Tomokazu Moritani

Substructure and crystallography of degenerate pearlite in an Fe-C binary alloy

Microstructures formed by degenerate pearlite transformation in an Fe-0.38mass%C alloy were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Degenerate pearlite which contains fine cementite particles even at the growth front was observed with other structures such as proeutectoid ferrite, lamellar pearlite and bainite in a temperature range between 773K and 923K. As the isothermal transformation temperature is lowered, a fraction of the degenerate pearlite increases. The degenerate pearlite consists of ‘block’ (a region in which ferrite orientations are nearly the same) and ‘colony’ (a region containing cementite particles of nearly the same orientation), both of which are similar to those in lamellar pearlite. Block boundaries within an austenite grain are generally of high-angle type and their misorientations deviate largely from intervariant relationships for the K-S orientation relationship. In contrast, colony boundaries are of low-angle type. Cementite films are formed along those ferrite boundaries in the degenerate pearlite, presumably formed by encounter of the blocks or colonies.

TEM Observations of Two-Phase Microstructure Formed by Phase Separation of Gamma-Prime Precipitates in Ni-Al-Si Alloys

The phase-separation behaviour of γ’ precipitates in Ni-7.1Al-6.7Si alloy was investigated by means of transmission electron microscopy (TEM). When the alloy is aged at 1173K, coherent spherical γ’ particles having ordered L12 structure appear in γ matrix having disordered A1 structure. When the two-phase microstructure of γ + γ’ is aged at 973K, spherical γ particles precipitate in the individual γ’ precipitates. In the course of ageing at 973K, the new γ particles grow keeping the spherical shape, their number gradually decreases and finally γ particles aging at 1173K gradually change their shape from sphere to cuboid, but do not practically change their size, i.e. such phase-separation behaviour brings the decelerated growth of γ’ precipitates.

TEM Observations of the Precipitation of A2 Particles in D03 Precipitates in Fe-Si-V Alloy System

The present study examined the effects of heat treatment and the addition of Cu-Ni alloy on the corrosion resistance of the matrix of spheroidal graphite cast iron in aqueous environments. Test materials of white cast iron and carbon steel were used for comparison with spheroidal graphite cast iron. The alloy spheroidal graphite cast iron that added Cu and Ni was prepared. The spheroidal graphite cast iron was subjected to three kinds of heat treatment to adjust the matrix: annealing, oil quenching, and austemper heat treatment. In electrochemical tests, measurements of corrosion electrode potential and cathode and anode polarization were used. The following was clarified from the relationship between the electrode potential and current density of each of the materials in each of the solution. The alloy spheroidal graphite cast iron had a high corrosion electrode potential owing to the addition of Cu-Ni, and tended to have a low corrosion current density. This demonstrates that in any of the materials having a matrix adjusted by heat treatment, the addition of Cu-Ni increased the corrosion resistance. The corrosion current density was highest in a sulfuric acid environment.

Precipitation of gamma-prime and gamma-double prime coherent phases in three-phase Ni-V-Si alloys

Phase separation of γ (A1) supersaturated solid solution into A1, γ’ (L12) and γ” (D022) phases was investigated in two Ni-rich Ni-V-Si ternary alloys by means of transmission electron microscopy. When the alloys are annealed at 1073K, two different sequences of the phase separation are observed, depending on the chemical composition of the alloy: In Ni-17.0at%V-6.9at%Si alloy (A) at the D022 corner of three-phase field, first many D022 particles precipitate aligning along the <110> direction of the matrix and the so-called chessboard pattern is observed, followed by the formation of L12 phase at the interface between D022 and A1 phases. In Ni-12.1at%V-11.3at%Si alloy (B) at the L12 corner of the Gibbs triangle, cuboidal L12 particles precipitate arranging along the <100> direction, and then D022 phase is formed. As the phase separation proceeds, a selective growth/formation of the third phase (L12 in the alloy A, D022 in the alloy B) occurs: In the alloy A, L12 phase grows into D022 particle inside along the diagonal direction of D022 cube which is parallel to the a-axis of D022 tetragonal phase. In the alloy B, D022 forms on the {100} cube face of cuboidal L12 particle, arranging the c-axis of D022 perpendicular to the {100} cube face of L12 phase. As a result of such a selective growth/formation, the first phase D022/L12 is split off into two particles, which results in the formation of laminated structure consisting of D022 and L12 phases. The selective growth/formation is considered to occur so as to maintain the less elastic strain state.

Two-Phase Microstructures of Gamma+Gamma-Prime Formed by Phase-Separations due to Heat-Treatments of Elastically Constrained Ni-Al-Ti Alloys

In the elastically constrained Ni-Al-Ti alloy system, three kinds of phase-separations, i.e. microstructure changes, take place to bring the two-phase state of γ+γ’ depending on the alloy compositions and heat treatments: 1) in Ni-8at%Al-6at%Ti, the phase-separation of γ phase takes place and γ’ particles appear in the γ matrix, 2) in Ni-13at%Al-9at%Ti, the phase-separation of γ’ intermetallic phase takes place and γ particles appear in the γ’ matrix, 3) in Ni-8.5at%Al-5.4at%Ti, the phase-separation of γ’ precipitate phase takes place and γ particles appear in the γ’ precipitate.

