The Effect of Cooling Rate on High-Temperature Precipitation in a Powder-Metallurgy, Gamma/Gamma-Prime Nickel-Base Super
- PDF / 1,518,405 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 39 Downloads / 201 Views
TRODUCTION
GAMMA-PRIME strengthened, nickel-base superalloys are among the most common materials used for high-temperature applications in land-base and aerospace power and propulsion systems. As such, understanding and control of precipitation behavior is very important. The size and volume fraction of gamma-prime precipitates play a key role in controlling strength, fatigue resistance, and other properties.[1–3]
S.L. SEMIATIN, D.W. MAHAFFEY and J.S. TILEY are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433-7817. Contact e-mail: [email protected] N.C. LEVKULICH and O.N. SENKOV are with UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. Manuscript submitted May 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The synthesis of nickel base superalloys hardened by the fcc gamma-prime phase (based on Ni3(Ti, Al)) is usually accomplished by one of three distinct routes based on ingot casting, powder production, or part casting. The ingot-metallurgy (IM) route is typically applied to alloys having low-to-moderate amounts (~ 10 to 30 pct) of gamma prime in the fully-hardened state and for which macrosegregation and thermal cracking can be avoided. In such materials, precipitation is often minimal during cooling following solution treatment at temperatures either high in the gamma + gamma-prime phase field or above the gamma-prime solvus. Hardening relies therefore on a final aging heat treatment. In contrast to IM superalloys, more-highly-alloyed, powder-metallurgy (PM) superalloys (having ~ 40 to 65 pct gamma prime) often decompose during cooling following solution treatment. For material that is supersolvus solution treated, for example, precipitation usually occurs in several ‘‘bursts’’. At a relatively-small undercooling relative to the gamma-prime solvus, secondary gamma prime (so-called to differentiate it from
the coarse, primary gamma-prime dispersion developed during subsolvus extrusion or isothermal forging) nucleates over a narrow temperature interval. During continued cooling, the secondary gamma prime grows via diffusion to a size typically in the range of 100 to 500 nm. At temperatures several hundred kelvins below the solvus, diffusional growth becomes sluggish, matrix supersaturation increases again, and an additional burst of gamma-prime precipitates, typically referred to as tertiary (with a size of approximately ~ 10 to 50 nm), is formed. Following cool-down, an isothermal aging treatment at a temperature approximately 50 K to 200 K (50 C to 200 C) above the service temperature is common. This aging treatment produces additional fine (~ 10 to 20 nm) gamma-prime precipitates. Cooling following the solution treatment of large PM components must be carefully controlled to avoid thermal (quench) cracking; the propensity for such cracking increases with gamma grain size and solvus temperature.[4] Thus, the problem is most severe for alloys having a high volume fraction of gamma prime and a high solvus temperature. Durin
Data Loading...
