Precipitation twinning

Abstract

The paper describes the heterogeneous nucleation of mechanical twins in a TiAl alloy containing Ti₃AlC perovskite precipitates. Examination of the deformation structure by high-resolution electron microscopy has revealed that overlapping planar faults were generated at the perovskite particles. These faults can apparently easily rearrange into embryonic twins and subsequently grow into large twins. The nucleation mechanism was found to be closely associated with the misfit character and stress state of the precipitates. The structural data serves as input for the modelling of the precipitation-induced twinning process. Based on these calculations, the thickness and length of the embryonic twins can be predicted, which agree well with the experimental observations.

Introduction

Precipitation hardening is a widely applied technique for improving the high-temperature performance of intermetallic alloys. In γ(TiAl) alloys precipitation reactions are often implemented by carbon additions because the optimum dispersion can be achieved by homogenization and ageing procedures [1], [2]. Suitable particle dispersions reduce the secondary creep rate over at least one order of magnitude, when compared with the homogenized reference state. However, the attributes that are desirable for high-temperature service might be counter to low-temperature ductility and damage tolerance. TiAl alloys suffer from brittle fracture that persists up to relatively high temperatures (700–800 °C). The brittleness is mainly associated with the existence of {1 1 1} cleavage planes [3], [4] and generally low dislocation mobility [5]. A significant toughening can only be obtained by the implementation of lamellar α₂(Ti₃Al) + γ(TiAl) microstructures [6]. Nevertheless, for crack propagation parallel to the lamellae, i.e. parallel to the {1 1 1} planes, the fracture toughness is extremely low [7]. As in γ(TiAl) the dislocations exclusively propagate on {1 1 1} planes, immobilization of glide processes may easily lead to fracture. Any further increase in the glide resistance by precipitation hardening can therefore be extremely harmful for the fabrication and handling of the material. As described in extensive reviews [3], [8], [9], [10], [11] and detailed experimental [4], [5] and theoretical [12], [13] studies, deformation of γ(TiAl) can also be supported by mechanical twinning along $1/6〈11\overline{2}]\{111\}$. Thus, this mechanism provides auxiliary slip systems and plays an important role in γ(TiAl) alloy design strategies for mitigating the problems associated with the poor damage tolerance. As with other materials, in γ(TiAl) the spontaneous formation of a large twin is difficult to envisage. Thus, separate consideration is usually given to the nucleation of a small twinned region (embryonic twin) and to its subsequent growth into a large twin [9]. While the heterogeneous twin nucleation at dislocation structures [9], [10] and internal boundaries [4], [13] of γ(TiAl) alloys is well documented, there is little information on whether precipitates may act as nucleation centres.

The goal of the paper is to present both an experimental basis and a modelling frame-work for the understanding of precipitation (induced) twinning. In the experimental part a high-resolution electron microscope study of defect structures at precipitates is reported. Here, it is shown that the observed faults can easily be rearranged into embryonic twins. The modelling part consists of the kinematics of twinning, studying the twinning shear eigenstrain in a physically adequate coordinate system. Then the stress state due to the misfit of the precipitate is investigated with the current methods of micromechanics in the same coordinate system. Finally, the thermodynamics of precipitation twinning is analysed in a similar way to that in Ref. [13] for twinning induced by distinct dislocations.