Structural stability of the rod-like iron-iron sulfide eutectic at elevated temperatures

A study has been made of the thermal stability of microstructures within unidirectionally solidified ingots of the rod-like Fe?Fe_1-xS eutectic. The ingots were heat treated at temperatures of up to 98.7 pct of the eutectic’s melting point for times of up to 500 hr. At the higher temperatures the iron rods broke-up and spheroidized; these processes were accompanied by diffusion and precipitation of iron at surfaces and grain boundaries of specimens. The greater termalinstability of the Fe?Fe_1-xS eutectic compared with that of the Al?CuAl₂ and Al?Al₃Ni eutectics was attributed to the higher interfacial energy of the former system, the absence of preferred crystallographic orientations and the higher temperatures of heat treatment.     

Thermal stability of direct dental esthetic restorative materials at elevated temperatures

With increasing use of direct esthetic restorative materials, the identity of a body may rely upon knowledge of temperature effects on this class of dental restorations. This research examined the effect of atmospheric gas on thermal decomposition and color change of a wide variety of direct esthetic restorative materials. Cured discs (4 x 1 and 8 x 1 mm) were made using manufacturer's directions: traditional glass ionomer (Fuji II), light-curable resonomer (Fuji II LC), compomer (Geristore), and three types of resin composites--highly filled, urethane-based (Occlusin), and two Bis-GMA/TEGDMA resins: hybrid (Herculite XRV) and microfill (Silux Plus). Three replications of each material were heated at 5 degrees C/min in a thermogravimetric analysis unit using either room air or nitrogen purge to simulate different thermal environments. First derivative values of percent weight loss with respect to temperature were obtained to determine temperatures associated with increased decomposition rates. Room-air heating showed greater numbers of decomposition events than did nitrogen-heated discs. The only material decomposing less than 200 degrees C in either atmosphere was traditional glass ionomer. The majority of decomposition occurred between 200 degrees and 500 degrees C for all materials. Only products containing glass ionomer components decomposed between 600 degrees and 800 degrees C. Room-air heating resulted in ash white discs at 800 degrees C and higher. Specimens heated in nitrogen were gray to black at 600 degrees C and higher. Heating atmosphere greatly affected color, and some products demonstrated distinguishing color changes: glass ionomers, in particular, showed characteristic color features. An atlas was constructed from color change of specimens recovered after 200 degrees, 400 degrees, 600 degrees, 800 degrees, and 1000 degrees C compared with non-heated controls.     

Hardness of surfacing alloys at elevated temperatures

Conclusions Of all the surfacing materials tested the greatest hardness at elevated temperatures is exhibited by electrodes made by the cermet method using chromium carbide as a base, i.e., GK-10 and GK-15 with a metal binder.Translated from Poroshkovaya Metallurgiya, No. 3 (51), pp. 80–83, March, 1967.     

Creep-rupture behavior of iridium at elevated temperatures

The creep-rupture properties of annealed arc-cast iridium were determined between 982°C and 1288°C for times less than 1000 h. It was found possible to fit the rupture and creep results by the Sherby-Dorn parameter method using an activation energy of 70,600 calJmole. It was found that under creep deformation, failure was at least partially intercrystalline, and the failure elongation decreased with increasing temperature and time.     

Plastic deformation of titanium at elevated temperatures

The deformation of iodide titanium single crystals containing 200 to 250 ppm O, was studied in compression at temperatures from 25° to 800°C. Reduction of about 5 pct along thec axis was accommodated almost entirely by \(\left\{ {11\bar 22} \right\}\) twinning from 25° to 300°C, and above 400°C by \(\left\{ {10\bar 11} \right\}\) twinning in combination with c+a slip. The stress for \(\left\{ {11\bar 22} \right\}\) twinning increased with increasing temperature, and twin formation was accompanied by a load drop, while the stress for \(\left\{ {10\bar 11} \right\}\) twinning decreased with increasing temperature and twinning was not accompanied by a load drop. Crystals reduced normal to thec axis deformed by a combination of prism slip and \(\left\{ {10\bar 12} \right\}\) twinning at 25°C and by prism slip alone above 500°C.     

