A model for the anomalous mechanical properties of nickel-base L12 ordered alloys—I. Dislocations dynamics

A model for the anomalous mechanical properties of nickel-base L12 ordered alloys — II. Cross-slip processes and mechanical properties

Publisher Summary This chapter discusses the dislocation cores and yield stress anomalies. In addition to high yield stresses and good creep properties up to high temperatures, materials with attractive mechanical properties for high-temperature applications must have sufficiently good fracture toughness at both low and high temperatures. These properties are however a priori incompatible, simply because high flow stresses generally result from high frictional forces acting on mobile dislocations. When the frictional stress is high, the classical laws that describe the movement of dislocations and the plasticity of materials indicate that owing to the thermal activation the deformation stress steeply increases when the temperature is decreased. High yield stresses at high temperatures are thus coupled with such high yield stresses at low temperatures that little plastic deformation is allowed and brittle fracture occurs more easily.

Chapter 50 Dislocation Cores and Yield Stress Anomalies

This paper will provide an overview of the current status of the research and development of ordered alloys for high temperature structural applications, which were conducted in P.R.China. The physical and mechanical properties of ordered alloy will be reviewed with enphasis on aluminides. Current research programs in P.R.China will be outlined, and the expections of the future will be discussed.

The ordered intermetallic research in p.r-china

Many intermetallic alloys with L12, L10 and BCC related structures exhibit yield-stress anomalies, which have strong influence on their mechanical properties at intermediate temperatures. A short review is made of the anomalous glide systems in various intermetallics. They are shown to exhibit several common characteristics, which allow it to classify them into three groups with possibly similar dislocation mechanisms. In the second half, the well-known case of octahedral glide in Ni3Al is considered again, and a new model is proposed, based on the formation and the unlocking process of incomplete Kear-Wilsdorf locks by a double cross-slip mechanism. This model is supported by several microscopic and macroscopic observations including “in situ” straining experiments in a TEM. It accounts for the positive temperature dependence of the yield-stress with fairly good orders-of-magnitude estimates of the stress. It also explains satisfactorily the small stress-strain rate sensitivity and the transient effects.

Mao Wen

Papers

Slip behaviours in a directionally solidified Ni3Al at 850°C along [001]