Tatsuo Tabaru

Tensile properties of a refractory metal base in situ composite consisting of an Nb solid solution and hexagonal Nb5Si3

Abstract Tensile properties and fracture behavior of an NbSS/Nb5Si3 in situ composite in which the silicide has a hexagonal structure has been investigated. Excellent tensile strength of 460 MPa is obtained at 1470 K. The fracture is attributed to cleavage of the Nb5Si3, NbSS/Nb5Si3 interface separation and ductile rupture of NbSS.

Effect of carbon on microstructure and high-temperature strength of NbMoTiSi in situ composites prepared by arc-melting and directional solidification

Abstract Improvement of high-temperature strength of Nb–(10–20)Mo–22Ti–18Si alloys by adding carbon and directional solidification technique has been investigated. Vickers hardness HV and compressive strength at 1470 and 1670 K were examined. The as-cast specimen without C composes of Nb solid solution (Nb SS ) and (Nb,Mo,Ti) 5 Si 3 silicide, while (Nb,Ti)C carbide obviously appears when contain more than 2.4 mol% carbon. The amount of carbide and silicide phases increases with increasing carbon content. The directionally solidified samples have the same phases as the as-cast ones and show coarse microstructure slightly oriented to the direction of growth. A small amount of carbon addition lowers both the HV and strength at 1470 and 1670 K, where corresponds with the appearance of the carbide phase. By adding more than 4.8 mol% carbon, however, the HV , 0.2% yield strength ( σ 0.2 ) and maximum strength ( σ max ) tend to increase. The directional solidification samples show higher strength and work hardening rate than the as-cast ones at corresponding C content. The 10 mol% Mo sample is weakened by C addition, but a considerable improvement in strength at elevated temperatures can be predicted when more than 10 mol% C is added to the 20 mol% Mo material. The sliding and debonding of the Nb SS /silicide interfaces is dominant in compressive damage at elevated temperatures.

Influences of Al content and secondary phase of Mo5(Si,Al)3 on the oxidation resistance of Al-rich Mo(Si,Al)2-base composites

Abstract The microstructures and oxidation behavior of Mo(Si,Al)2-base composites were investigated. A high Al content up to 38 mol% and the effect of the secondary Mo5(Si,Al)3 phase were specifically examined. Microstructure observations indicated that the Al solubility limit at 1680 K extends to 43 and 10 mol% for Mo(Si,Al)2 and Mo5(Si,Al)3, respectively. The composites containing about 10 vol.% Mo5(Si,Al)3 exhibits excellent oxidation resistance in the temperature range between 780 and 1780 K, so that the Mo5(Si,Al)3 up to about 10 vol.% is acceptable and has no detrimental influence on the oxidation resistance. At temperatures from 1480 to 1780 K, a continuous alumina layer develops on the Mo5(Si,Al)3 as well as on the Mo(Si,Al)2. The alumina growth rate is nondistinctive in the Al range investigated, and it is controlled by inward oxygen diffusion through the alumina at 1580 K and higher.

Oxidation behavior of Mo(Si0.6,Al0.4)2/HfB2 composites as aluminum reservoir materials for protective Al2O3 formation

Abstract Mo(Si0.6,Al0.4)2/HfB2 composites containing up to 18 vol% HfB2 have been shown to exhibit good oxidation resistance. Incorporated HfB2 has a minor influence on the scale growth behavior, and the overall behavior is similar to that of Mo(Si0.6,Al0.4)2. A further increase in HfB2 content, however, may lead to substantially fast scale growth.

Piezoelectric Response to Pressure of Aluminum Nitride Thin Films Prepared on Nickel-Based Superalloy Diaphragms

Aluminum nitride (AlN) thin films were prepared on Inconel 600 superalloy diaphragms by rf magnetron sputtering for the first time to our knowledge. The crystal structure of the AlN films is hexagonal, and the c-axis of the AlN films orients perpendicular to the diaphragm surfaces. The full-width at half-maximum (FWHM) of the X-ray rocking curves of the AlN films is 5.7°, and the piezoelectric constants d33 and d31 are 2.0 and 0.7 pC/N, respectively. We have investigated the influence of the diaphragm structure on the piezoelectric response to pressure of the AlN films. The AlN films sensitively generate electric charges to pressure changes, and the generated charges show an excellent linearity with increasing pressure. The AlN films indicate a high sensitivity of 723 pC/N. The sensitivity of the AlN films agrees with the result calculated using a method in which the electroelastic energy is differentiated from the voltage in AlN films for unimorph circular diaphragms.

