Formation and Stability of Extended Solid Solutions Made by Rapid Quenching from the Melt
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FORMATION AND STABILITY OF EXTENDED SOLID SOLUTIONS MADE BY RAPID QUENCHING FROM THE MELT HOWARD JONES Department of Metallurgy, University of Sheffield, St. George's Sq., Sheffield S1 3JD, England. ABSTRACT Extension of solid solubility by rapid quenching from the melt (RQM) enormously increases scope for alloy development in metals such as aluminium for which solid solubility is particularly limited under equilibrium conditions. The present contribution reviews mainly recent work concerned with matching of experimental observations with predictions affecting: (1)conditions for solidification without change in composition (2)temperatures and modes of deconposition on subsequent heating (3)hardening effects as-quenched and on heat treatment. The significance of such findings for bulk production and consolidation of wrought products for engineering applications is briefly discussed. INTRODUCTION The ability to accommodate alloying elements into solid solution is a basic feature in the design and performance of engineering alloys, both in terms of solid solution strengthening and of age-hardening following solution treatment. Solution treatment within the solid state is restricted however to compositions within the limits of equilibrium solid solubility and the scope for alloying can be enormously enlarged by employing rapid quenching from the melt (RQM) to retain in the solid state the much larger solubilities attainable by solution treatment in the liquid state. Aluminium-based solid solutions provide a particular opportunity in that only eight elements (all neighbouring aluminium in the periodic table) dissolve significantly (>Iat%) at equilibrium whereas this select group has already been trebled [1] by RQM. Possibilities range from beneficially accommodating relatively high levels of normally damaging additions such as iron accumulated e.g. during scrap recycling sequences, to developing quite novel alloys with enhanced characteristics such as improved thermal stability, which is a particular limitation of conventional wrought aluminium alloy compositions. The present purpose is to review progress primarily in the author's group in relating experimental observations to predictions concerning: (1)conditions for solid solubility extension by RQM (2)temperatures and modes of decomposition on subsequent heating (3)hardening effects both in as-quenched and heat-treated conditions. CONDITIONS FOR SOLIDIFICATION WITHOUT CHANGE IN COMPOSITION [2] For a given alloy of composition Co, the extreme condition of solid solubility extension occurs when solidification occurs effectively without solute partitioning or long-range diffusion to give single-phase solid of uniform composition Co, the same as in the parent melt. This corresponds to the
72 formation of predendrites identified by Biloni and Chalmers [3] at surfaces of chill-castings and correspondingly for RQM [4,5]. Thermodynamic considerations suggest that this solidification without change in composition should be possible at temperatures below To for the compo
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