Effect Of Cryogenic Treatment On Two Kinds Of As-cast Magnesium Alloys

Deep cryogenic treatment(DCT), as a process to change the microstructure and improve the properties of materials, has been widely used in tool and die steel, high speed steel, cemented carbide etc. However, the research in non-ferrous metals, especially about magnesium alloy is less. Magnesium alloys are being increasingly studied and widely used due to their low density, high specific strength and stiffness, superior damping capability, good electromagnetic shielding characteristics and easy recycled advantages. It is a new method to adopt DCT and cycle heat treatment in the field of Magnesium alloys. Therefore, the research of the effect of DCT on Magnesium alloys is of significance to both theory and practice aspects.In this paper, effect of DCT and cycle heat treatment on ZK60 and AZ91 as-cast magnesium alloys has been investigated. The specimens were analyzed by metallographic examination, X-ray diffraction, transmission electron microscope (TEM) and microhardness, tensile test. The mechanism of DCT was expored via observing and comparing the change of propertiy and the transformation of microstructure before and after DCT. The results showed:(1) To ZK60, hardness of the samples treated by cryogenic environment were higher than that of untreated samples. It was also worth noting that with the soaking time increasing, mechanical properties were not increased linearly. DCT caused the evolution of microstructure. The TEM analysis showed there were several shapes of precipitates in grain. One was rod-likeβ1′and another one was disc-shapedβ2′. After cycle heat treatment, the microhardness was improved more, after DCT16h+T6 treated, the hardness of ZK60 reached to 87.78, increased by more than 30%. Both the amount and kind of precipitates in ZK60 were also increased. The DCT acted as pre-aging process in cycle heat treatment.(2) The situation of hardness transformation in AZ91 was similar as in ZK60 after DCT. The execution of DCT on samples changed the distribution ofβprecipitates. The coarse divorced eutecticβphase penetrated into the matrix. After cycle heat treatment, the density ofβprecipitates increased observably. This microstructural modification resulted in a significant improvement on mechanical properties of the alloy. The hardness of AZ91 sample treated as DCT24h+T6 proceess reached to 108.80, increased by 85.54%. (3) The evolution of grain orientation from basal orientation plane (0002) to other pyramidal planes just as (1011), (1012) was found in both alloys.