Study On Phase Transformation And Thermophysical Properties Of High Aluminum Zn-Based Alloys

As a new kind of alloy, high Aluminum Zn-based alloys have found wide use in wear-resisting machinery in recent years. Not only does it feature a high tensile strength, high hardness, wear-resisting etc., but it is also characteristic of excellent processing properties and low energy consumption, no pollution in melting as well. Due to all of these advantages, this type of alloy has drawn many attentions of researchers in many countries.Solid transformation of high Aluminum Zn-based alloy has an important influence on its properties; so, the mechnism of the transformation has being investigated by researchers as a key subject. However, most of the researches were focused on alloys with aluminum content lower than 30%. Few researches have dealed with the alloys with aluminum content more than 30%. In the present research, a systematic study on solid transformation and thermophysical properties of alloy with aluminum content higher than 30% has been carried out. By means of metallographic, high temperature metallographic analysis, scanning electron microscope, thermal ananysis and thermal expansion experiment etc., some new phenomena have been observed as for solid transformation of the alloys. The main conclusions are:(1) The dymatic process of eutectoid transformation is observed continuously though high-temperature microscope for the first time.(2) The as-cast microstructure of high Aluminum Zn-based alloy consists of white a phase which takes on dendrite morphology, B phase which appears around a phase,and phase and eutectic.(3) The eutectoid transformation temperature of five alloys is identical , being about 285C. And ZA50's heat of eutectoid transformation is the lowest. With the increasing of aluminum content, the heat of eutectoid transformation reduces, while the melting point of alloy rises. The average heat of fusion of five testing alloys remains about 170 kJ/kg, so high Aluminum Zn-based alloy consume less energy in their melting.(4) As the temperature rises, the alloy's coefficient of thermal expansion and heat capacity also go up. With the increasing of Aluminum content, the number of a phase increase, and its coefficient of thermal expansion drops. Under every temperature, the coefficient of thermal expansion of ZA45 turns to be the smallest, and the heat capacity of ZA40 turns to be the biggest.(5) Through continual observations to different microstructure of ZA30 under isothermal heat treatment at the temperature 100C. It seems that spinnodal and celluar transformation have happened during ageing stage.