Materials Science Forum Vols. 539-543

Abstract: In the high temperature deformation window of the nickel base Alloy 80A the lower temperature region during open die forging was examined with regard to the materials formability. For that purpose, hot compression samples were investigated by means of EBSD and TEM in order to look at recrystallization, precipitations and ductile damage as well as their reciprocal effects. Further a microstructure model was used, which calculates the materials strengthening, softening and the particle kinetics. A micro mechanical damage model of the effective stresses was coupled with the grain structure development in order to describe a retarded damage rate due to the ongoing recrystallization.

Abstract: The effects of nickel content and heat treatment conditions on the creep strength of precipitation-strengthened 15Cr ferritic steel were investigated. The creep strength of the 15Cr ferritic steel was drastically improved by solution treatment and water quenching. However, over the long term, the detrimental effect of nickel on the creep strength was pronounced for water-quenched steels. The volume fraction of martensite phase increased with increased nickel content in both the furnace-cooled and water-quenched steels. The volume fraction of martensite phase in the water-quenched steel was smaller than that in the furnace-cooled type, even for the same nickel content. Fine particles, smaller than 500 nm, were precipitated homogeneously within the ferrite phase of the water-quenched steel. On the other hand, coarse block-like particles 1 $m in size were precipitated sparsely within the martensite phase. The creep strength of the steels decreased with increased volume fraction of the martensite phase caused by furnace cooling and nickel addition. The lower creep strength and microstructural stability of the martensite phase is attributable to less precipitation strengthening. To enable this steel to be put to practical use, it will be necessary to suppress the formation of the martensite phase caused by addition of nickel by optimizing the chemical composition and heat treatment conditions.

Abstract: The precipitation site, main metallic composition and number density of Z phase have been investigated in T91 in order to clarify the influence of Z phase formation on recovery of martensitic structure and creep strength degradation. The Z phase particles were mainly present around prior austenite grain boundaries and/or packet boundaries in the steels crept at 550oC and 600oC. The Z phase particles were found in specimens crept at 550oC to 650oC. There was no indication of Z phase formation up to about 62475.0 h at 500oC and 14106.5 h at 700oC. The Nb content of Z phase observed at 550oC was lower than that at 600oC. The number density of Z phase measured at 550oC was lower that that at 600oC, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550oC in contrast with 600oC.

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.

Abstract: Alloy design concept for the development of a new class of austenitic heat resistant steels strengthened by Fe2M Laves phases (M: transition metals) has been proposed. The phase diagram studies on Fe-Ni-M ternary systems demonstrate that Fe2Nb with C14 structure is the most promising, because more than 40at% Ni can dissolve into the Fe sublattice sites and large γ+Fe2Nb two-phase region exists along the equi-niobium concentration direction. The control of the c/a ratio of the Laves phase using the composition homogeneity region by alloying makes it possible to disperse the Laves phase finely in the γ matrix. Based on the knowledge, a model alloy Fe-20Cr- 30Ni-2Nb (at%) was proposed and the TTP diagram of the Laves phase was constructed. The Laves phase homogeneously nucleates in the matrix and its fine morphology remains almost unchanged even after long-term aging at 1073K.

Abstract: The platelike bainitic ferrite growth rates were calculated by two modified diffusional models. Good agreements between experimental and theoretical results are found in Fe-0.59C wt.%, Fe-0.81C wt.% and Fe-0.478C-4.87Ni wt.% alloys. A slowing down effect due to the alloying element Mo is emphasized in Fe-0.69C-1.8Ni-0.8Mo wt.% alloy. However, the experimental data are lower than theoretical ones about two orders in Fe-C-8.7Ni wt.% alloys. According to the discussion of the results, it is suggested that the bainite transformation mechanism may relates to steel composition and transformation temperature.

Abstract: It has been elucidated that the small creep strain must be essential to form the rafted γ/γ' structure using a single crystal nickel-based superalloy, CMSX-4. To confirm the amount of the strain required to make rafting, the single crystals interrupted the creep tests up to a strain of 0.01 at 1273K-250MPa was aged at 1273K without stress. To compare the difference in the morphology of the γ/γ' structure of the as-heat treated and the creep-interrupted single crystal with the simple aging, the as-heat treated single crystal was also aged. Microstructural observations by SEM were conducted for the specimens sectioned parallel to (100). The cuboidal γ' precipitates of the as-heat treated single crystal connected each other with three <100> directions after the simple aging. By employing the aging without stress on the specimen interrupted the creep test at a strain of 0.0074, the drastic morphological change in the γ/γ' structure was confirmed, that is, the γ/γ' structure changed to rafted one perpendicular to the creep-interrupted stress axis. The aspect ratios of the creep-interrupted specimens increased with increasing simple aging time, and attained to the maximum value at the simple aging time of 3.60x106s. The maximum value of the aspect ratios increase with increasing creep-interrupted strain and attained 4 at a strain of 0.0074. The dislocation density at the γ/γ' interface increases with increasing creep strain. Consequently, the formation of the rafted γ/γ' structure requires the dislocation substructure induced by the creep strain.

Abstract: Through the analysis of many creep rate-strain curves of γ-single phase Ni-20mass%Cr alloy single crystals with various stress axes, it has been elucidated that the ratio of transient stage to rupture life becomes larger with decreasing the stress. And the transient stage consists of Stage I and Stage II. In Stage I, the creep rate just after loading remains constant, and in Stage II, a steep decrease in creep rate continues. It is noticeable that there is a marked difference in transient stage among single crystals with different stress axes. The aim of this study is to elucidate the mechanisms leading to the different transient stages as the function of stress axes. The deformation during transient stage in the single crystals except for the single crystals with the stress axes of the [001] and [1,–11] poles in the standard stereographic triangle, proceeds using the primary slip plane. And they are divided into two groups of the single crystals with the angle between stress axis and primary slip plane, θ, less than 45° and the single crystals with θ more than 45°. The deformations of Stage I and Stage II in these single crystals proceed using the slip system of (111)<1,–01> and the slip system of (111)<1,–10>, and in Stage I, the former slip system acts mainly except for that of single crystals with stress axis of [011]. While, in the single crystal with stress axis of [011], two slip systems above described operate at the beginning of Stage I, and the stress axis moves along [011]-[1,–11] line. And this moving gives slight increase in the Schmid factor, therefore, in Stage I slight increase in creep rate was confirmed. The {111} pole figure of the single crystal with stress axis of [1,–11] whose deformation proceeds using the plural slip planes are obtained by SEM-EBSD method. It becomes clear that the smallest strains of Stage I and Stage II derived from the increase in the torsion with creep deformation.

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.