Variation in the degree of order and its influence on mechanical behavior in complex B2 aluminides of refractory metals

This paper deals with the variation in the degree of order and its influence on mechanical behaviour in complex B2 aluminides of refractory metals, comprising three groups of metallic elements: X ≡ Ti, Zr, Hf; Y ≡ Nb, Ta, Mo, W, Cr, V; Al. During high-temperature high-rate deformation (extrusion), many of these alloys, which are B2 single phase, seem to experience plastic instabilities because of a local variation in hardness (soft and hard zones). This variation in hardness stems presumably from the variation in the degree of order of the B2 phase associated with a change in the local chemistry due to dendritic segregation. Consequently, the cross-section of the extruded bars exhibits a surprisingly irregular shape. Metallographic examination of the cross-section shows a wavy contrast bearing a resemblance to “van Goghʹs sky” (VGS) paintings, although the materials are B2 single phase. Furthermore, waves of VGS follow closely the irregular contour of the cross-section. When tensile tests are conducted at room temperature on specimens machined from the extruded products, a very large tensile elongation (10%–28%) can be obtained in many cases. This unusually high elongation at room temperature for intermetallic materials is, in some cases, accompanied by a composite-like deformation behaviour indicating the existence of soft and hard zones even during tensile straining.