Phase Decomposition of Heat Resistant Magnesium Alloys Containing Heavy Rare Earth Elements and Its Control

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The aim of this study is to develop a new magnesium alloy which has more excellent heat and corrosion resistance than that of the conventional heat resistant aluminum and magnesium alloys that could be applicable to piston and other engine parts undergoing the highest thermal load among automobile parts. The Mg-rich portion of phase diagrams, aging characteristics, hot workability, corrosion behavior and microstructural control of magnesium alloys containing heavy rare earth elements were investigated. Finally, the alloys were hot-forged into a piston shape and their tensile properties were also evaluated. (1) The solid solubilities of heavy rare earth elements in magnesium shift largely to lower concentrations than those of Mg-Gd or -Dy binary alloys by Nd or Y addition with decreasing temperature. As a result, alloys containing Gd or Dy of more than 6% and Nd or Y of more than 3% show marked age hardening even at 250ﾟC and have higher tensile strength than the conventional magnesium … More alloys. (2) The precipitation sequences of the investigated alloys are as follows : Magnesium supersaturated solid solution* beta" (DO_<19>) *beta' (bco) *Equilibrium beta (Nd containing alloy : fcc, Y containing alloy : bcc). The remarkable age hardening is caused by the precipitation of metastable beta"-phase. The beta"-phase precipitate preferentially on the dislocations introduced by the hot rolling or hot forging and, therefore, precipitation free zone near the grain boundaries is reduced. As a result, tensile properties are enhanced by the hot working. (3) It is possible for the investigated alloys to suppress grain coarsening and to reduce the amount of remained eutectic compound by lowering the homogenizing temperature to just above the solubility limit and, therefore, to directly dieforge all specimens into a piston shape without an intermediate upset forging. Die-forged Mg-10%Gd-3%Y alloys exhibits a tensile strength of 290MPa and 0.2% proof stress of 250MPa at 250ﾟC,which are superior to those of the conventional heat resistant magnesium alloys and are more than double those of the conventional heat resistant aluminum alloys. (4) A fine Mg (OH)_2 layr is formed as the stable protective film on the surface of all investigated alloys by the additions of rare earth elements under a corrosive environment containing a chloric ion. Less