Materials Science Forum Vols. 706-709

Abstract: Spark plasma sintering has been used for decades in order to consolidate a wide variety of materials and permitting to obtain fully dense specimens. This technique has been mainly applied to ceramics. This paper concentrates on an unusual use of spark plasma sintering system: obtaining innovative materials especially architectured ones. Different applications are presented. Firstly, the SPS technique has been used to elaborate nanometers grain size materials or containing nanoscale microstructure. This is possible since the sintering temperature and the holding time are far lower in the SPS compared to other techniques. Then SPS has been used to realize diffusion bonding. In that case again, bonding can be realized at low temperature and for short time. It permits for example to realize bonding between two copper layers which is of a great importance for microelectronic applications. It is worth noting that this bonding can have the same mechanical strength as pure copper even for diffusion time of a few minutes. Secondly, bonding has been also carried out between a metallic layer and a ceramic one. This could lead to design of new layered materials combining interesting properties in terms of mechanical strength but also in terms of electrical resistance. The SPS machine has also been used to obtain porous materials (cobalt alloys or copper) with an adapted microstructure (porosity, tortuosity,). These structures could open new perspectives for biomedical or for microelectronic applications. All these examples lead to a better understanding of the physical processes which happen during spark plasma sintering.

Abstract: If Mn could be partly substituted by Fe, Ti-Mn-Fe alloys would be less costly than Ti-Mn alloys. Furthermore, the use of iron as a beta-stabilizing element is more suitable than the use of manganese from a situation of an element strategy. In this study, 4.26 was admitted as the average ratio of valence electrons to atoms, e/a. The compositions of Mn and Fe were chosen under 4.26 as e/a. We investigated the influence Fe in selected Ti-Mn-Fe alloys by performing electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In solution-treated and water-quenched 10Mn alloy, the beta and athermal omega phases were identified, while only the beta phase was identified in 8.7Mn-1Fe, 6.1Mn-3Fe, and 3.5Mn-5Fe alloys. In all alloys, equiaxial beta grains were observed by optical microscope. The resistivities at room and liquid-nitrogen temperatures and the Vickers hardness were relatively invariant across all Ti-Mn-Fe alloys, except for the Vickers hardness of the 5Fe alloy. During aging at 773 K, an isothermal omega phase precipitated in only the 3.5Mn-5Fe alloy, whereas only the alpha phase precipitated in the others.

Abstract: The effect of reduction rate on the deformation texture of cold-rolled Ti-26mol%Nb-3mol%Al shape memory alloy was investigated. The alloy is the parent phase (β: bcc) at room temperature (RT) and the martensite start temperature is much lower than RT; no residual martensite was detected after cold-rolling. The reduction rate, r, was varied in the range of 60 ~ 99%. Texture evolution was as following; γ-fiber à {001}<110> à {001}<110> + {112}<110> (α-fiber). The strength of {001}<110> was maximized at about r = 97%. The recrystallization texture is expected to be controlled by the reduction rate; optimization of r is supposed to be promising to obtain the {001}<110> recrystallization texture that is preferred for superelastic deformation.

Abstract: The purpose of this work is to establish a deposition process of Ti-Ni-Cu films showing shape memory effect in the as-deposited state. 5-µm-thick Ti₅₀Ni₃₅Cu₁₅ films have been deposited onto thermally oxidized (001) Si wafer by triple-source dc magnetron sputtering at various substrate temperatures. Their shape memory behavior were characterized by XRD, DSC measurements and thermal cycling tests under various constant tensile stresses. We have confirmed that crystalline films can be grown directly when the substrate temperature exceeds 400°C. The films deposited at higher than 450°C showed thermoelastic martensitic transformation and their M_s temperature slightly increased with increasing substrate temperature. Since their M_s temperature were found to be higher than 30°C, they can be used as an actuator at RT. These films were also found to have higher critical stress against plastic deformation than the post-deposition crystallized films. We have also tried to fabricate a prototype of micro-actuator and to characterize their actuation behavior and have confirmed that TiNiCu/SiO₂ double layered diaphragm showed an actuation response to a pulsed current of more than 100Hz.

