H. Harada

Rh-base refractory superalloys for ultra-high temperature use

In the previous paper, the authors proposed a new class of superalloys, namely, refractory superalloys. This new concept is defined as alloys with fcc and L1{sub 2} coherent two phase structures similar to Ni-base superalloys, and yet with considerably higher melting temperatures. In this paper, Rh-Nb and Rh-Ti systems were selected to compare with Ir-Nb and Ir-Ti systems which were shown in the previous paper. Rh-Ta system was also selected because of its highest melting temperatures among above binary systems. The microstructure evolution and high temperature strengths of these Rh-base alloys were investigated.

The Origin of the Difference in the Mechanical Strengths of Czochralski-Grown Silicon and Float-Zone-Grown Silicon

The mobility of individual dislocations was measured by means of in-situ observations, including the use of X-ray topography, for both Czochralski- and float-zone silicon crystals. No difference in mobilities was found between the two types of crystals. Stress-strain characteristics were also measured for both types of crystals. On the basis of observed facts, it is concluded That the difference in the mechanical strengths of the two types of silicon crystals is associated with the locking effect of dislocations by oxygen atoms.

Microstructure dependence of strength of Ir-base refractory superalloys

Abstract Precipitation hardening was investigated in Ir–Nb and Ir–Zr alloys with a two-phase structure consisting of the fcc matrix and L1 2 coherent precipitate phases, similar to that in Ni-base superalloys. Cuboidal L1 2 precipitates and plate-like L1 2 precipitates were formed with coherent interfaces in the fcc matrix in the Ir–Nb and Ir–Zr alloys, respectively. Effects of precipitate shape and coherency strains on precipitation hardening are discussed in terms of lattice misfit. Plate-like precipitates forming a 3-dimensional maze structure in the Ir–Zr alloys were profitable to precipitation hardening in both factors, that is precipitate shape and coherency strains.

NiTi-base intermetallic alloys strengthened by Al substitution

Abstract A series of NiTi-base alloys with Al additions substituting the Ti were designed and evaluated in terms of the microstructure and mechanical properties. It was found that the compression strength is improved drastically by the Al additions, especially when the Al amount is high enough to precipitate Ni 2 TiAl (Heuslar compound) phase which is coherent to the NiTi(B2) phase matrix; an alloy with 8.4 mol.% Al showed compressive yield strengths as high as 2300 and 200 MPa at room and high (1000°C) temperatures, respectively. When the Al content exceeds 11 mol.%, however, Ni 2 TiAl phase started to deposit in a dendritic manner to reduce the strength. Although compressive ductility declined with the increase in Al content, 5.2% deformation was achieved with the 8.4 mol.% Al-containing alloy at room temperature.

Nanoindentation tests on diamond-machined silicon wafers

Nanoindentation tests were performed on ultraprecision diamond-turned silicon wafers and the results were compared with those of pristine silicon wafers. Remarkable differences were found between the two kinds of test results in terms of load-displacement characteristics and indent topologies. The machining-induced amorphous layer was found to have significantly higher microplasticity and lower hardness than pristine silicon. When machining silicon in the ductile mode, we are in essence always machining amorphous silicon left behind by the preceding tool pass; thus, it is the amorphous phase that dominates the machining performance. This work indicated the feasibility of detecting the presence and the mechanical properties of the machining-induced amorphous layers by nanoindentation.

High temperature tensile properties of a series of nickel-base superalloys on a γ/γ′ tie line

Abstract A series of nickel-base superalloys with γ′ fractions changed from 0 to 100 mol% and with γ and γ′ compositions kept constant were designed and examined in terms of high temperature tensile properties. In the temperature range from 900 to 1000°C, 0.2% proof stress shows a maximum at about 85 mol% in a corrected amount of γ′ , whereas at 1100°C, it shows maximum at about 65 mol%. The maximum shift of proof stress from the law of mixture in strength was constant for these temperatures. In the strain rate range from 10 −8 to 10 −2 /s at 1000°C, the maximum shift of proof stress increased with the strain rate. The proportion of the shift stress to the proof stress becomes larger as the temperature increases.

