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 inf luence on mechanical behaviour in complex B2 aluminides of refractory metals, comprising three groups of metallic elements: X equivalent to Ti, Zr, Hf; Y equivalent to Nb, Ta, Mo, W, Cr, V; Al. During high-temp erature 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 variatio n in hardness stems presumably from the variation in the degree of ord er of the B2 phase associated with a change in the local chemistry due to dendritic segregation Consequently, the cross-section of the extru ded bars exhibits a surprisingly irregular shape. Metallographic exami nation of the cross-section shows a wavy contrast bearing a resemblanc e to ''van Gogh's 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 mater ials is, in some cases, accompanied by a composite-like deformation be haviour indicating the existence of soft and hard zones even during te nsile straining.