Microstructure Approach to Property Optimization in Wrought Superalloys

Abstract

The effects of structure control available from the precipitating phases in wrought superalloys were demonstrated in a recent review. It was shown that criteria for selection of superalloys should be reviewed periodically to establish that they reflect the state of the art. For example, expensive materials may have been specified where lower cost alloys could do a job equally well. This is especially true for superalloys in applications where properties up to about 1200°F are critical. This paper reviews some results of the continuing study by superalloy metallurgists to reveal means of “retrograding” elevated temperature requirements to lower cost compositions that, combined with proper processing, will meet property requirements. It is shown that microstructure studies, combined with knowledge of phase relationships, prove to be an invaluable tool. Recent examples of improved hot-work behavior, heat-treat response, and property development are reviewed. Effects of thermomechanical processing are also discussed.

The Time-Temperature-Transformation Behavior of Alloy 706

“…By similar argument, prior precipitation of a phase richer in Ti (such as q) would likely enhance localized precipitation of Nb-rich y" (and possibly s). Further, strain induced rl precipitation will occur if mini-grain processing is used for grain refinement (9). Therefore, the precipitation responses encountered during the multistep heat treatment of large commercial parts may differ from the isothermal transformations observed in this controlled study.…”

Microstructural Features Leading to Enhanced Resistance to Grain Boundary Creep Cracking in ALLVAC 718Plus

“…As can be seen in Table I, ALLVAC 718Plus can be considered a cross between 718 and WASPALOY. The reduction of Fe and the addition of 10 wt pct Co along with the increase in Al content by 1 wt pct were found to promote the formation of the ordered L1 2 c¢ phase (Ni 3 (Al,Ti)) as opposed to the DO 22 bct c¢¢ phase (Ni 3 Nb), [2][3][4] which is the predominant strengthening phase in 718. Also, 1 wt pct tungsten is added in order to increase the high-temperature deformation resistance of ALLVAC 718Plus.…”

The Role of Niobium in Wrought Precipitation-Hardened Nickel-Base Alloys

“…While it can contribute to grain control, its contribution to strength is doubtful (10). Another phase that can form directly from γ′ is eta phase, η (Ni 3 Ti), due to high levels of titanium and niobium in the alloy.…”

“…To establish guidelines for forging, the re-solution temperatures for the common phases present in alloy 706 have been established (23). Both γ′ and γ′′ re-solution at ∼885°C (slightly lower for longer times), phases η and δ at ∼955°C (somewhat higher if the material is adversely segregated), grain boundary Laves at ∼1065°C or higher if the particles are large.…”

“…There is compositional transition of γ′′ (Ni 3 Nb) [orthorhombic, DO 22 24 . Precipitation of η to control grain size during forging is a common practice (23,27,31). Experience has shown that working alloy 706 below the η solvus temperature (∼954°C) significantly increases flow stress and consequently forge press requirements.…”