Strain–Age Hardening and Brittleness in Chromium

Abstract

CURRENT theories of the cause of brittleness in ‘pure’ chromium ascribe the phenomenon to Cottrell locking¹, invoking the Mott–Stroh^2,3 theory of crack formation as the operative mechanism of brittle fracture, and nitrogen as the associated interstitial solute atom. There are several features of the tensile behaviour of annealed chromium (containing approximately 0.002 per cent nitrogen and 0.015 per cent oxygen), however, which this picture does not wholly explain, namely: (1) the very high temperature up to which embrittlement extends (about 350° C.) compared with other body-centred cubic metals of similar structure such as α-iron, molybdenum and tungsten; (2) the abrupt nature of the transition from a highly ductile condition to a completely brittle one, behaviour which again differs markedly from other body-centred cubic metals; (3) the fact that embrittlement occurs (at this nitrogen level) at the bottom of a trough in the maximum stress/temperature and yield stress/temperature curves just below 350° C.⁴, and not on the steeply rising part of the yield stress/temperature curve, which occurs only below 200° C., and which has been ascribed directly to Cottrell locking.