The Importance of Microstructural Instability in Determining The Mechanical Behaviour of Cubic Titanium Trialuminides
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THE IMPORTANCE OF MICROSTRUCTURAL INSTABILITY IN DETERMINING THE MECHANICAL BEHAVIOUR OF CUBIC TITANIUM TRIALUMINIDES
D.G. MORRIS, S. GUNTHER and R. LERF Institute of Structural Metallurgy, Avenue de Bellevaux 51, University of Neuchatel, NEUCHATEL 2000, SWITZERLAND. ABSTRACT Cubic trialuminides deform by the movement of dislocations which are clearly dissociated as APB superdislocations at high temperatures, with debate about whether these are dissociated as APB or SISF superdislocations at low temperatures. These materials are characterized by the following mechanical behaviour: (i) strength variable with ternary element addition or titanium content; (ii) mild strength anomaly at high temperature, sometimes; (iii) serrations in stress-strain curve at intermediate temperatures; (iv) low tensile ductility (.,0) at room temperature, increasing at higher temperatures, with an intermediate temperature minimum. These properties are explained by the fine microstructure and its variation locally and with temperature: (a) a tetragonal component of order in ternary alloys in addition to the basic L12 order, varying with ternary element and content; (b) precipitation (sometimes on dislocations) during high temperature testing - accounting for the strength anomaly; (c) extra-ordinarily rapid solute collection at dislocations and APB's - accounting for strain aging and minimum ductility phenomena; (d) strain relaxation at crack tips restricted to single slip planes by the structural modification produced by shear - making a major contribution to brittleness. All these processes are particularly acute in the titanium trialuminides because of the structural instabilities involved, and the fast kinetics of atom rearrangement; thus low temperatures during testing or during cooling after prior heat treatment play a major role. Similar effects may be of importance in other intermetallics such as FeAI, TiAl. RESULTS AND DISCUSSION Strength of Cubic AlTi Alloys The yield or flow strength of cubic AI 3Ti materials has been reported on numerous occasions (e.g. 1-5). The most important characteristics of strength variations are: - (i) the observation of a weak stress anomaly at test temperatures of about 500-600'C (1,3,4,5) - sometimes observed as a strength peak, other times as a stress plateau, other times not clearly detected; (ii) a significant flow stress increase on testing at low temperatures (77K and below) (2,4); (iii) significant variations from one alloy system to another and from one observer to another (1,3,4,5,6). These results have received various interpretations;: - (i) operation of cube cross slip processes leading to Kear-Wilsdorf locks causing the stress anomaly (5); (ii) superdislocation dissociation into a pair of superShockley partials separated by a SISF fault - the cores of these dislocations are known to be non-planar and lead to significant low temperature strengthening, as in the case of the Pt 3 A1 and Co3 Ti intermetallics (7-9); (iii) differing bond strengths according to choice of ternary element (6) or
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