Transmission Electron Microscopy (TEM) Observations of Phase-Separations of Gamma-Prime Precipitates in Ni-Al-Fe and Ni-Si-Fe Ternary Alloys

In Ni-13.0at%Si-3.1at%Fe alloy, when γ/γ’ two-phase microstructure formed at 1123 K is isothermally heated at 923 K which is lower than the temperature where the initial γ/γ’ microstructure forms, the phase-separation of γ/γ’ precipitate phase occurs and γ particles newly appear in each cuboidal γ’ precipitate. While in Ni-10.2at%Al-10.8at%Fe alloy, when γ/γ’ two-phase microstructure formed at 1023 K is isothermally heated at 1123 K which is higher than the temperature where the initial γ/γ’ microstructure forms, the phase-separation of γ’ precipitate phase takes place and γ particles newly appear in each cuboidal γ’ precipitate. Such appearance of new γ particles in γ’ precipitates can be explained by the difference in the volume fraction of γ phase that should exist in the γ/γ’ two-phase system depending on the heating temperature.

Microstructural Analysis of Butterfly-Type Martensite by Using EBSD

Electron backscatter diffraction (EBSD) is effective method to investigate crystal orientation distribution in materials. Especially, EBSD has given a lot of novel knowledge for phase transformation of Fe and Fe alloys. In this study, microstructural analysis of martensite (α’) with butterfly shape in Fe-Ni alloys are performed by conventional EBSD observation and in-situ EBSD observation during tensile test. By using EBSD, it is found that crystal orientation in the butterfly-type α’ is locally changed. Moreover, the butterfly-type α’ has both features of lath and lenticular α’. This comes from the change of accommodation process for shape strain of the butterfly-type α’ during its growth. Furthermore, the tensile deformation influences the change of accommodation process for shape strain of the butterfly-type α’. From the obtained results, formation process of the butterfly-type α’ is discussed.

Microstructural Evolution of Martensite Phase in Fe-Ni Alloy Depending on Temperature of Tensile Deformation

Martensite (α’) formed in Fe-Ni alloy is mainly separated into four morphologies (thin plate, lenticular, butterfly and lath). The morphology of the α’ depends on its formation temperature because accommodation process for transformation strain in the α’ is changed with temperature. Also, crystal orientation relationship between α’ and austenite (γ) is depended on the morphology of the α’. In this study, temperature dependence of the microstructural evolution of the butterfly-type α’ in Fe-30mass%Ni alloy by tensile deformation is investigated. When the tensile test is performed at room temperature (RT), morphology of the butterfly-type gets close to lath-type α’. On the other hand, the morphology of the butterfly-type α’ is kept under the tensile test at lower temperature. These microstructural changes of the butterfly-type α’ depending on the tensile temperature are discussed in term of the change of the accommodation process for the transformation strain.

In Situ Scanning Electron Microscopy Observation of the Pattern Formation on the Surface of Al/Ge Bilayer Films

When the Al/Ge/SiO2 bilayer films are annealed in-situ in a scanning electron microscope (SEM) at the temperatures lower than the crystallization temperature of amorphous Ge itself, the so-called metal-mediated-crystallization (MMC) takes place. In the course of MMC, crystalline Ge aggregates (Ge clusters) form in the bilayer films, which results in the formation and the evolution of impressive fractal patterns with branching on the free surface. In-situ SEM observations of annealed Al/Ge/SiO2 bilayer films indicate that the grain size of polycrystalline Al-layer influences the nucleation of Ge clusters and hence of fractal patterns. For the bilayer films containing larger Al grains, the nucleation rate of fractal patterns (Ge clusters) is faster and the number of patterns is larger.

Two-Phase Microstructures Formed by Phase-Separations of Ordered Precipitates in Fe-Si-V Alloys

Phase-separation of D03 precipitates in A2 matrix of Fe-Si-V alloys was investigated with TEM. When Fe-14.5at%Si-12.9at%V alloy is aged at 873 K, the phase-separation of cuboidal D03 precipitates occurs and A2 particles newly appear in each D03 cuboid. The A2 particles grow to become plates, then the A2 plates elongate along {100} to reach the A2 matrix, and finally the split of D03 cuboid is realized to form smaller cuboids. When Fe-15.5at%Si-5.0at%V alloy is aged at 873 K, the phase-separation of rod-shaped D03 precipitates occurs and A2 particles newly appear in each D03 rod. The A2 particles elongate along the long axis of D03 rod to reach the A2 matrix, and the split of D03 rod is realized to form thinner rods. The split in each alloy brings the refinement of two-phase microstructure, which is a result of not only the elastic energies but also the chemical free energy.