Creep-rupture behavior of iridium at elevated temperatures

The creep-rupture properties of annealed arc-cast iridium were determined between 982°C and 1288°C for times less than 1000 h. It was found possible to fit the rupture and creep results by the Sherby-Dorn parameter method using an activation energy of 70,600 calJmole. It was found that under creep deformation, failure was at least partially intercrystalline, and the failure elongation decreased with increasing temperature and time.     

Fatigue limit of ceramic materials at elevated temperatures

Translated from Poroshkovaya Metallurgiya, No. 2(350), pp. 42–47, February, 1992.     

Fatigue of Ni-Ai-Mo aligned eutectics at elevated temperatures

The elevated-temperature mechanical behavior of two aligned eutectics (Ni-8.1 wt pct Al-26.4 wt pct Mo and Ni-6.3 wt pct Al-31.2 wt pct Mo) has been investigated utilizing monotonic and cyclic testing in vacuum. Tensile yield strength and fatigue resistance increased from 25 to 725 °C, but then were reduced at 825 °C. The fatigue lives of specimens tested at 725 °C decreased sharply with decreasing frequency. A shift from surface to internal crack initiation was observed upon increasing the test temperature from 725 to 825 °C. Stage II crack propagation was observed at both temperatures, in contrast to stage I cracking at 25 °C. The test results are compared to those for other nickel and cobalt-base aligned eutectics to show that the frequency effect on fatigue life is not limited to the Ni-AI-Mo system. formerly Graduate Assistant in the Department of Materials Engineering, Rensselaer Polytechnic Institute     

The deformation and fracture of TiAl at elevated temperatures

The tensile properties of the intermetallic compound TiAl have been determined at several temperatures in the range 25 to 1000°C. Additional variables studied were the influence of strain rate and the effect of exposure to oxidizing conditions prior to testing. The modes of deformation under the various testing conditions were studied in the electron microscope, the modes of fracture were studied in the scanning electron microscope, and these data were correlated with the mechanical properties. The results indicate that the ductilebrittle transition behavior of TiAl at about 700°C is controlled by the trailinga/6 [112] partial dislocation components of thea [011] superdislocations overcoming their pinning barriers. It was also shown that prior exposure to oxidizing conditions does not markedly influence the mechanical properties of TiAl.     

Slow crack growth in ceramic materials at elevated temperatures

Techniques for studying slow crack growth at high temperatures are described. The techniques are used to obtain crack growth data for a range of silicon nitrides between 1100 and 1400°C. For these materials the data suggest that the slow crack growth may be effectively characterized by the relation between crack velocity and stress intensity factor. Data obtained for mechanical and thermal fatigue modes indicate that these behaviors can be predicted with moderate accuracy from the isothermal, static stress parameters (for the range of conditions investigated). Finally, the application of slow crack growth data to failure prediction is described and illustrated using data for one of the materials tested.     

Prismatic slip in zirconium single crystals at elevated temperatures

Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form:γ = AT ⁿ e-Q/rT where y is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m²)^•4.3. The stress exponentn = 4.3 ± 0.3 and the activation energy Q = 2.05 = 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.     

Prismatic slip in zirconium single crystals at elevated temperatures

Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form: {ie1217-05} where {ie1217-06} is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m²)^−4.3. The stress exponentn = 4.3 ± 0.3 and the activation energyQ = 2.05 ± 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.     

Slow crack growth in ceramic materials at elevated temperatures

Techniques for studying slow crack growth at high temperatures are described. The techniques are used to obtain crack growth data for a range of silicon nitrides between 1100 and 1400°C. For these materials the data suggest that the slow crack growth may be effectively characterized by the relation between crack velocity and stress intensity factor. Data obtained for mechanical and thermal fatigue modes indicate that these behaviors can be predicted with moderate accuracy from the isothermal, static stress parameters (for the range of conditions investigated). Finally, the application of slow crack growth data to failure prediction is described and illustrated using data for one of the materials tested.