Tohru S. Suzuki

A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

Abstract Single crystal elastic constants of Ti–Ni alloys without (quenched) and with (aged) Ti 3 Ni 4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti–Ni–Cu and Ti–Ni–Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19′ martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c 44 , in additional to c ′, is a common feature for all Ti–Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19′, suggesting that softening in c 44 is responsible for the unique B19′ martensite, which found no analogy in other β phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2–B19′ transformation anisotropy shows a decrease with lowering temperature; prior to B2–B19 an anisotropy increase, while prior to B2–R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19′) are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti–Ni-based alloys: B2–R–B19–B19′.

Orientation amplification of alumina by colloidal filtration in a strong magnetic field and sintering

The controlled development of texture in ceramics is a growing focus of interest in connection with processing because it leads to improved electrical, piezoelectric, mechanical, and other properties. We demonstrate in this communication, that the highly textured microstructure of pure dense alumina can be manipulated by a strong magnetic field applied to alumina powder in a suspension, followed by heating. Macroscopic interaction between the agglomerated particles in a suspension prevents the powder in the suspension from rotating when a magnetic field is applied, so it is necessary to disperse the powder in the suspension to allow the magnetic field to exert its effects. Colloidal processing using repulsive surface forces was used in this study to prevent heterogeneous agglomerates from forming.

Band-Gap Energy Anomaly and Sublattice Ordering in GaInP and AlGaInP Grown by Metalorganic Vapor Phase Epitaxy

The previously reported photoluminescence(PL)-peak-energy anomaly problem for Ga0.5In0.5P grown on GaAs by metalorganic vapor phase epitaxy (MOVPE) was studied in detail. X-ray microprobe analysis, and optical transmission spectra measurements were carried out to examine alloy compositions and band-gap energies (Egs), respectively. The MOVPE growth condition dependence of {1/2, 1/2, 1/2} superlattices (SLs) on the cation sublattice in Ga0.5In0.5P was studied in detail, using transmission electron microscopy. The correlation between the Eg anomaly and the SLs was examined in detail and established. Raman spectra seemed to show zone-folding effects due to the monolayer SL. A similar Eg anomaly was reported for AlGaInP. AlGaInP and AlInP were also found to show the same SLs.

Control of texture in alumina by colloidal processing in a strong magnetic field

Abstract Electrical, mechanical and other properties of ceramic materials can be controlled by designing their microstructures. It had generally been difficult to utilize a magnetic field for tailoring the microstructure in feeble magnetic ceramics, such as Al2O3; however, the possibility of controlling the microstructure by a magnetic field occurred with the development of superconducting magnets. In this review paper, we introduce a novel processing for the microstructual design in ceramics by colloidal processing in a strong magnetic field and an electric field. We demonstrate that the textured alumina can be fabricated by slip casting in a strong magnetic field and the production of alumina/alumina laminar composites with different crystalline-oriented layers can be achieved by electrophoretic deposition in a strong magnetic field. In order to control the texture using a magnetic field, a good dispersion of powder in a suspension is necessarybecause a strong attractive force between the agglomerated particles prevents each particle in a suspension from rotating in the magnetic field. The degree of orientation depends on the processing factors, such as heating temperature, viscosityof suspension, etc. And the grain growth in Al2O3 matrix enhances crystallographic texture development. The bending strength of the laminar composite depended on the direction of the multilayered microstructure with alternate crystalline-oriented layers. Crack propagation and fracture mode depend on the direction of microstructure in the laminar composite with controlled crystalline orientation.

Fabrication of Textured Titania by Slip Casting in a High Magnetic Field Followed by Heating

Anisotropic susceptibility is very small in feeble magnetic ceramics such as titania; therefore, it had been very difficult to develop a textured microstructure using a magnetic field. However, we demonstrate in this paper, that textured titania can be prepared by slip casting in a high magnetic field and heating. After the suspension was compacted by slip casting in a high magnetic field, a green solid with a slight degree of crystallographic orientation was obtained. The degree of texture increased with increasing temperature, and crystallographic texture development accompanied grain growth in the specimens.

