Kristin Breder

Dimensional changes and creep of silica core ceramics used in investment casting of superalloys

Dimensional changes and creep deformation of a silica/zircon (74%/24%, respectively) and a high silica (93% silica and 3% zircon) ceramic were characterized and compared. All specimens were tested with a thermal profile that consisted of a 300°C/h heating rate to 1475 or 1525°C, followed by a one-hour isothermal hold (where each specimen was compressively crept under a static stress of 2.07, 4.14, or 6.21 MPa). The specimens were cooled at a rate of 900°C/h under stress. Dimensional changes were interpreted from apparent thermal expansion behavior during heating as well as before-and-after dimensional measurements. The silica/zircon ceramic generally exhibited less total contraction than the high silica ceramic for a specific test condition even though it crept faster at all stresses and temperatures during the one-hour isothermal/isostress segment. This indicates that the total contraction for both was dominated by reinitiated sintering and subsequent cristobalite formation that occurred during the heating segment. Minimum creep rate during the one-hour isothermal/isostress segment was examined as a function of stress and temperature for both ceramics using a power-law creep model. Creep-rate stress exponents (n) and activation energies (Q) were equivalent (within 95% confidence) for both ceramics showing that their different contents of zircon (3 vs. 24%) did not affect them. Lastly, n ≈ 1.3–1.4 and Q ≈ 170 kJ/mol indicate that diffusion-assisted crystallization of cristobalite, combined with power-law sintering owing to the high concentration of porosity (28–30%) was likely the rate-limiting mechanism in the creep deformation for both ceramics.

Dynamic fatigue behavior of two SiC and a SiCp reinforced Al2O3 at elevated temperatures

Abstract Dynamic fatigue behavior of a siliconized SiC, a sintered β-SiC, and a SiCp (silicon carbide particle)-reinforced Al 2 O 3 formed by directed oxidation of Al metal, which are considered for use in heat exchangers in coal-fired power plants, were evaluated at 1100 and 1400 °C in air. Four-point flexure specimens were tested at five stressing rates from 37 MPa s −1 to 0.0001 MPa s −1 resulting in total times to failure up to 1200 h. Thirty specimens of each material were tested at the fast-fracture condition and 10 specimens were tested at the four dynamic fatigue conditions at each temperature. At 1100 °C none of the materials exhibited any loss of strength as a function of stressing rate and very little tendency to creep was observed. At 1400 °C the sintered β-SiC exhibited no strength loss, while the siliconized SiC showed a significant loss of strength and some signs of creep at stressing rates less than 0.01 MPa s −1 . The SiCp reinforced Al 2 O 3 exhibited extensive creep at stressing rates ranging from 0.01 to 0.0001 MPa s −1 at 1400 °C, in fact at the slower stressing rates the creep was dominant and the specimens could not be brought to fracture in the four point flexure fixtures. Extensive fractography showed that the failure mode for the sintered β-SiC was indeed a fast-fracture mode at all temperatures and stressing rates, the specimens mostly failing from pores in the microstructure. The siliconized SiC failed partly from pores and partly from metal inclusions at 1100 °C and fast stressing rates, and at 1400 °C at slower stressing rates slow crack growth was observed to occur with the Si-metal inclusions as starting points. The failure modes in SiCp reinforcedAl 2 O 3 changed from fast fracture from residual Al-alloy rich areas to a creep failure at intermediate stressing rates at 1400 °C.

Strength Distribution Changes in a Silicon Nitride as a Function of Stressing Rate and Temperature

Machining damage (a surface flaw) and porous-region-flaw (a volume flaw) populations limited the flexure strengths of a commercially available silicon nitride at 25°C, while these same flaws, along with inclusions, limited flexure strengths at 850°C. The machining damage and porous region flaws were the primary interest in the present study because they caused failure at both temperatures. Censoring revealed that the two-parameter Weibull strength distributions representing each flaw population changed as a function of stressing rate (i.e., dynamic fatigue) and temperature. A decrease in the Weibull scaling parameter is recognized as an indication of slow crack growth or time-dependent strength reduction in monolithic ceramics. Available life prediction codes used for reliability predictions of structural ceramic components consider the slow crack growth phenomenon. However, changes in the Weibull modulus are infrequently observed or reported, and typically are not accounted for in these life prediction codes. In the present study, changes in both Weibull parameters for the strength distributions provided motivation to the authors to survey what factors (e.g., residual stress, slow crack growth, and changes in failure mechanisms) could provide partial or full explanation of the observed distribution changes in this silicon nitride. Lastly, exercises were performed to examine the effects of strength distribution changes on the failure probability prediction of a diesel exhaust valve. Because the surface area and volume of this valve were substantially larger than those of the tested bend bars, it was found that the valve’s failure probability analysis amplified some slight or inconclusive distribution changes which were not evident from the interpretation of the censored bend bar strength data.© 1998 ASME

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained ∼90% α-SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10 × 10−10 s−1) at temperatures ≤1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (∼0.8 to 3 × 10−9 s−1) at stresses below a threshold value of ∼190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n = 17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature ≤1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.