Papers by Keyword: Fe-32%Ni Alloy

Abstract: The effect of strain on the microstructure evolution of Fe-32%Ni alloy during multi-axial forging at the temperature of 500°C and a strain rate of 210-2 s-1 was investigated by optical microscope (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scatter diffraction (EBSD) observations. The results show that the austenite grains were greatly refined with increasing cumulative strain, and the microstructure evolution during multi-axial forging can be summarized as such a process that deformation bands crossing each other subdivide the original austenite grain into several sub-grains and then these sub-grains are subdivided into more small ones and gradually angled to new independent grains with their boundaries transformed into large angle boundaries in subsequent compression.

Abstract: Hot deformation behavior and microstructure evolution of Fe-32%Ni alloy were investigated when compressed at the temperature of 1000°C and a strain rate of 2×10^-3s^-1. The microstructures were analyzed using optical microscope (OM), electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). The results show that the generation and development of dynamic recrystallization (DRX) can obviously refine the grains of Fe-32Ni% alloy and the DRX reached dynamic equilibrium when the strain was high. According to the TEM observations, the DRX microstructure can be categorized into three kinds: grains with low dislocation density, which are DRX nucleations; grains with low dislocation density around the grain boundary and high dislocation density in its interior which means that grains with dislocation density gradient and which are DRX grains in growth; grains with high dislocation density, which are fully work-hardened DRX grains.

Abstract: The microstructure evolution during annealing in large strain deformed Fe-32%Ni alloy was investigated by transmission electron microscope (TEM). Firstly, the Fe-32%Ni alloy was subjected to multi-axial forging at temperature of 773K and a strain rate of 10-3 s-1 to cumulative strain of 9.0, and then the large strain deformed specimens were annealed at temperature of 973K with different time. The results show that the grains of Fe-32%Ni alloy were obviously refined by severe plastic deformation, and the ultra-fine grains with mean size of about 300nm were obtained when the cumulative strain amounted to 9.0. The large strain deformed microstructure evolves homogeneously and gradually to equiaxed structures with straight and smooth grain boundaries when annealed at temperature of 973K, and there is no observable nucleation stage found during annealing. The annealing process involves two sequential processes i.e. recovery followed by normal grain growth, and the microstructure evolution mechanism is considered as continuous recrystallization.

Abstract: The microstructure evolution taking place in Fe-32%Ni alloy during multi-axial forging was investigated by electron backscattered diffraction (EBSD). The samples were compressed with loading direction changed through 90º from pass to pass at temperature of 650°C and a strain rate of 10-1/s. The results show the microstructure evolution is characterized by continuous grain subdivision process, i.e. the multi-axial forging promotes the development of deformation bands in various direction followed by their frequent intersection in grain interiors with changing of strain path, which results in continuous fragmentation of coarse grains into subgrains. Concurrently the misorientations of subgrain boundaries rise gradually with repetitive deformation followed by their progressive transformation into high angle boundaries. The ultra-fine grains are concluded to evolve by continuous dynamic recrystallization (CDRX).

Abstract: The Fe-32%Ni alloy was multi-axially forged at the temperature of 873K and strain rate of 10-2s-1, then the microstructure evolution in Fe-32%Ni alloy during deformation was investigated by the transmission electron microscopy (TEM). The results show that the grain size decreases with strain. The severe plastic deformed microstructure is characterized by the ultra-fine equiaxed grains and high internal stresses. The microstructure evolution mechanism is presented as the following: firstly, the dislocations accumulate as deformation bands in some directions with the progress of deformation; then the cellular structured subgrains are formed by continuous intersecting of deformation bands for the changing of strain path; eventually, the ultra-fine structured grains are formed by the subgrains rotation and the dislocations rearrangement.

Abstract: Recrystallization behaviors have been investigated with respect to two different kinds of the initial structures, original austenite and martensite, in an Fe-32%Ni alloy. The recrystallized austenite grain size from the martensite is much smaller than that from the original austenite, and decreases linearly with increasing the initial hardness, independent of the initial structure. The recrystallization sequences are different between the two structures: only one step due to recrystallization appears in hardness-temperature curve of the original austenite, whereas two steps corresponding to reverse transformation of α’ to r’ and recrystallization are shown in that of the martensite.