| Magnesium alloys have bright future in aviation, aerospace, automotive industry for their advantages such as low density, high specific strength and so on. While the application of conventional magnesium alloys such as AZ and AM series have always been limited due to their low creep resistance.The addition of rare earth to magnesium alloys can greatly improve creep resistance. Because the heavy rare earth elements such as Nd, Dy, Ho and so on are being extensively used in functional materials, leading to the resource depletion. However, the mish metals with high abundance containing cerium or lanthanum are surplus. So it is a reasonable way to replace expensive pure rare earth elements with surplus mish metals, in order to ensure high temperature creep resistant properties of magnesium alloys. What is more important is that can balance the consumption of rare earth resources of domestic.In this paper, the study object was WE93 (Mg-9Y-3MM-0.6Zr) alloy which was added with mish metal. The reasonable way of heat treatment of this alloy was achieved through the study of microstructures and performances under the states of as-cast, homogenized and as-aged. The creep behavior and the creep mechanism of the extruded and aged alloy at 150℃～250℃under different stresses were also discussed. In addition, the creep resistant property of WE93 and high cost EW75 (Mg-7Gd-5Y-1Nd-0.5Zr) alloy were compared. Some main contents were as follows:The microstructures of as-cast and homogenized of WE93 alloy were studied, obtaining the suitable homogenization way. The microstructures of as-cast of WE93 alloy were consisted of a-Mg, Mg12 (MM) and Mg24Y5 with an average grain size of 45μm, the eutectic structure was discontinuously distributed along the grain boundaries. After homogenization, the Mg-Y phase dissolved completely and the Mg-MM phase was found at the grain boundaries. With the homogenization time increasing, the grains did not grow up obviously. The suitable homogenization treatment methods of WE93 alloy was 535℃×18h.The influence of aging treatment to WE93 alloy as extruded was also studied. The mechanical properties of the extruded alloy (extrusion ratio= 20) were as follows:σb= 305MPa,σ0.2=245MPa andδ=12.5%. After aged, the mechanical properties changed dramatically. The reasonable aging treatment technology of WE93 alloy was:225℃×10h withσb,σ0.2,δrespectively were 385 MPa,315MPa and 6.5%.The effects of temperature and stress on the creep resistance of WE93 alloy as peak aged were analysised. Under the stress of 100～150MPa, the influence of temperature to the creep resistance of WE93 alloy was significantly. When at 150℃, the alloy did not show obvious creep phenomenon, while the temperature increasing to 200℃, the steady creep rates were 6.44×10-8～2.12×10-8 s-1. While at 250℃, the steady creep rates were 1.69×10-8～4.36×10-7 s-1 with the stress varing from 50MPa to 100MPa, the alloy's steady creep rates increased rapidly when the stress beyond 50MPa.At 200℃the creep stress exponent n was 2.97 with the stress varing from 100MPa to 150MPa and the creep activation energy was 175010J/mol, suggesting that the main creep mechanism of the alloy was dislocation's movement which was blocked by precipitation. The creep rate formula of WE93 alloy is:εs= 2.65×10-13σ2.97exp (-175010/RT).The creep resistance comparison test showed the steady creep rates of WE93 alloy were both lower than EW75 alloy when at 200℃and 250℃with the stress respectively varying from 100-150MP and 50-100MPa.
|
PDF Full Text Request