Abstract:
This thesis describes the results of an investigation into the effects of low temperatures and high strain-rates on the properties of Armco iron in single- and poly-crystalline forms. The experimental programme may be divided into two parts: (a) Developing apparatus suitable for carrying out the type of work envisaged. This apparatus included equipment for the production of single crystals of Armco iron and an impact tensile tester. (b) Assessing existing knowledge of the temperature and strain-rate dependent properties of iron. From this a testing programme was established aimed at contributing further to this knowledge. As a result of this investigation the following conclusions can be made concerning the effects of temperature and strain- rate on Armco iron in single- and poly-crystalline form: (a) The rate-controlling mechanism in iron appears to be the thermally activated nucleation of kinks to overcome the Peierls-Habarro lattice friction force as long as deformation is initiated by slip. (b) A Critical resolved Stress for Twinning which depends on crystal orientation exists under dynamic and static loading conditions at low temperatures. Results from dynamic tests at room temperature indicate that a resolved shear stress criterion alone is insufficient co explain the initiation of twinning in single crystals of iron. (c) The rate of work-hardening in dynamic tests is less than that in static tests at temperatures above 77°. (d) A definite minimum appears to exist in the relationship between fracture elongation and strain-rate. In the present work this minimum occurred at a mean plastic strain-rate of 4.5 x 102 sec-1. (e) Tensile tests at high rates of strain are adiabatic in nature and give rise to large rises in specimen temperature with strain.