On the microstructural stability of TiAl/Ti3Al polysynthetically twinned crystals under creep conditions

• Published in: Philosophical magazine. A, Physics of condensed matter. Defects and mechanical properties Vol. 80; no. 10; pp. 2283 - 2298
• Main Authors: Wegmann, Gerhard, Maruyama, Kouichi
• Format: Journal Article
• Language: English
• Published: London Taylor & Francis Group 01.10.2000; Taylor and Francis
• ISSN: 0141-8610; 1460-6992
• DOI: 10.1080/01418610008216473

The lamellar structure of polysynthetically twinned (PST) specimens has been characterized before and after creep testing in terms of a quantitative description of the microstructural variables: lamellar spacing, α 2 spacing and α 2 volume fraction. An apparent α 2 → γ phase transformation taking place when the samples were held at 1150k was encountered not only in creep-deformed specimens but also in a simply heat-treated specimen. The transformation of the α 2 phase into the γ phase took place by partial dissolution of α 2 lamellae. resulting in a substantial increase in the spacing of the α 2 lamellae. The driving force for the α 2 decomposition was identified as the chemical force to complete the thermodynamic equilibrium that was not reached in the as-grown PST crystals. Two processes of lamellar refinement during creep deformation were identified, namely mechanical twinning parallel to the lamellar interfaces and secondary precipitation of γ phase in α 2 lamellae. Mechanical twinning parallel to the lamellar interfaces was much more pronounced in soft-oriented PST specimens than in hard-oriented PST specimens. Both refinement processes led to an increase in the total number of interfaces even though some α 2 -γ interfaces disappeared as a consequence of α 2 dissolution. Both refinement processes may contribute to the self-strengthening of lamellar α 2 + γ alloys in the course of creep and hence the superior creep resistance of the lamellar microstructure.