Publisher Summary This chapter discusses the microscopy and plasticity of the L1 2 γ´ Phase. Single-phase ordered intermetallic alloys exhibit a variety of macroscopic mechanical properties that are dominated by the individual behavior of dislocations and, more precisely, by transformations that occur within the core of superdislocations. Positive temperature dependence of the flow stress has been reported in some metals and ceramics, but it is certainly in intermetallic systems that this behavior is the most frequently encountered. This chapter describes macroscopic mechanical properties, including deformation under constant strain rate, creep, and the analysis of transient tests. From one alloy composition to another, the measured orientation dependence may vary in a significantly inconsistent manner as shown by uncertainties on the position of maximum asymmetry. The actual orientation dependence appears, in fact, to be more complicated than a simple theoretical analysis could predict, unless of course this theory is appropriately parameterized.

Chapter 53 Microscopy and plasticity of the L12 γphase

In recent years, instabilities in plastic deformation of metals and alloys became a subject of intensive experimental and theoretical investigations. They were mainly directed towards developing a sound phenomenology, based upon the knowledge of elementary mechanisms, which would adequately describe the manifestations of the instabilities in the stress vs. strain curve and in the form and properties of strain localizations. A classification of plastic instabilities with regard to the underlying mechanisms makes it possible to identify two broad types of instabilities occurring in monotonic loading: (1) instabilities which are related to local strain softening and (2) instabilities which are associated with a negative strain rate sensitivity of the flow stress, referred to as type S instabilities. However, very little is known about the behaviors of systems where microscopic mechanisms giving rise to both type h and type S instabilities may be operative. To be more specific, a relation between precipitate shearing (expected to give a type h instability) and discontinuous yielding, commonly attributed to the PLC effect, is unexplored. Several experimental findings have led the authors to think that the effect of precipitate shearing on the strain rate sensitivity of the flow stress needs to be investigated. In the presentmore » note the authors will focus on the case of an alloy whose solid solution has a positive strain rate sensitivity (and thus does not exhibit the PLC effect proper) and which contains in addition a homogeneous distribution of shearable precipitates. Their aim is to investigate the possibility of deformation induced precipitate reversion and to elucidate the relevance of the effect for the occurrence of discontinuous yielding. The influence of precipitation on the characteristics of the PLC effect in an alloy whose dilute solid solution exhibits a negative strain rate sensitivity will be considered in a separate paper.« less

On a pseudo-Portevin-Le Chatelier effect

The Al 3 Ti-based intermetallics stabilised in the L1 2 structure by additions of transition metals are known to show stress-strain serrations during high temperature deformation. The dislocation pinning phenomenon behind this effect is examined by a study of static strain ageing. The extent of strain-age yield point varies with ageing time and temperature in a manner that confirms pipe diffusion of solute along dislocation cores as the process controlling dislocation pinning. The solute species responsible for pinning is identified as titanium present in excess of stoichiometry.

Static strain ageing of L12 Al3Ti-based alloys

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.

G Lapasset

Papers

Jerky flow (the PLC effect) in Ll2 Al3Ti-based alloys