Environment-induced embrittlement: Stress corrosion cracking and metal-induced embrittlement; Environmental embrittlement of iron aluminide alloys

This research program has included two thrusts. The first addressed environment-induced embrittlement in a parallel study of stress corrosion cracking and metal-induced embrittlement. This work has examined (1) mechanical properties as influenced by embrittling environments, (2) fractography and crystallography or transgranular cracking, (3) the mechanics of cracking, (4) the extent and role of local plastic flow, and (5) local chemistry within stress corrosion and metal-induced cracks. The embrittlement of iron aluminide alloys by air was addressed by determining the effect of water and hydrogen upon the mechanical properties. Slow strain rate testing in aqueous environments was carried out at controlled anodic and cathodic potentials. The effect of cathodically charged hydrogen and the effect of subsequent baking were measured. Environmental susceptibility was measured as affected by alloy composition, microstructure and degree of ordering

High Frequency Fatigue Crack Propagation Behavior of a Nickel-Base Turbine Disk Alloy

Fatigue crack propagation tests were conducted on the powder metallurgy nickel-base superalloy KM4 at room temperature. Two different heat treatments were investigated, one which produced a relatively coarse grain size around 55 m, and another which produced a very fine grain size around 6 m. Tests were conducted at 50 Hz and 1000 Hz in an advanced servohydraulic testing machine at R-ratios between 0.4 and 0.7. There was no effect of frequency on the fatigue behavior at room temperature, which is expected in this type of alloy, and this result yields confidence in the reliability of the servohydraulic fatigue testing system. The threshold stress intensity for fatigue crack propagation decreased with decreasing grain size and with increasing R-ratio, again as expected. With increasing grain size, the crack path tortuosity and the crystallographic facet size on the fracture surface both increased substantially, leading to increases in roughness-induced closure and a higher apparent thres..

Thresholds for high-cycle fatigue in a turbine engine Ti-6Al-4V alloy

The characterization of critical levels of microstructural damage that can lead to fatigue-crack propagation under high-cycle fatigue loading conditions is a major concern for the aircraft industry with respect to the structural integrity of turbine engine components. The extremely high cyclic frequencies characteristic of in-flight loading spectra necessitate that a damage-tolerant design approach be based on a crack-propagation threshold, #K TH . The present study identifies a practical lower-bound large-crack threshold under high-cycle fatigue conditions in a Ti--6Al--4V blade alloy (with #60% primary a in a matrix of lamellar a+b). Lower-bound thresholds are measured by modifying standard large-crack propagation tests to simulate small-crack behavior. These techniques include high load-ratio testing under both constant-R and constant-K max conditions, performed at cyclic loading frequencies up to 1 kHz and R-ratios up to 0.92. The results of these tests are compared to the near-thr..

On the mechanism of cross slip in Ni \u3c inf\u3e 3 AI

The mechanical properties of Ll2 intermetallic alloys have been previously described by models based on the assumption that cube cross slip is the rate-limiting step. In this study, it was demonstrated that the cube cross-slip event must be reversible under a change in loading direction. This observation allows the cross-slip models to remain consistent with cyclic deformation data. Additionally, this observation was used as a critical test of the available cross-slip models. It was demonstrated that the rate-limiting step cannot be a total cross-slip event, in which both α/2(110) superpartial dislocations cross slip to the cube plane. Conversely, the limited cross-slip event proposed by Paidar, Pope, and Vitek (PPV) was demonstrated to be consistent with the reversibility constraint. This lends additional experimental support to the PPV model. © 1989 The Metallurgical Society of AIME

The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA 1480

Deformation microstructures in PWA 1480 nickel-base superalloy single crystals were studied in the range of 20 °C to 1100 °C. Similar to previous investigations, superlattice stacking faults were observed after slow strain rate deformation at temperatures between 700 °C and 950 °C. Unlike previous studies, a high density of superlattice stacking faults was observed after deformation at 200 °C and below. The mechanisms of fault formation in the two temperature regimes were different. In the range of 700 °C to 950 °C, single isolated superlattice-intrinsic stacking faults (SISFs) were produced by the decomposition of an a/2(110) matrix dislocation in the γ/γ′ interface. The a/3(112) partial shears the particle, while the a/6(112) Shockley remains in the interface. At 200 °C and below, a high density of faults was produced on closely spaced parallel planes. The most common feature after deformation in this range is the faulted loop, which is most often observed to be a superlattice-extrinsic stacking fault (SESF). These low-temperature faults, along with their temperature dependence, were quite similar to those observed in single-phase Ll22 materials. The available evidence suggests that the low-temperature faults were produced by the dissociation of an a \u3c 11\u3e unit superdislocation into a pair of a/3 \u3c 112\u3e partials. The temperature dependence of the faulting (at low temperatures) was modeled by linear isotropic elasticity, and the results suggest that the SISF energy increases significantly from 20 °C to 400 °C. Multiplanar, overlapping superlattice faults were analyzed with respect to bond violations. This analysis suggested that an antiphase boundary (APB) on top of an SISF has a very high fault energy, similar to that of the complex stacking fault. Therefore, the presence of SISF loops on glide planes promotes further dissociation by the SISF scheme instead of the APB scheme and explains the high density of SESFs and microtwins observed in the deformation structures. © 1991 The Minerals, Metals and Materials Society, and ASM International