording machinery, where their high electrical resistivity and, for particular compositions, high magnetic permeability and good corrosion resistance are also critical for the application. Plastic flow at stress σ > σ _y and temperature T < T _g , the glass transition temperature, is localized in shear bands giving way to more homogeneous time-dependent flow at high temperatures, allowing hot forming to be carried out with or without inducing crystallization. The corresponding microcrystalline products of rapid solidification can also exhibit ultrahigh strengths and durability because of combinations of very fine matrix grain size, high volume fraction of hard intermetallic precipitates or dispersoid phases, and/or extended concentrations of hardening alloy additions in solid solution in the matrix phase. Such ultrafine dual or multiphase microstructures are also ideal candidates for superplastic forming and/or diffusion bonding at elevated temperature and low stress, while intermetallic dispersoids can be incorporated that exhibit excellent resistance to dissolution or coarsening, so imparting quite exceptional micro-structural stability at elevated temperature. Wide ranging reviews of the mechanical performance of rapidly solidified materials have been published by Gilman [5.1], by Davis [5.2], by Li [5.3], by Rama Rao and Radhakrishnan [5.4], by Taub [5.5], by Das and Froes [5.6]and by Davis et al. [5.7]. References [5.1–5.4]are confined to metallic glasses, [5.5]and [5.6]feature microcrystalline alloys while [5.7]embraces both categories.