William E. Luecke

High-temperature properties of liquid-phase-sintered α-SiC

Abstract We have characterized the high-temperature subcritical crack growth and oxidation resistance of a liquid-phase-sintered (LPS) SiC with 20% volume fraction yttrium aluminum garnet (YAG) second phase. Constant stress-rate testing in air in the temperature range 1100–1300°C yielded a crack growth exponent, n=38.9±9.9 and an activation energy, Qscg=(380±237) kJ mol−1. Oxidation followed parabolic kinetics in the temperature range 1100–1300°C with an activation energy, Qox=(246±33) kJ mol−1. At 1350°C reaction between the growing oxide layer and the YAG second phase produced a low-melting eutectic, resulting in accelerated oxidation. Below 1100°C, oxidation rates were also anomalously high for reasons we do not understand. In the intermediate temperature range, both the oxidation and subcritical crack growth resistance compare favorably with other silicon carbides.

Mechanical Properties of Austenitic Stainless Steel Made by Additive Manufacturing

Using uniaxial tensile and hardness testing, we evaluated the variability and anisotropy of the mechanical properties of an austenitic stainless steel, UNS S17400, manufactured by an additive process, selective laser melting. Like wrought materials, the mechanical properties depend on the orientation introduced by the processing. The recommended stress-relief heat treatment increases the tensile strength, reduces the yield strength, and decreases the extent of the discontinuous yielding. The mechanical properties, assessed by hardness, are very uniform across the build plate, but the stress-relief heat treatment introduced a small non-uniformity that had no correlation to position on the build plate. Analysis of the mechanical property behavior resulted in four conclusions. (1) The within-build and build-to-build tensile properties of the UNS S17400 stainless steel are less repeatable than mature engineering structural alloys, but similar to other structural alloys made by additive manufacturing. (2) The anisotropy of the mechanical properties of the UNS S17400 material of this study is larger than that of mature structural alloys, but is similar to other structural alloys made by additive manufacturing. (3) The tensile mechanical properties of the UNS S17400 material fabricated by selective laser melting are very different from those of wrought, heat-treated 17-4PH stainless steel. (4) The large discontinuous yielding strain in all tests resulted from the formation and propagation of Lüders bands.

Creep Damage Mechanisms in Si3N4

Cavitation was characterized on commercial grades of silicon nitride by transmission electron microscopy, small angle X-ray scattering and precision density measurements. The type of cavitation depends on both the microstructure and the test temperature. Three types of cavities are observed: isolated lenticular cavities at two-grain boundaries; crack like cavities also at two-grain boundaries; and interstitial cavities at multi-grain junctions. These cavities form in all grades of silicon nitride studied. As a result of cavitation, silicon nitride creeps up to 100 times faster in tension than in compression for the same applied stress. As much as 85% of the deformation in tension can be accounted for by cavity formation.

Instrumented indentation and ultrasonic velocity techniques for the evaluation of creep cavitation in silicon nitride

Instrumented indentation and ultrasonic wave velocity techniques combined with precise density change measurements and transmission electron microscopy (TEM) were used to investigate the changes of elastic moduli in silicon nitride after tensile deformation up to 3%. Linear dependencies on strain were also found for the degradation of the indentation modulus, longitudinal and transverse ultrasonic wave velocities, Youngs, shear and bulk moduli and Poissons ratio. The results obtained by indentation technique and ultrasonic method were essentially identical. TEM observation confirmed that multigrain junction cavities were responsible for the density changes and the elastic moduli degradation. The density changes were linearly proportional to tensile strain with the slope of 0.75. Thus, cavitation is the dominant creep mechanism in silicon nitride studied. Instrumented indentation and ultrasound velocity techniques are suitable for non-destructive monitoring of creep damage accumulation in ceramic components.

Importance of Cavitation to the Creep of Structural Ceramics

Current interest in structural ceramics for heat exchangers, power generation turbines and automotive engines is based on their superior behavior at high temperature. For the same stress and expected lifetime, structural ceramics can be used at temperatures as much as 200 to 300°C higher than the best metallic alloys available today.1 Ceramics are also more inert chemically and have a greater resistance to wear and erosion than structural alloys. The principal impediment to widespread use of structural ceramics is their brittle behavior, which at low temperatures results in thermal and mechanical shock sensitivity and in a large variability in strength, principally due to the presence of mechanical defects in the ceramics. At high temperatures, the same brittle behavior results in the generation of mechanical damage that leads to a time-dependent failure. Although mechanisms of damage nucleation and growth may be complex, the relationship between lifetime at high temperature and creep rate is given by the Monkman-Grant2 relationship, in which lifetime is inversely proportional to creep rate. Thus, the lifetime of high-temperature ceramics can be improved by increasing creep resistance, a fact that has been recognized and used over the past 20 years to improve the operating temperature of silicon nitride.