Evaluation of mechanical properties of a low-cobalt wrought superalloy

In the late 1970's and early 1980's, cobalt was subjected to significantly supply and market pressures. Those pressures caused renewed attention to the use of cobalt in aircraft engines. A NASA-sponsored program called Conservation of Strategic Aerospace Materials (COSAM) was created in response to the supply problems with cobalt and other aerospace metals. Among the work performed in the COSAM program and simultaneously by others were several studies on laboratory-size heats of wrought nickel-base super-alloys. These studies suggested that the cobalt levels of the alloys might be reduced by about half, with minimal negative impact on mechanical properties. The Lewis Research Center procured a 1,365-kg (3,000-lb) heat of a modified Waspaloy having a reduced cobalt level. This article reports the results of a program performed at four gas turbine manufacturers which evaluated the mechanical properties of forgings fabricated from that heat. The alloy chemistry selected reduced the nominal cobalt level from 13.5 to 7.75 wt %. To compensate for the anticipated strength reduction caused by a slight reduction in the amount of [gamma][prime], the nominal aluminum was increased from 1.3 to 1.5% and the titanium was increased from 3.0 to 3.2%. The increase in aluminum and titanium were intended to increase the amount of [gamma][prime] in the alloy. Tensile, creep-rupture, low-cycle fatigue, and cyclic crack growth tests were performed. In addition the effect of hydrogen on the alloy was determined. It was concluded that, in the event of a cobalt shortage, a low-cobalt modification of Waspaloy alloy could be substituted for Waspaloy with little development in those applications that are not creep-rupture limited. With some additional development to better control the grain size, it is probable that most of the current Waspaloy requirements might be met with a lower cobalt alloy.