Authors: Junji Yamanaka, Chiaya Yamamoto, Yasuhiro Kuno, Minoru Doi
Abstract: We have been studying the microstructure change of B2 cubic precipitates into an A2+B2 complex structure in Fe-Al-Ni alloy. In this study, we carried out detailed observation using focused ion beam (FIB) and scanning transmission electron microscopy (STEM). First, Fe-14.3at%Al-10.3at%Ni solid solution was prepared. Secondly, the specimens were heated at 1173 K, at which they formed B2 cubic precipitates (ordered bcc) dispersed in an A2 matrix (disordered bcc). After that, the B2/A2 two-phase specimen was annealed at 973 K. Then we fabricated STEM specimens using FIB, followed by high-resolution secondary electron imaging. We repeated this slice-and-observation procedure to determine the detailed microstructure of this heat-treated alloy. At the early stage of the 973 K annealing, the A2 phase appeared in the original B2 precipitates and showed a spongelike structure, whereas small nanometer-order B2 particles appeared in the A2 matrix. The A2/B2 interface at this stage showed no anisotropic morphology. Therefore, the main driving force of this process may not be strain energy, but chemical and interface energies. Further annealing at 973 K decreased the number of small B2 particles in the A2 matrix, and these particles dissolved into the matrix eventually. The annealing also changed the A2/B2 spongelike structure, which was observed in the original B2 precipitates, into simple structures such as the A2 core and B2 crust. Then the B2 phase showed ordinal coarsening behavior. When B2 precipitates, which had hollow cubic morphology, were observed to be very close to each other, the face-centered area of the B2 crust tended to dissolve and only large B2 precipitates remained.
2496
Authors: Yasuhiro Kuno, Yasuo Nakane, Takao Kozakai, Minoru Doi, Junji Yamanaka, Chiaya Yamamoto, Shizuo Naito
Abstract: When Fe-10.3mol%Ni-14.3mol%Al alloy is heated at 1173 K for 8.64104 s, a number of B2 precipitates are dispersed in the A2 matrix. When the two-phase microstructure of A2+B2 is aged at 973 K, the phase-separation of B2 precipitate particles takes place to form a new A2 phase in each B2 particle. In the course of further ageing at 973 K, the new A2 phase grows but decreases in number, and finally only one A2 particle is left in the individual B2 particles. The appearance of new A2 phase in each B2 precipitate is due to the difference in the volume fraction of A2 phase that should exist in A2+B2 two-phase system depending on the heating temperature: i.e., the phase-separation of B2 precipitates starts with the aid of chemical free energy.
2274
Abstract: Coherent two-phase microstructures consisting of ordered precipitate and disordered matrix phases sometimes exhibit a phase-separation, which brings the split and/or the decelerated coarsening of precipitates. When the coherent two-phase microstructure of A1+L12 (+’) in Ni-base alloys are aged inside the two-phase region of A1+L12 , the L12 precipitate sometimes exhibit a phase-separation and A1 phase newly appears and grows in each L12 precipitate. Phase-separations of the same type to the above also take place due to ageing of coherent two-phase microstructures of A2+D03 and A2+B2 in Fe-base alloys: D03 and B2 precipitates sometimes exhibit phase-separations and A2 phase newly appears and grows in both precipitates. These types of phase-separation take place under the influence of chemical free energy. In the course of further ageing, the new disordered phases of A1 and A2 change their morphology in various ways depending on the elastic constraint: i.e. the morphology of new A1 or A2 phase is influenced by the elastic energies and the surface energy.
2215
Authors: M. Senga, H. Kumagai, Tomokazu Moritani, Minoru Doi
Abstract: 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.
1311
Authors: Yusuke Inuzuka, Shogo Ito, Takao Kozakai, Minoru Doi
Abstract: Phase separations of A1 (γ) supersaturated solid solution into γ, cubic-L12 (γ') and
tetragonal-D022 (γ") phases were investigated in Ni-V-Ge alloys by means of transmission electron
microscopy (TEM). When Ni-15.8at%V-9.0at%Ge alloy is aged at 1073 K, at the early stage of
ageing γ" phases are observed in the γ matrix as triangle- or diamond-shaped precipitates. With
further ageing, colonies of lamellar structure consisting of two variants of γ" phase are dispersedly
formed in the γ matrix and then γ" plates grow along the <110> direction. When
Ni-14.5at%V-8.8.at%Ge alloy is isothermally annealed at 1023 K, first spherical γ' particles
precipitate homogeneously in the γ matrix, followed by the formation of lamellar structure of γ"
phases. In the course of further ageing, the lamellar structure develops, and only γ' particles around
the lamellar structure grow and others gradually fade out.
2365
Authors: Tomokazu Moritani, Masahiro Ota, Takao Kozakai, Minoru Doi
Abstract: 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.
2361
Authors: Minoru Doi, Tomokazu Moritani, Takao Kozakai
Abstract: 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.
2357
Authors: H. Kumagai, M. Shibata, Tomokazu Moritani, Takao Kozakai, Minoru Doi, Makoto Takagi, Toru Imura
Abstract: 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.
3568
Authors: Takao Kozakai, C. Aoki, Tomokazu Moritani, Minoru Doi
Abstract: 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.
3036
Authors: Minoru Doi, Takao Kozakai, Tomokazu Moritani, Shizuo Naito
Abstract: 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.
3006