| The microstructures and hardness values of thixoformed ZA27 alloys in naturally aged state for 5 years and heat treated state were observed and examined by optical microscope, scanning electron microscope and X-ray diffraction. The results are shown as followings:The microstruture of as-thixoformed alloy is composed of aluminum-richα′dendrites and interdendritic secondarily solidified structures. On cooling after casting and subsequent natural aging, theβphase decomposes eutectoidally into well-formed lamellae ofαandηor irregularly complex morphologies. Some coarse lamellar colonies spread from the edges of the primary particles towards the centerα′-phase cores with low-aluminium. The growth of these lamellae is always retarded by the continuous decomposition of the centerα′phase and the center cores decompose into the mixture of fine zinc-richηphase in an aluminum matrix. The eutectic structures are mainly dominated by divorced eutectic and honeycomb rod-like eutectic. In some regions they appear in an irregular lamellar form.During solution treatment at 360℃, theα,ηandεphases transform into the uniqueβphase particles for the thixoforming and permanent mold casting alloys. Theα′phase andβphase all experience a similar process process: lamellae→discontinuous lamellae→dot-like form→uniform grey structure. The eutectic structures change from the original net shape to discontinuous and coarse net shape and finally disappear. The concentrations of Al and Zn in theβphase is about 28.2﹪,68.5﹪respectively when it reaches the final equilibrium state. The microstrcture of permanent mold casting ZA27 alloy changes in turn of small dendrites, coarse dendrites, petal-like microstructure, dot-likeηphase distributed in gray matrix and uniform grey structure.With the increase in solution time, the hardness values of the thixoromed and permanent mold casting ZA27 alloys all first decrease, then sharply increase and finally tend to a mild way. When solutionzed for 24 h, the heardness is up to a peak value and the hardness begain decreasing when the matrix become into a unique saturatedβphase. The hardness of the thxiformed alloy reaches the minimun value when heated for 12 while that of the permnant mould casting alloy reaches its minimun value when heated for 30min. As the solution temperature increases, the minimun hardness increases as the sequence of the alloys heated at 340℃, 360℃and 380℃, and the needed time decreases in the same sequence. The needed time to reach the maximum values is all approximately 24h. During aging at 150℃, the amounts of precipitatedαandηphase of the thixoformed alloy increase as the ageing time increases and the microstrcuture morphology experiences this process, from the granular form to coexistence of lump and lamellae and lamellar structures which spread towards the whole grains. The permanent mould casting alloy has a similar process to the thixoformed alloy. The hardness vaule of the thixoformed alloy first sharply decreases and finally tends to a constant. The minumum hardness values of the thixoformed alloys are always higher than those of the permanent mould casting alloys. The amount and size of the precipitated products increase with rising the ageing temperature. The time that the hardness reaches minimum value decreases as the the temperature rises.
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