Effect of HfC Addition on Mechanical Properties of Nb-5Mo-2W-18Si In-Situ Composites

Nb base in-situ composites with the base composition of Nb-5Mo-2W-18Si were prepared by conventional arc-melting and induction heating floating zone melting followed by directional solidification. To investigate the effect of HfC addition, Nb was replaced with 0, 1 and 2 mol% HfC. The in-situ composites predominantly have an eutectic microstructure consisting of Nb solid solution (NbSS) and (Nb,Mo,W))5Si3 (5-3 silicide). The strength at 1470 K and 1670 K increases without fracture toughness decreasing, with increasing the HfC content. Directional solidification also improves the strength at the high temperature. The slip band under the shearing stress occurs in the NbSS during plastic deformation, which contributes to suppress microcrack propagation. It seems that HfC addition reinforces the bonding strength at grain boundary or NbSS/5-3 silicide interface.

Effects of tungsten addition on tensile properties of a refractory Nb−18Si−10Ti−10Mo−xW (x=0,5, 10 and 15 mol.%)in-situ Composites at 1670 K

To investigate the effect of tungsten addition on mechanical properties, we prepared refractory (62−x)Nb−18Si−10Mo−10Ti−xW (x=0, 5, 10 and 15 mol.%)in-situ composites by the conventional arc-casting technique, and then explored the microstructure, hardness and elastic modulus at ambient temperature and tensile properties at 1670 K. The microstructure consists of relatively fine (Nb, Mo, W, Ti)5Si3 silicide and a Nb solid solution matrix, and the fine eutectic microstructure becomes predominant at a Si content of around 18 mol.%. The hardness of (Nb, Mo, W, Ti)5Si3 silicide in a W-free sample is 1680 GPa, and goes up to 1980 GPa in a W 15 mol.% sample. The hardness, however, of Nb solid solution does not exhibit a remarkable difference when the nominal W content is increased. The elastic modulus shows a similar tendency to the hardness. The optimum tensile properties of the composites investigated are achieved at W 5 mol.% sample, which exhibits a relatively good ultimate strength of 230 MPa and an excellent balance of yield strength of 215 MPa, and an elongation of 3.7%. The SEM fractography generally indicates a ductile fracture in the W-free sample, and a cleavage rupture in W-impregnated ones.

Development of High-Temperature Acoustic Emission Sensor Using Aluminium Nitrade Thin Film

Aluminum nitride (AlN) is a promising Acoustic Emission (AE) sensor element for high-temperature environments such as gas turbines and other plants because AlN maintains its piezoelectricity up to 1200°C. Highly c-axis-oriented AlN thin-film sensor elements were prepared on silicon single crystals by rf magnetron sputtering. Both ordinary-temperature AE sensors and high-temperature AE sensors have been developed using these elements. In this paper, to study effects of d33 and thickness of AlN elements on sensor sensitivity, AlN elements with d33 from 2 to 7 pm/V and thickness from 3 to 9 /m were prepared. It is confirmed that the AE sensor sensitivity increased with d33 and thickness of AlN elements. The sensitivity of the high-temperature AE sensor was also improved by a design of the sensor structure. The sensor characteristics were evaluated at elevated temperatures from 200 to 600°C. It was confirmed that the AE sensor works well at 600°C and does not deteriorate.

Spatial structure of radio frequency ring-shaped magnetized discharge sputtering plasma using two facing ZnO/Al2O3 cylindrical targets for Al-doped ZnO thin film preparation

Spatial structure of high-density radio frequency ring-shaped magnetized discharge plasma sputtering with two facing ZnO/Al2O3 cylindrical targets mounted in ring-shaped hollow cathode has been measured and Al-doped ZnO (AZO) thin film is deposited without substrate heating. The plasma density has a peak at ring-shaped hollow trench near the cathode. The radial profile becomes uniform with increasing the distance from the target cathode. A low ion current flowing to the substrate of 0.19 mA/cm2 is attained. Large area AZO films with a resistivity of 4.1 – 6.7×10-4 Ω cm can be prepared at a substrate room temperature. The transmittance is 84.5 % in a visible region. The surface roughnesses of AZO films are 0.86, 0.68, 0.64, 1.7 nm at radial positions of r = 0, 15, 30, 40 mm, respectively, while diffraction peak of AZO films is 34.26°. The grains exhibit a preferential orientation along (002) axis.

Feasibility study of detection of dielectric breakdown of gate oxide film by using acoustic emission method

An in situ detection method for the dielectric breakdown of oxide films for MOS gates has been required in the plasma etching process. In this feasibility study, a conventional MOSFET device is used and an acoustic emission (AE) method is employed for the detection of the dielectric breakdown of a gate oxide film. A thin type AE sensor is attached at the backside of an electrostatic chuck (ESC), and the dielectric breakdown in a MOSFET, which is set on the ESC, is detected. The results demonstrate that the thin type AE sensor can detect the dielectric breakdown with an energy on the order of µJ.