The reheating-cooling method: A technique for measuring mechanical properties in the nonequilibrium mushy zones of alloy
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I. INTRODUCTION
HOT tears are defects that form during the solidification of castings. The formation of hot tears is closely related to the mechanical properties of the alloy at small liquid fractions in the mushy zone.[1–9] The common approaches for measuring the mechanical properties of alloys at mushy-zone temperatures include the reheating method[2,3,4] and the solidification method.[5–9] The most widely used approach is the reheating method, which involves heating a specimen to the desired mushy-zone temperature, holding the specimen at the test temperature for a certain amount of time to obtain a uniform temperature distribution in the gage length of the specimen, and then measuring the properties. The method is straightforward and relatively easy to use. A fair amount of useful data has been generated using this method. The reheating method is fairly accurate when the mushy zone is in equilibrium, but has limitations when the mushy zone is in nonequilibrium and the liquid fraction in the mushy zone is small. However, the mushy zone in an alloy is usually not in equilibrium during solidification, and hot tearing occurs at small liquid fractions (0.1).[8] Duncan et al.[10] demonstrated that the liquid fraction decreases rapidly when a specimen is held at mushy-zone temperatures due to backdiffusion in the solid (backdiffusion is a special term referring to diffusion in the solid). During the heating and holding stages of the reheating method, the liquid fraction in the specimen decreases with time. The liquid may completely disappear if the liquid fraction is small and the heating/holding time is long. As a result, it is inadvisable to use the reheating method to measure the mechanical properties of alloys in the mushy zone at temperatures below the equilibrium solidus temperature of the alloy. Q. HAN, Research Staff Member, is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6083. Contact e-mail: [email protected] M.I. HASSAN, Research Assistant Professor, and K. SAITO, Professor, are with the Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506-0108 S. VISWANATHAN, Senior Research Staff Member, is with Sandia National Laboratories, Albuquerque, NM 871851134. S.K. DAS, President, is with Secant Inc., Lexington, KY 40511. Manuscript submitted March 5, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
The solidification method involves cooling a specimen from liquid to solid in a mold and measuring the mechanical properties of the alloy during cooling.[5–9] It can be used for measuring mechanical properties in the nonequilibrium mushy zone, but its accuracy is influenced by interactions between the specimen and the mold and the nonuniform temperature distribution in the specimen. In order to increase the accuracy of the method, the specimen must be solidified slowly in the mold, resulting in the formation of large grains. Since hot tearing is sensitive to the grain size, the solidification method has limitations in its application to casting processes in which the cooling r
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