The Effect of Thermomechanical Processing on the Microstructure and Creep Behavior of Udimet Alloy 188

Abstract

Udimet 188 alloy was subjected to thermomechanical processing in attempt to understand the effects of cold-rolling deformation on the microstructure and tensile-creep behavior. Commercially available sheet was cold rolled to varying amounts of deformation (between 5%-35% reduction in sheet thickness) followed by a solution treatment at 1191ºC for one hour followed by air cooling. This sequence was repeated four times to induce a favorable grain boundary character distribution containing a high volume fraction of low-energy grain boundaries. The resultant microstructure was characterized using electron backscattered diffraction. The effect of the thermomechanical processing treatment on the hightemperature (760-815ºC) creep behavior was evaluated. Conventional lever-arm creep experiments were performed in an open air environment. The measured creep stress exponents (5.7-6.4) suggested that dislocation creep was dominant at 760ºC for stresses ranging between 100-220MPa. The material exhibited a significant extent of grain boundary cracking. The thermomechanical processing treatments which resulted in the greatest fractions (~0.8) of special grain boundaries (low-angle boundaries + coincident site lattice boundaries) also exhibited the lowest creep rates. Thus a correlation was exhibited between the grain boundary character distribution and the minimum creep rates. Creep rupture experiments were performed at T=815ºC and σ=165MPa and the thermomechanical processing treatment which resulted in the greatest special boundary fraction also resulted in the greatest average time-to-rupture.

In-Situ Tensile-Creep Deformation Observations of a Cobalt-based Superalloy

“…The surface of the sample was imaged in-situ during the creep experiment without interrupting the test. The alloy was thermomechanically processed using a recipe that has previously shown a beneficial increase in creep resistance compared to other thermomechanical processing treatments [2]. In particular, the alloy sheet was subjected to three cold rolling passes of 35% deformation and a fourth pass of 10%.…”