D. Banerjee

A new ordered orthorhombic phase in a Ti3AlNb alloy

An ordered orthorhombic phase has been detected in a Ti-25at.% Al-12.5at.%Nb alloy which is one of a family of Ti3Al base alloys under development for high temperature applications. Convergent beam electron diffraction and channelling enhanced microanalysis have been used to assign a space group to the structure and determine atomic positions. The ordered orthorhombic phase has a Cmcm symmetry and is isomorphous to the NaHg type of structure. Preliminary studies on the effect of heat treatment on the formation of this new phase suggest that the Ti3Al free-energy composition curve develops an inflection at low temperatures with Nb addition leading to a phase separation reaction in which the Nb rich component has the orthorhombic symmetry.

Microstructure and mechanical properties of orthorhombic alloys in the TiAlNb system

The current understanding of the metallurgy of the orthorhombic alloys in the TiAlNb system is reviewed with emphasis on tensile and creep properties of ternary alloys. It is shown that increasing the Nb content of alloys from 15 to 27 at% at a constant Al level significantly increases both the tensile and creep properties of equiaxed as well as lath structures, while small changes in Al content have a large effect on creep. For a given alloy composition, the amount of B2(β) phase and its distribution and the scale of O laths influences tensile properties, while creep properties depend on the volume fraction of equiaxed α2O phase present in the structure as well as the size of O laths.

Phase stability and ordering behaviour of the O phase in TiAlNb alloys

This paper describes a vertical section Ti-27.5Al up to 25 at% Nb in the ternary TiAlNb system. This section is shown to be of fundamental importance in understanding the origin and relationship between the various phases which constitute the microstructures of an important class of engineering alloys based on the Ti3AlNb system, the α2, B2 and O phases. In carrying out this work, the presence of two distinct forms of the O phase with the same space group and lattice periodicity, but different site occupations, has been confirmed.

Structure, tensile deformation, and fracture of a Ti3Al-Nb alloy

The development of Ti3Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α2 phase (based on Ti3Al, DO19) and βo, (based on Ti2NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D019 (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α2 phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α2 and βo phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the βo phase. A rationale incorporating the failure modes of the two phases—cleavage of α2 and slipband decohesion of βo—has been evolved to explain the trends in ductility with heat treatment.

Transformations in a Ti-24AI-15Nb alloy: Part I. Phase equilibria and microstructure

A variety of heat treatments have been employed to explore phase equilibria and the development of microstructure in a Ti-24Al-15Nb alloy. These include solution treatments both above and below the β-transus, followed by controlled cooling and aging at temperatures high enough to preclude ω-phase formation. The phase fieldsβ, βo, α2 + βo, α2 +βo + O, andβo + O have been identified in the alloy, and schematic time-temperature-transformation (TTT) curves are proposed for continuous cooling transformations from theβ phase. The composition of the α2 andβo phases in the α2 +βo region and of the α2,βo and O phases in the ternary phase field have been obtained by analytical electron microscopy.

Deformation structure in a Ti-24Al-11Nb alloy

AbstractA Ti-24Al-11Nb alloy has been heat-treated so as to obtain a microstructure of coarse α2 particles (D019 structure based on Ti3Al) in a matrix of the ordered βo phase (B2 structure based on Ti2AlNb). Dislocation structures generated by tensile strains of ∼2 pct at room temperature have been analyzed by transmission electron microscopy The βo phase is shown to deform inhomogeneously on {110}, {112}, and {123} planes by α/〈211〉 slip. The slipband structure is complex, consisting of segments of heavily pinned edge dislocations with periodic cross slip of screw components on to secondary slip planes. Incompatibility stresses at α2/βo interfaces can generate fine α[100] slip as well. The α2 phase deforms independently by α dislocation slip. Slipbands in the βo phase can shear the α2 phase by activatingc +a/2 slip on

A resolution of the interface phase problem in titanium alloys

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A structural study of ordered precipitates in an ordered matrix within the FeAlNb system

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