Phase stability and yield stress of Ni-base superalloys containing high Co and Ti

Ni-base superalloys containing high Co (>20 wt pct) and Ti (>5.5 wt pct) were designed in order to study the effects of Co16.9 wt pct Ti addition on phase stability and mechanical property. These new alloys, though they contained high Ti, mainly consisted of γ and γ′ phases. Ni3Ti (η) phase was observed along the grain boundaries in some of the alloys. The formation of η phase was mainly related to the Ti/Al ratio, Ti content, and alloy composition. Tensile and compression tests showed that these new alloys exhibited higher yield stress than that of the baseline alloy, TMW-1(U720LI). The possible strengthening mechanisms were discussed in terms of solid-solution and precipitation strengthening effects by the Co16.9 wt pct Ti additions. Preliminary results show promising trends for the development of new superalloys for turbine disc applications.

Precipitation hardening of Ir-Nb and Ir-Zr alloys

The authors previously developed refractory superalloys based on platinum-group metals with fcc and L1{sub 2} two-phase coherent structures. The refractory superalloys are potentially useful at ultra-high temperature, at which Ni-based superalloys can not be used. From among the platinum-group metals, they selected Ir as the base material because its melting temperature (2447 C) is higher than that of Ni (1455 C) and because Ir, with an fcc structure, can be equilibrated with the L1{sub 2} structure phase. Preliminary results demonstrated the superior strength of Ir-based refractory superalloys above 1200 C. Those results also revealed that precipitation hardening occurs and is affected by the shape of the precipitate. The precipitate shape of Ir-based alloys heat-treated at 1200 C is affected by the lattice misfits between the matrix and precipitates. In this study, the authors investigated the precipitation hardening mechanism by observing dislocation structures in deformed samples of Ir-Nb and Ir-Zr alloys using bright-field imaging and dark-field weak-beam imaging techniques with a transmission electron microscope (TEM).

Evaluation of defects generation in crystalline silicon ingot grown by cast technique with seed crystal for solar cells

Although crystalline silicon is widely used as substrate material for solar cell, many defects occur during crystal growth. In this study, the generation of crystalline defects in silicon substrates was evaluated. The distributions of small-angle grain boundaries were observed in substrates sliced parallel to the growth direction. Many precipitates consisting of light elemental impurities and small-angle grain boundaries were confirmed to propagate. The precipitates mainly consisted of Si, C, and N atoms. The small-angle grain boundaries were distributed after the precipitation density increased. Then, precipitates appeared at the small-angle grain boundaries. We consider that the origin of the small-angle grain boundaries was lattice mismatch and/or strain caused by the high-density precipitation.

Finite element modeling of high-pressure deformation and phase transformation of silicon beneath a sharp indenter

Modeling of the mechanical response of single crystalline silicon to a sharp indenter is an essential step for the optimization of wafer manufacturing processes. In this paper, deformation of silicon during indenter loading and unloading was analyzed by the finite element method, and the changes of stress field and high-pressure phase distribution were dynamically simulated. We found that the deformation of silicon in nanoindentation can be simply characterized by two factors: one is the elastic strain of each high-pressure phase and the other is the equivalent elastic strain of each phase transformation. In loading, indentation energy is absorbed mostly by phase transformation, and accumulated as the elastic strain of the high-pressure phases. The distribution pattern of the high-pressure phases beneath the indenter is independent of the indentation load, and the depth of the phase-transformed region is approximately twice the indentation depth. In unloading, high-pressure phases except the β-Sn phase undergo reverse phase transformations. The β-Sn phase does not transform back to the diamond phase but changes to other non-equilibrium phases, which becomes the dominant reason for residual strain. During unloading, the non-equilibrium phase expands from the diamond phase region toward the indenter tip, while the boundary between the non-equilibrium phase and the diamond phase remains unchanged. The unloading mechanism is independent of the change in the maximum indentation load and the presence/absence of pop-out events.