Hardening mechanisms in a dynamic strain aging alloy, HASTELLOY X, during isothermal and thermomechanical cyclic deforma
- PDF / 1,750,089 Bytes
- 11 Pages / 630 x 792 pts Page_size
- 75 Downloads / 269 Views
INTRODUCTION
I. HASTELLOY*
X
is
a
solid-solution-strengthened
*HASTELLOY is a trademark of Stoody Deloro Stellite, Inc., Industry, CA.
Ni-base high-temperature alloy. Its nominal composition in weight percent is Ni-22Cr-18.5Fe-9Mo-l.5Co-0.6W0.1C. H A S T E L L O Y X is readily wrought into sheet and bar and has had wide usage in gas turbine engine combustors and tail pipes as well as in furnace parts. It was chosen as a "simple" high-temperature alloy to be the basis for development of an initial model of isothermal and nonisothermal, or thermomechanical, cyclic deformation response. However, the observed behavior was complex. Cyclic hardening as a function of test temperature showed a dramatic peak between about 200 ~ and 700 ~ [~j Also, hardening response in some thermomechanical cycles was found not to be bounded by the isothermal behaviors at the temperature extremes of the cycle, t21 This work was conducted to identify the physical mechanisms responsible for cyclic hardening in HASTELLOY X to guide constitutive model development. H A S T E L L O Y X is usually supplied solution-treated at 1175 ~ and rapidly cooled in air. It consists only of the face-centered cubic solid solution matrix and a few Mo-rich M6C carbides. The M 6 C carbide is unstable with respect to a Cr-rich M23C 6 carbide during aging at inter-
R.V. MINER, Chief, Advanced Metallics Branch, is with the Materials Division, NASA Lewis Research Center. M.G. CASTELLI, Structures Research Engineer, is with Sverdrup Technology Corporation, NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted June 20, 1991. METALLURGICAL TRANSACTIONS A
mediate temperatures, r3j Ultimate tensile strength (UTS) increases somewhat with aging in the 600 ~ to 800 ~ regime, reaching a m a x i m u m increase of about 30 pct after 8000 hours at 650 ~ or 20 pct after 1000 hours at 760 ~ Cr23C 6 precipitation occurs in many Ni-Cr-base hightemperature alloys in the temperature range of about 800 ~ to 1050 ~ t4~ and in austenitic stainless steels in the range of about 600 ~ to 1000 ~ [51 Also, Cr and/ or C in austenite can produce dynamic strain aging (solute drag) effects during deformation. A broad peak in UTS due to solute drag effects occurs in a lower temperature range, about 200 ~ to 700 ~ than that for actual carbide precipitation, and substantially increased work-hardening rates occur in the center of the regime. [6,7,81 In the solute drag regime, strain rate sensitivity of the flow stress decreases with increasing strain. [7] Within a somewhat narrower region of the solute drag regime, the strain rate sensitivity of the flow stress becomes negative after some critical strain, and unstable flow, or serrated yielding, is initiated. The first explanation of the critical strain was offered by Cottrell. tgJ He proposed that vacancy generation by dislocation interaction was necessary to increase the diffusivity ofAhe solute sufficiently to allow interaction with the moving dislocations. However, problems with the original model later became apparent, a
Data Loading...