AlGaInP double heterostructure visible-light laser diodes with a GaInP active layer grown by metalorganic vapor phase epitaxy

AlGaInP double heterostructure laser diodes with a GaInP active layer constitute a basic laser structure for visible-light lasers using an AlGaInP alloy system. This paper gives a detailed description of (Al x Ga 1-x ) 0.5 In 0.5 P metalorganic vapor phase epitaxial growth, laser-fabrication processes, and basic device-characteristics for these lasers. The obtained pulsed-threshold-current was about 3.8 kA/cm2(3.2 kA/cm2minimum) for laser diodes with an 8-10 \mu m wide and 150-300 \mu m long injection stripe. High characteristic-temperature T 0 for the temperature dependence of pulsed threshold current was obtained and was found to be dependent on band-gap-energy differences between active layers and cladding layers. The maximum value for T 0 was 222 K. The lasing wavelength of an AlGaInP double heterostructure laser diode with a GaInP active layer was found to depend on growth conditions and dopant behavior during the growth, and it varied in a range from 664 to 690 nm, while the GaInP active layers were lattice-matched to GaAs substrates. This lasing wavelength difference caused by a crystal growth-condition difference, including a dopant effect, can be explained by the Ga 0.5 In 0.5 P band-gap-energy difference due to the crystal structure difference of Ga 0.5 In 0.5 P. Lasing operations (3-5 mW) over 2000 h at room temperature without significant degradation were obtained, demonstrating the highly promising nature of visible-light laser diodes using an AlGaInP alloy system.

Alignment of Titania Whisker by Colloidal Filtration in a High Magnetic Field

Whisker alignment is one possible way in order to improve the properties of whisker reinforced materials. Susceptibility is very small in diamagnetic ceramics such as titania and alumina, etc.; therefore, the effects of a high-magnetic field on these ceramics had been generally neglected. Recently, superconducting magnet technologies have been developed and used for applications for such feeble magnetic materials. We demonstrate that the alignment of titania whiskers can be controlled by colloidal filtration of a well-dispersed suspension of the whiskers in a high magnetic field (10 T) when the direction of the magnetic field was perpendicular to the direction of the fluid.

P-Type Doping Effects on Band-Gap Energy for Ga0.5In0.5P Grown by Metalorganic Vapor Phase Epitaxy

Photoluminescence properties of Mg- or Zn-doped Ga0.5In0.5P grown by metalorganic vapor phase epitaxy were studied as a function of hole concentration (p). The band-gap energy (Eg) value for Mg- or Zn-doped Ga0.5In0.5P, grown under a condition in which undoped Ga0.5In0.5P shows an anomalously low Eg, showed a steep increase for p~>1×1018 cm-3. This anomalous behavior was attributed to the Mg or Zn diffusion-enhanced randomization of the previously observed naturally formed monolayer {1/2, 1/2, 1/2} superlattices on the column III sublattice.

Novel Window-Structure AlGaInP Visible-Light Laser Diodes with Non-Absorbing Facets Fabricated by Utilizing GaInP Natural Superlattice Disordering

Window-structure AlGaInP visible-light ( λL=680 nm) laser diodes (LDs) have been fabricated, for the first time, by utilizing GaInP natural superlattice (NSL) disordering with selective Zn diffusion. The bandgap energy for the active layer near the mirror facets is increased by 70 meV by the NSL disordering. An 80 mW output power in a fundamental transverse-mode has been achieved for the uncoated window LDs under 1 µsec long pulsed operations. The maximum output power density for the window LDs is estimated to be 10 MW/cm2, which is five times higher than that for conventional LDs.

Understanding the martensitic transformations in TiNi-based alloys by elastic constants measurement

Abstract The origin of the unique monoclinic martensite B19′ in TiNi and TiNi-based alloys has remained obscure for many years. The formation of B19′ martensite also challenged the well-known basal-plane shear/shuffle theory of martensitic transformation in β phase alloys. Recently we proposed that the B2–B19′ transformation stems from a strong coupling between a non-basal-plane shear (c44 shear) with the basal-plane shear (c′ shear), being manifested by a low-lying and decreasing anisotropy factor (c44/c′) towards Ms. This model has gained experimental support from elastic constants measurement on Ti50Ni30Cu20 alloy which exhibits B2–B19 transformation (i.e. absence of the monoclinic shear). In the present study, we attempt to further verify this model by measuring the elastic constants of Ti50Ni40Cu10 alloy which undergoes a two-stage transformation B2–B19–B19′. The results clearly demonstrated once again that whenever the parent phase B2 does not transform directly into B19′, the anisotropy factor exhibits an increase towards Ms, indicating that the c44 shear is not included into the transformation. Therefore, the present study gives additional support to the coupling model of TiNi-based alloys. Furthermore, we tried to understand the multi-stage transformation in terms of coupling strength between the c′ shear and c44 shear.