Work-hardening and recovery mechanisms in gamma-based titanium aluminides
- PDF / 1,022,636 Bytes
- 9 Pages / 606.24 x 786 pts Page_size
- 26 Downloads / 218 Views
8/30/04
11:54 AM
Page 2103
Work-Hardening and Recovery Mechanisms in Gamma-Based Titanium Aluminides J.D.H. PAUL and F. APPEL The work-hardening mechanisms in two-phase g-titanium aluminide alloys were characterized in terms of the glide obstacles determining the velocity and slip path of dislocations, utilizing transmission electron microscopy (TEM) observations and thermodynamic-glide parameters. There was clear evidence that short-range obstacles in the form of dislocation debris and dipoles were produced during plastic deformation at room temperature. These dislocation obstacles contributed significantly to work hardening. The observed strong strain hardening arose from long-range elastic dislocation interactions and the production of dipole and debris defects. The thermal stability of these deformation-induced defects was assessed by isothermal and isochronal annealing. The results indicated that the dipole and debris defects were relatively unstable upon annealing at moderately high temperatures, which led to significant recovery of work hardening.
I. INTRODUCTION
EXTENSIVE investigations of structure/property relationships have led to a detailed understanding of the behavior and mechanical properties of g-titanium aluminides.[1,2,3] A topic that has received far less attention is the effect of work hardening. This is certainly in part due to the high brittleness of TiAl alloys, which persists to test temperatures in excess of 973 K and limits tensile ductility. However, in compression, it is possible to produce work hardening over large plastic strains and a wide range of temperatures, which apparently provides a significant potential to strengthen the material. The scientific interest in this field arises mainly for two reasons. The strain energy imparted during cold or hot working is determined by the micromechanisms behind work hardening. Thus, these processes are important for the initiation of static and dynamic recrystallization, techniques that are often used in order to refine the microstructure. Secondly, the rate at which the material hardens during cold working influences both the power required in shaping operations and the frequency with which the material must be annealed to enable further working to be performed. Work hardening can be utilized in real engineering applications. For example, a common method to increase the fatigue strength of materials involves shot peening, which locally increases the hardness of the surface layer of a component. If this method is to be become standard for titanium aluminide components, it is important that the induced obstacles remain stable at operating temperatures for the full benefit of the technique to be obtained. This, in turn, requires a quantitative assessment of the thermal stability of the defect structure.
J.D.H. PAUL, Researcher, and F. APPEL, Group Leader, are with the Institute for Materials Research, GKSS Research Center, Geesthacht D-21502, Germany. Contact e-mail: [email protected] This article is based on a presentation made in the s
Data Loading...
