Robert L. Goetz

Unconstrained and constrained tensile flow and fracture behavior of an Nb-1.24 At. Pct Si alloy

The unnotched and notched tensile behavior of the β-phase constituent (Nb with Si in solid solution) of the (Nb)/Nb5Si3 composite has been investigated at room temperature and -196 °C. At room temperature, the unnotched tensile behavior comprises significant strengthening due to Si, low strain-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s−1. At −196 °C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at −-196 °C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with un-notched tensile behavior, indicates that plasticity plays an important role in nucleating cracks, while the high-axial tensile stress component governs crack propagation. These results are used to rationalize the observed toughening and fracture behavior of a (Nb)/Nb5Si3 composite.

Notch fracture inγ-titanium aluminides

The notch fracture behavior of twoγ-titanium aluminide alloys, having duplex and fully lamellar microstructures, has been investigated as a function of notch geometry and test temperature. The unnotched tensile properties and notch fracture loads are used to perform finite element analysis (FEA) to determine triaxial tensile stresses and effective plastic strains in the vicinity of notch roots. These results, together with fractographic examinations of notch failures, indicate that a crack nucleates in the triaxial tensile field when the effective von Mises stress just exceeds the uniaxial tensile yield stress. The high tensile stress component then propagates the nucleated microcrack to failure with local stress intensity reaching the toughness of the material. Thus, both plasticity and high tensile stress are required to cause notch failure.

Effect of flow softening on ring test calibration curves

Abstract The application of the ring test to assess die–workpiece interface friction during forging of a flow softening material was investigated. Calibration curves were generated by conducting finite element method (FEM) simulations of ring forging with different friction factors. The method was validated by conducting isothermal, hot compression tests on rings of an α 2 titanium aluminide alloy for both lubricated and unlubricated interface conditions. The FEM results for the flow softening material were compared to those generated assuming no flow softening. In all cases, the differences between the curves predicted for the flow softening and non-softening behaviors were quite small. Furthermore, for given values of the interface friction factor, the calibration curves were essentially independent of the strain rate sensitivity ( m ) and flow softening.

The role of texturing and recrystallization during grain boundary engineering of Ni-based superalloy RR1000

The application of grain boundary engineering (GBE) techniques to enhance the physical and mechanical properties of Ni-based superalloys could potentially increase the efficiency of turbine engines. Compared to traditional GBE processes that require multiple iterations of room temperature deformation followed by annealing, novel techniques for GBE based on the optimization of the thermal–mechanical processing parameters exhibit more potential for producing complex-shaped Ni-based superalloys components. To date, the formation and microstructural evolution of Σ3 boundaries during thermal–mechanical processing have yet to be fully understood. In this investigation, the effects of deformation texture and strain were systematically investigated in an advanced Ni-based superalloy, RR1000. Using various strains and annealing temperatures, the effects of recrystallization and texturing were quantified. Although texturing was often associated with recrystallization that caused the length fraction of Σ3 boundaries to decrease, the formation of Goss type texture during deformation was found to promote the formation of Σ3 boundaries upon annealing when compared to deformation texturing 〈111〉 parallel to the rolling direction.

Comparative study of high-temperature grain boundary engineering of two powder-processed low stacking-fault energy Ni-base superalloys

Results of high-temperature grain boundary engineering of an experimental, low stacking-fault energy (LSF) Ni-base superalloy were compared to a commercially available superalloy RR1000. Deformation mechanism maps for thermal-mechanical processing were compared along with the resulting length fractions of Σ3 boundaries following sub-solvus and super-solvus annealing. Compared to the hot deformation processing characteristics of RR1000, lowering the stacking-fault energy reduces dislocation mobility and expands the range of temperatures and strain rates over which dislocation-based plastic flow mechanisms were operative in the LSF alloy. For both alloys, processing conditions conducive to dislocation-based plasticity allowed for the storage of strain energy within the microstructure that was utilised for strain-induced boundary migration (SIBM) and the formation of Σ3 boundaries upon annealing. Based on the results of this study, alloying changes that serve to reduce the stacking-fault energy of Ni-base superalloys also make the alloys more amenable for grain boundary engineering techniques that utilised hot deformation.

Deformation and Recrystallization during Thermomechanical Processing of a Nickel-Base Superalloy Ingot Material

Abstract : The deformation response and recrystallization behavior of a coarse, columnar-grain superalloy ingot material, Waspaloy, with a fiber texture were established. For this purpose, isothermal hot compression tests were performed on cylindrical and doublecone samples at supersolvus temperatures under both monotonic (constant strain rate) and multi-hit conditions. Plastic flow showed a noticeable dependence on test direction relative to the columnar-grain orientation; the observed anisotropy in peak flow stress and flow softening were explained on the basis of the evolution of crystallographic texture during recrystallization. Similarly, anisotropy in dynamic recrystallization kinetics with respect to test direction was interpreted in terms of the effect of initial texture on the plastic work imposed per increment of macroscopic strain. Nevertheless, the broad kinetics for the coarse-grain, ingot material deformed under both monotonic and multi-hit conditions were comparable to those previously measured for fine-grain, wrought Waspaloy. Such an effect was attributed to the beneficial influence of the nucleation of recrystallization at both grain boundaries and carbide particles in the ingot material. In addition, a spatial non-uniformity in recrystallization was found in the ingot material and was interpreted in the context of the grain-boundary character and non-uniform strain at the grain/intragrain scale. A suite of tools being developed to model recrystallization phenomena during the breakdown of superalloy ingots is described. These tools include a mechanistic cellular automata; a mesoscale, mechanism-based model; and the crystal-plasticity finite-element method.

An experimental and theoretical investigation of the rapid consolidation of continuously reinforced, metal-matrix composites

The feasibility of the rapid consolidation of T-14Al-21Nb/SCS-6 foil/fiber/foil composites using a forging approach was established as an alternative to slower and more expensive processes such as those based on hot isostatic pressing (HIP) or vacuum hot pressing (VHP). A firm basis for the technique was developed through theoretical analyses of temperature transients, forging pressures, and fiber fracture. These analyses demonstrated that there exists an optimal forging speed at which the consolidation stresses are a minimum. It was also shown that the flow stress of the encapsulation material relative to that of the densifying layup is an important consideration in achieving full consolidation during forging. Specifically, the difference in flow stress between the two materials influences the magnitude and sign of the in-plane (secondary) stresses that are developed during forging and therefore the rate of pore closure during the latter stages of the process. With regard to fiber fracture, analyses were performed to estimate the axial and tangential stresses during rapid consolidation. The theoretical work was validated by experimental trials using the Ti-14Al-21Nb matrix/silicon carbide fiber system. Measured forging pressures were in good agreement with predictions. Fiber fracture observations indicated that tangential tensile stresses developed in the fiber control failure; a forging window to avoid such failures was thus developed. Finally, it was demonstrated that matrix microstructures and mechanical properties similar to those of conventionally consolidated Ti-14Al-21Nb/silicon carbide composites can be achieved by the forge-consolidation technique.