The mechanical properties of ordered alloys

Abstract

It has been our aim to demonstrate that the mechanical behavior of alloys that form superlattices can be understood in terms of the change in dislocation configuration with the degree of order. Fully ordered materials deform by the movement at relatively low stresses of superlattice dislocations, which consist generally of closely spaced pairs of unit dislocations. Since these dislocations are constrained to move as a group in order to preserve the ordered arrangement of the lattice, cross-slip is hindered. Long-range order thus results in a high rate of strain hardening, and in many cases to brittle fracture. Since cross-slip is thermally activated, however, at elevated temperatures the rate of strain hardening decreases and ordered materials become more ductile. The degree of order in superlattice alloys can be altered by thermal treatment (except for alloys ordered to the melting point) or by departing from the stoichiometric composition. Increases in the separation of superlattice dislocations brought about by partial order can explain the peak in yield stress manifested by most superlattice alloys near the critical ordering temperature, and the minimum in strength observed at the stoichiometric composition at low test temperatures. Another type of deviation from perfect long-range order is the antiphase domain boundary, which contributes to the strength primarily in the early stages of isothermal ordering. While preparing this review it has become apparent that certain areas of study have not received sufficient attention. In particular, single crystals of superlattices with structure other than L1 2, and to a very limited extent L2 0, have not been studied. Also, it has not yet been established whether sources emit unit dislocation or superlattice dislocations; careful electron transmission studies might resolve this question, which has direct bearing on theories of yielding in solid solution alloys. Studies of creep and diffusion in superlattice alloys have demonstrated a large increase in activation energy with order. It may be anticipated therefore that other diffusion controlled processes such as recrystallization and grain growth may be retarded by order, although there is virtually no information on this subject in the literature. It is in the latter category of diffusion controlled properties that ordered alloys appear to reveal the most promise for commercial applications. While the strength of superlattice alloys at low temperatures generally is low compared to other solid solution alloys, ordered alloys demonstrate superior strength at elevated temperatures because dislocation climb is restricted. However, in order for this property to be exploited it is necessary to utilize alloys with a high critical ordering temperature. One of the major obstacles to more widespread use of inter-metallic compounds and other ordered alloys is their extreme brittleness at low temperatures. The origin of this brittleness is now understood to lie in the restriction of cross-slip by long-range order and/or in the segregation of interstitial impurities to grain boundaries. Future research should now be directed towards means of ameliorating brittleness, with techniques such as ternary alloying additions, or by appropriate thermal processing to produce very fine grain sizes while avoiding contamination by interstitials.