Anne Denquin

Phase transformation mechanisms involved in two-phase TiAl-based alloys—I. Lambellar structure formation

Abstract The present paper deals with the mechanism operating in one of the three major transformation modes appearing in two-phase TiAl-based alloys: the formation of the so-called γ α 2 lamellar structure. This microstructure, the most typical of the alloys of this category, results from precipitation of γ (Ll0) lamellae in the α (A3) or α2 (DO19) matrix. This reaction is achieved through (i) pre-nucleation stage corresponding to the formation of a local f.c.c. type stacking sequence by the movement of Shockley partials; and (ii) nucleation and growth stages involving both ordering and ledge movement. The presence of numerous order domain boundaries in a given γ lamellar precipitate is explained by the encounter of separately nucleated order domains.

Phase transformation mechanisms involved in two-phase TiAl-based alloys. II: Discontinuous coarsening and massive-type transformation

Abstract This paper is directed towards examination of the mechanisms involved in two of the three major transformation modes appearing in two-phase TiAl-based alloys: discontinuous coarsening and massive-type transformation. The discontinuous coarsening implying grain boundary migrations is essentially promoted by highly stable interfaces in the primary lamellar structure, which render the normal lamellae coarsening of continuous type extremely difficult. During rapid cooling, the massive-type transformation with a direct transition α → γ replaces the primary γ lamellae precipitation. The nucleation takes place most likely on grain boundaries through the formation of either an ordered nucleus or a disordered f.c.c. nucleus followed by ordering, and the nucleus is coherent with one of the two grains. Since such a coherency renders diffusion across the nucleus/matrix interface a difficult process, the growth of the massive phase occurs into the opposite grain through thermally activated short-range jumps of individual atoms across an incoherent α γ interface.

Dissimilar Friction Stir Welding of 2014 to 6061 Aluminum Alloys

Welding cheap and ductile 6xxx Al alloys with high strength 2xxx Al alloys is desirable for instance in specific aeronautical applications. These alloys present different rheological behaviors and melting temperatures which affect the ability to produce sound dissimilar friction stir welds. Dissimilar friction stir butt welds made of 2014-T6 and 6061-T6 Al alloys were performed with various welding parameters including shifts of the tool from the initial separation between the plates to be welded and placing one alloy either on the advancing, or on the retreating side of the weld. Temperature measurements during welding, mechanical characterization (transverse tensile tests and hardness profiles) and macrographic observations were performed. Macrographies on sections perpendicular to the welding direction reveal different metal flow patterns in the weld nugget. If the 2014 alloy is placed on the advancing side of the weld, an abrupt transition between the weld nugget and the 6061 alloy is observed on macrographs leading to premature fracture in tension. Dissimilar welds are cooler on the 6061 side of the weld, i.e. the weakest side of the weld, than the corresponding 6061 similar weld, limiting the growth of the hardening precipitates. This leads thus to higher strength of the dissimilar welds. Dissimilar welds with the weld center shifted towards the 2014 alloy present lower temperatures than unshifted welds on the 6061 side of the weld, also leading to higher strength.