Coarse-grained Condition Research Articles

Influence of precipitation on recrystallization and texture development in an iron-1.2% copper alloy

The development of preferred orientation has been studied in this alloy for a wide range of annealing conditions which included single and two-stage treatments as well as continuous heating at various rates. The most strongly pronounced textures are formed after high rolling reductions and in annealing treatments which allow precipitation of copper to occur in the cold-worked structure prior to recrystallization. Fine-scale precipitation which can be produced by ‘light’ preliminary annealing treatments results in a coarse-grained condition after recrystallization with a high value of the texture ratio I 222/I 200. The same effect may be achieved by heating at an optimum rate of about 10 K min−1. This behaviour resembles closely that of commercial aluminium-killed steels and is attributed to a reduction of the nucleation rate of recrystallized grains and an enhanced orientation selectivity.Prolonged low-temperature annealing or very slow heating rates lead to a fine-grained recrystallized state with a strong {l00} <all> texture component. During these treatments, sub-boundary migration is initiated in regions of sharp lattice curvature resulting in a high concentration of coarse copper particles which inhibits continued movement of the boundaries. It is proposed that under these conditions, nucleation of recrystallized grains may occur by a process of extensive recovery within volumes of the deformation structure which exhibit very small lattice curvature.

Effects of microstructure and strain ageing on fatigue-crack initiation in steel

Evidence of the effects of grain size, precipitation hardening, and dynamic strain ageing on the long-life fatigue endurance of smooth mild-steel testpieces is reviewed. The fatigue limit behaviour of annealed mild steel involves a grain-size transition. When dynamic ageing is suppressed, fine-grained conditions still retain a well-defined fatigue limit but coarse-grained samples do not. In strongly locked fine-grained conditions the limit stress corresponds closely with the cyclic stress required to spread plasticity across grain-boundary barriers, but its grain-size dependence is much less than that of discontinuous yielding in monotonic straining. Tests using prestrained and coarse-grained conditions show that dynamic strain ageing can be a sufficient cause of the fatigue limit, but the usefulness of room-temperature ageing as a general fatigue-strengthening mechanism is limited. Atmosphere-locked substructures developed at low cyclic stresses are unstable at higher stress amplitudes. Elevated-temperature ageing which causes carbide precipitation can be more effective, but quench-aged mild steels with very fine dispersions of intermediate precipitates have relatively poor long-life fatigue endurance, owing to the effects of progressive precipitate dissolution during cyclic straining. In contrast, stable dispersions which maintain diffuse slip are remarkably efficient strengtheners. Such observations of the behaviour of mild steel have provided clues to the relative importance of strengthening mechanisms which contribute to the fatigue resistance of tempered martensitic steels. It is likely that the contribution from the matrix substructure is limited by instability in much the same way as the strengthening effects of a strain-aged substructure are limited. Dynamic ageing will promote a fatigue limit, but strengthening by stable dispersion hardening appears to be the dominant source of high fatigue resistance.