Study On Microstructure And Mechanical Properties Of Ultrafine CoCrW Alloy

CoCrW alloys are widely applied to industries whose operating conditions are harsh and which require extremely high performance of materials, such as the nuclear power industry, gas turbine industry, aerospace industry and the medical field.Despite the numerous advantages of CoCrW alloys, their ductility and toughness are poor, and because of that, CoCrW alloys are used only in the as-casted state for a long term. However, with the development of the industries, the performance of the as-casted CoCrW alloys can’t meet the needs of some operating conditions. New molding means need to be developed urgently to improve the properties of CoCrW alloys. The wrought CoCrW alloys have already been developed abroad, such as Stellite 6B CoCrW alloy. But only components of simple shape can be wroght, and the subsequent processing costs are too high due to the high hardness and brittleness of CoCrW alloy. So the wrought means are not ideal. In recent years, researchers used the means of powder metallurgy to mold CoCrW alloys, and found that CoCrW alloys molded by powder metallurgy had excellent properties. For the further research of the microstructure and mechanical properties of the CoCrW alloy molded by powder metallurgy, this work used a hot vacuum pressing way to mold CoCrW alloys, and studied the microstructure and mechanical properties of them.Based on the DSC data of CoCrW alloys powder, a vacuum hot pressing process were developed. CoCrW alloys were formed by the process using the homemade vacuum hot pressing furnace. In order to improve the mechanical properties of as-hot pressed alloy CoCrW alloys, the solid solution treatments were made on the basis of the DSC data of the as-hot pressed CoCrW alloys. To study the microstructure and properties of the vacuum hot pressing CoCrW alloys, CoCrW alloys were molded by the casting means too.The phases of CoCrW alloys were studied by SEM, EDS, XRD and TEM, and the changes of CoCrW alloys’ microstructure were researched too. A comparison of the microstructure of vacuum hot pressing CoCrW alloys and casting CoCrW alloys was made. The changes of the mechanical properties of the as-hot pressed, as-solid solutioned and as-casted CoCrW alloys were studied by hardness test, room temperature tensile test and ball-on-disc reciprocating wear test. It turns out that both the as-hot pressed and as-solid solutioned CoCrW alloys have the same kinds of phases: M23C6, M12 C, Cr Co and g-Co matrix. The phases of as-casted CoCrW alloys are M23C6, M12 C and g-Co matrix. The proportion of M23C6 and prior particle boundaries decrease remarkably after solid solution treatment. It also resultes in coarsening of individual phases and solid solution strengthening of the matrix. The ductility and wear resistance of CoCrW alloys improve after solid solution treatments. The hardness will increase by solid solution treatments of appropriate temperature and time, but it will decrease by solid solution treatments of too high temperature and too long time. The ultimate tensile strength of CoCrW alloys increases first and then decreases with the increase of solid solution temperature and time. The ductility and ultimate tensile strength of as-casted CoCrW alloys are better than that of as-hot pressed CoCrW alloys. But the ductility and ultimate tensile strength of as-solid solutioned CoCrW alloys which go through proper solid solution treatments are superior to that of as-casted CoCrW alloys. The hardness and wear resistance of as-hot pressed CoCrW alloys are superior to that of as-casted CoCrW alloys.