The absence of relative grain translation during superplastic deformation of an Al-Li-Mg-Cu-Zr alloy
- PDF / 1,351,338 Bytes
- 9 Pages / 597.28 x 785 pts Page_size
- 62 Downloads / 179 Views
I.
INTRODUCTION
THE phenomemon o f superplasticity in polycrystals is o f great practical as well as academic interest. It involves a resistance to strain localization that is due to a consistently high sensitivity o f the flow stress to the strain rate. This is usually achieved at high homologous temperatures and low strain rates, which normally result in quite low flow stresses. Forming processes similar to those employed with thermoplastics and glass are then possible with metallic materials. Much work on superplasticity has been published, o-a] Of particular practical significance has been the development o f superplastic aluminum alloys, and one o f these, the A1-Li-Mg-Cu-Zr alloy AA8090, has attracted considerable interest for aerospace applications, tsj The attraction o f this material lies in the fact that, compared to the 2xxx and 7xxx series aluminum alloys against which it was designed to compete, it is approximately 10 pct less dense and has a 10 pct higher elastic modulus. Various mechanisms have been proposed to account f o r the high rate sensitivity, and it has been noted that there is an apparent requirement f o r a fine and stable grain size, typically not greater than about 1 0 / z m mean grain diameter. A mechanism w h i c h is often invoked is based on the primary importance o f grain boundary sliding. t61 Evidence f o r this mechanism has come from in situ observations o f specimen surfaces during superplastic deformation f7] and from changes in crystallographic preferred orientationJ 81 Mechanisms based on grain boundary sliding, in particular, would indicate that certain types o f fine-grained microstructures would not be associated with superplasticity. One example is the structure present initially in unrecrystallized AA8090 sheet P.L. BLACKWELL, Research Fellow, and P.S. BATE, Senior Research Fellow, are with 1RC in Materials for High-Performance Application, T h e University of Birmingham, Birmingham B15 2TT, United Kingdom. Manuscript submitted June 1 6 , 1992. METALLURGICAL TRANSACTIONS A
intended for superplastic forming. In this, an insufficient population and distribution o f high-angle boundaries for grain boundary sliding to o c c u r are known to exist, and a suitable structure only develops following some period o f nonsuperplastic deformation, tgl The present work examines the superplastic deformation o f that alloy, particularly the development o f the grain microstructure and the grain boundary population in the microstructure and the associated rate sensitivity o f the flow stress.
II.
EXPERIMENTAL M E T H O D S
A. Material
The composition o f the material is given in Table I; this conforms to AA8090. The material was supplied in the form o f plate, with thickness o f 12.5 mm. This was heat-treated at 545 °C for 1 hour and cooled to room temperature at about 25 °C per hour; this treatment serves to form large stable precipitates and so reduces the solute content and prevents cracking during roiling. The heattreated p l a t e was rolled at a n o m i n a l temperature o
Data Loading...
