Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Lecture Demonstration Packages Deformation twinning in low symmetry metals Deformation twinning in low symmetry metals (all content)

Deformation twinning in low symmetry metals (all content)

Note: DoITPoMS Teaching and Learning Packages are intended to be used interactively at a computer! This print-friendly version of the LDP is provided for convenience, but does not display all the content of the LDP. For example, any video clips and answers to questions are missing. The formatting (page breaks, etc) of the printed version is unpredictable and highly dependent on your browser.

Contents

Introduction

The package provides information needed to set up and present a short practical demonstration in a lecture.  This LDP (Lecture Demonstration Package) is centred on tin cry phenomenon, the characteristic sound heard when a bar of tin is bent. Variously described as a "screaming" or "crackling" sound, the effect is caused by the crystal twinning in the metal. The sound is not particularly loud, despite terms like "crying" and "screaming". Although the cry is most typical of tin, a similar effect occurs in other metals, such as niobiumindiumzinc and gallium.

 Twinning is common in structures of low crystallographic symmetry. They tend to have fewer independent slip systems, making it less likely that an imposed strain can be fully accommodated by dislocation glide. For example, hexagonal metals are much more prone to deformation twinning than cubic metals. Similarly, deformation of tin (tetragonal) occurs predominantly by twinning, causing the so-called “tin cry” (acoustic waves generated by twin nucleation and growth).

Three points are illustrated:
•   The nature of the stress field in the wall of a pressurised thin-walled tube
•   Fast fracture under conditions such that crack propagation is energetically favoured   -  ie there is a net release of energy
•   By inhibiting the relaxation of regions around a crack, the energy balance can be altered such that fact fracture does not occur (but stable crack growth does)

IT resources

Video of the demonstration

Academic consultant: Bill Clyne (University of Cambridge)
Content development: Anton Sazhin
Photography and video: Steve Penney, Jess Gwynne
Web development: Lianne Sallows and David Brook

ThisDoITPoMS LDP was funded by the UK Centre for Materials Education and the Department of Materials Science and Metallurgy, University of Cambridge.