Abstract: The establishment of rapid synthesis process at a low temperature of thermoelectric non-doped and Y-doped SrTiO₃ was tried by using the polymerized complex process and the pulsed current heating method with addition of TiB₂ powder. The TiB₂ addition was quite effective for reduction in the electrical resistivity of the thermoelectric SrTiO₃, and the power factor of the Sr_0.9Y_0.1TiO₃/TiB₂ composite was similar to that of the Sr_0.9Y_0.1TiO₃ polycrystal synthesized by the conventional solid state reaction method. It was also found that the addition of TiB₂ significantly reduced densification temperature during sintering. This rapid consolidation caused by TiB₂ addition was available even when the composite powder was sintered by conventional hot pressing.

Abstract: To enhance the mechanical properties of Mg alloys, powders of pure Mg (Al₂O₃/Mg) dispersed with over 10 vol% Al₂O₃ particles were formed by ball milling powder mixtures of pure Mg and Al₂O₃ particles with particle sizes of 0.3 and 1 μm. The effect of the Al₂O₃ content of hot-pressed discs of Al₂O₃/Mg powders on the mechanical properties was investigated. The hardness increased sharply at certain Al₂O₃ content. This is considered to be because of the development of Al₂O₃ particle strengthening due to the excellent interfacial bonding between Mg and Al₂O₃ particles when the Al₂O₃/Mg discs contain a sufficient amount of Al₂O₃ particles and have a sufficient interparticle distance. The maximum hardness was 220 HV. The tensile strength increased as Al₂O₃ content was increased. The Al₂O₃/Mg plate with Al₂O₃ particles sizes of 0.3 and 1 μm indicated maximum values of the bending strength at 10 and 14vol%Al₂O₃ content, respectively.

Abstract: The stability of shape memory properties was investigated by thermal cycling tests in Ti-23Ta-(2-4)Sn alloys. The martensitic transformation temperature decreased considerably after aging at 573 K for the Ti-23Ta-2Sn alloy. The decrease in the martensitic transformation temperature by the aging effect was suppressed by the addition of 3at.%Sn and more to the Ti-23Ta alloy. The Ti-23Ta-3Sn alloy exhibited better cyclic stability compared with a Ti-32Ta alloy which has a similar transformation temperature.

Abstract: Photonic crystals with periodically arranged structures of ceramics can reflect light or electromagnetic waves through Bragg diffraction and exhibit forbidden gaps in transmission spectra. We have successfully fabricated micro diamond crystals including twined lattices with plane defects to realize wavelength selections in terahertz frequency ranges. Novel smart processing composed of computer aided design, manufacturing and evaluation was established. The terahertz waves are expected to detect micro cracks in material surfaces and structural defects in electric circuits by fine wave interferences, and to analyze cancer cells in human skins and toxic bacteria in natural foods through high frequency excitations. In the fabrication processes using micro patterning stereo-lithography, the photo sensitive resin paste including alumina fine particles were spread on a glass substrate with 10 µm in layer thickness by moving a knife edge, and cross sectional images of ultra violet ray were exposed by using a digital micro mirror device with 2 µm in part accuracy. Through the layer stacking process, micrometer order structures were formed exactly. Dense ceramic components could be obtained through dewaxing and sintering heat treatments. The electromagnetic wave transmission spectra were measured by terahertz wave spectroscopy. The micro diamond lattices could form perfect photonic band gaps opining for all crystal directions. The introduced plane defects realized the wave select resonations. This resonation behavior was visualized and analyzed by finite difference time domain simulations.

Abstract: The processes of structure formation in Ti-Ni and in Ti-Nb-Zr, Ti-Nb-Ta shape memory alloys (SMA) under thermomechanical treatment (TMT) were studied. The TMT comprised cold rolling with true strains from e=0.25 to 2 and post-deformation annealing. Differences in these processes between two groups of alloys are considered. The main conclusions are as follows: nanostructures created by TMT are useful for radical improvement of the SMA functional properties, and an optimum nanostructure (nanocrystalline structure, nanosubgrained structure or theirmixture) should be selected by taking into account other structural and technological factors.

Abstract: In this study, the relationship between magnetostriction and microstructure for a Fe-30at-%Pd alloy was investigated. The alloy was fabricated by vacuum arc melting, followed by 24 h annealing under vacuum at 1000 °C and then by quenching in ice water. Crystal structure, orientation mapping, and phase identification were obtained by employing X-ray diffraction and electron back-scatter diffraction. Magnetic properties were measured using vibrating sample magnetometer. Magnetostriction of a Fe-30at-%Pd alloy was obtained using the digital image correlation technique under different magnetic fields. The maximum strain of Fe-30at-%Pd alloy was 5×10^-4 at a magnetic field of 7500 Oe.