Overview no. 50: Cleavage-limited maximum strength of work-hardened b.c.c. polycrystals

Abstract

b.c.c. metals and alloys maintain a non-decaying work-hardening rate up to very high plastic strains when deformed monotonically at low temperatures ( T < 0.3 T M ), specially by axisymmetric elongation (wire drawing). This fact seems to offer the possibility of production of strong metallic filaments which are tougher than other fibre reinforcing materials. However, b.c.c. metals being prone to cleave, their rising stress-strain curves could be trimmed by brittle failure beyond some point. This paper studies such contingency and shows that two failure modes are possible: a microscopic-type failure, when the flow stress meets the (strain-dependent) tensile cleavage stress, σ f T , and, for large strains, a macroscopic-type failure when the critical stress intensity factor (also strain-dependent) is reached for cracks associated to inclusions or for crack-like surface defects. From the rather limited information gathered on the evolution of the brittle failure stress with strain it is deduced that σ f T is mainly controlled by the instantaneous—strain-induced—grain size and that it will rarely limit the b.c.c. wire strength directly. On the contrary, macroscopic-type failures can constitute the absolute strength limit for alloys with very high workhardening rate. Guidelines to further improve such strength limit arise from the present overview.