Papers by Keyword: High Temperature Strength

Abstract: Enploying unidirectional solidification, Al2O3/GdAlO3(Gadolinium Aluminum Perovskite:GAP) eutectic composite was fabricated. The Al2O3/GAP eutectic composite obtained has a flexural strength of over 600Mpa above 1500
C. The composite also showed plastic deformation above 1550
C on the flexural test. The effect of microstructure on mechanical strength at high temperature was investigated. Al2O3/GAP eutectic composites having different microstructure were fabricated by controlling the temperature gradient on unidirectional solidification. It was found that the flexural strength at high temperature of the composite became higher with refining the microstructure.

Abstract: Two types of joining specimens with and without Ni foil interlayer between 316L–SS bar have been prepared by diffusion bonding in a temperature range of 850- 1050°C, under a uniaxial pressure of 10MPa for 1hours. The relationship between the bonding parameters and the tensile strength of the joints at elevated temperature was studied. Optimized processing parameters were suggested based on the testing results. It was found that the introduction of the interlayer may reduce the room temperature strength but increase the high temperature strength. This was attributed to the transformation of Fe0.64Ni0.36 formed in bonding process into FeNi3 at high temperature.

Abstract: Previous studies have demonstrated that dense coatings of CVD mullite (3Al2O3×2SiO2) provide excellent oxidation protection for Si3N4 and SiC in a high pressure, steam environment. In this study the mechanical properties of CVD mullite coated silicon nitride materials from different vendors (AS800, NGKSN88, Kyocera SN281) were evaluated following ASTM test procedures. The dynamic fatigue tests werep erformed in ambient air at temperatures of 850 and 1200°C under fast (30 MPa/s) and slow (0.003 MPa/s) load rates. The static fatigue tests were carried out at a constant load of 350 MPa for 1000h at 1200°C. The cyclic fatiguetests at 850°C consisted of a loading ramp from 20 to 400 MPa in 30 seconds followed by unloading ramp from 400 to 20 MPa. A total of 10,000 cycles were applied to the fatigue test specimens before fast fracture tests were conducted at room temperature. The strength test results indicated that CVD mullite coatings showed excellent adhesion during dynamic fatigue tests and exhibited no creep behavior. Minor flexure strength reduction observed at low stressing rate and at high temperatures appeared to be related to Si3N4 properties such as SCG (slow crack growth) susceptibility. During cyclic and static fatigue tests, a glassy silica/aluminosilicate phase was formed due to oxidation. This resulted in localized coating separation and buckling. However, accumulation of this corrosion layer was not critical since the coated specimens showed a flexure strength increase of ~7-9.5%.

Abstract: The effect of glassy-phase, using AlN and Lu2O3 as sintering additives, on the microstructure and mechanical properties of liquid-phase-sintered, and subsequently annealed SiC ceramics was investigated. The microstructure was strongly influenced by the sintering additive composition, which determines the intergranular phase (IGP). The average thickness of SiC grains increased with increasing the Lu2O3 /(AlN + Lu2O3) ratio, whereas the average aspect ratio decreased with increasing the molar ratio. The homophase and heterophase boundaries of the SiC ceramics were completely crystalline in all specimens. The room temperature (RT) strength decreased with increasing the molar ratio whereas the RT toughness showed a minimum at the molar ratio of 0.6. The best results at RT were obtained when the molar ratio was 0.2. The flexural strength and fracture toughness of the ceramics were >700 MPa and ~6 MPa.m1/2 at RT. The high temperature strength was critically affected by the chemistry, especially the content of Al in the IGP. The best strength at temperatures ³ 1500oC was obtained when the molar ratio was 0.5. Flexural strengths of the ceramics at 1500oC and 1600oC were 610 ± 80 MPa and 540 ± 30 MPa, respectively. The beneficial effect of the new additive compositions (Lu2O3-AlN) on high-temperature strength of SiC ceramics was attributed to the crystallization or removal of IGP and introduction of Al into SiC, i.e., removal or reduction of Al content from the IGP, resulting in an improved refractoriness of the IGP.

Abstract: A2O3/YAG/ZrO2 eutectic Melt-Growth-Composites (MGCs) were unidirectionally solidified by the modified-pulling-down method (MPD) and the Bridgman type method, in which a crucible was brought down at different speeds. The microstructures and crystallographic textures were studied by field emission scanning electron microscopy (FE-SEM) and electron backscattered pattern (EBSP) method. The high-temperature strength was investigated by compression tests. All MGC rods show strong preferred growing orientation, although the structural size of eutectic microstructure among MGC rods was different. The high-temperature strength of MGC rods is dependent on orientation, compression temperature and strain rate. The high-temperature strength of MGC rods is controlled by the anisotropic strength of constituent Al2O3, as well as the structural size of eutectic microstructure.

Abstract: Effects of aging treatment on high temperature strength of Nb added ferritic stainless steels for automotive parts were investigated. Hot tensile tests were carried out at 700 °C after the aging at 700 °C for different aging times using Gleeble 1500. High temperature strength of all steels decreased as the aging time increased. In Nb free steels, the reduction in high temperature strength is mainly due to grain growth. On the other hand, in Nb added steels, the reduction in high temperature strength occurred by Nb precipitation. It was observed that Fe2Nb (Laves phase), Nb(C,N) and Fe3Nb3C were precipitated out during the aging at 700 °C in Nb added steels. The coarsening rate of Fe2Nb was higher than that of Nb(C,N). Fine Fe2Nb precipitates formed during at the early stage of aging contributed to high temperature strength in 0.01C-0.38Nb steel. However, coarse Fe2Nb particles formed during the aging were very detrimental to high temperature strength. The coarsening of Fe2Nb was relatively retarded by adding Mo.

Abstract: 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.