MECHANICAL PROPERTIES AND DEFORMATION MECHANISMS OF TERNARY γ-γ’-δ NICKEL-BASED SUPERALLOYS

The primary drive towards the continued development of nickel-based superalloys lies in the turbine industry, both for propulsion and power generative uses, where approximately 90% of such materials are used. The ongoing trend is to increase the temperature, increase the rotational speed, and decrease the weight of turbines to increase their efficiency. Thus, much of the improvement of the efficiency of these machines rests on what materials are available to the industry. This has led to the design of superalloys specifically tailored to withstand the varied and increasingly demanding properties of turbine parts. Turbine discs, as one such part, experience substantially lower temperatures than the turbine blades, thus the creep properties of materials to be used in such an application are not quite as important. However, in the drive to increase turbine rotation speed and reduce weight, the industry is looking for materials which can withstand higher stresses. It is with this goal in mind that the material focused on in this work, V204H, was designed. The material utilizes a novel, three-phase microstructure consisting of γ-γ’-δ rather than a conventional two-phase γ-γ’ microstructure. It has been theorized that the addition of δ precipitates, formed through the addition of niobium, will behave as a reinforcement phase as in a particulate composite. The material was also heavily alloyed to increase strengthening and improve creep behavior via the addition of elements which decrease diffusion rate and deter grain boundary damage. It was discovered that this material has a fourth phase between the γ’ precipitates, and this was identified using SAD to be γ”. Flow stress tests over a range of temperatures and creep tests, both performed in compression, were conducted. These properties were seen to be exemplary for use in a turbine disc. It was, however, found that such properties deteriorated when the testing was conducted in tension. This xvM.S. in in Materials Science and Engineering, December 201