On the influence of grain morphology on creep deformation and damage mechanisms in directionally solidified and oxide di
- PDF / 5,295,678 Bytes
- 12 Pages / 594 x 774 pts Page_size
- 94 Downloads / 221 Views
I.
INTRODUCTION
T H E strength of modern cast nickel-base superalloys like CM 247 LC depends on a mechanism of precipitation hardening in which a high volume fraction of 7' cuboids are coherently precipitated in the 3' matrix, which appears as a channel phase between the 7' cuboids. Other elements are soluble in these two basic phases. All nickel-base superalloys contain chromium, primarily for oxidation and sulfidation resistance at elevated temperatures. Directionally solidified (DS) superalloys exhibit large, elongated grains which grow during solidificationtl.2] in an appropriate temperature gradient at the solid/liquid interfaceJ2] In the direction of the elongated grains, DS materials have a higher creep strength than conventionally cast (CC) materials with an equiaxed grain microstructure. On the other hand, DS materials are weaker than superalloy single crystals (SX).tl] But since it is easier to produce DS microstructures than it is to produce SX ones, especially in large geometric dimensions,t1] there still is considerable interest in DS superalloys, mainly for blade materials in land-based gas turbines. The DS superalloys are generally creep ductile with elongations to rupture of the order of 20 to 30 pct. Oxide dispersion strengthened (ODS) alloys like MA 760 represent another class of nickel-base superalloys with elongated macrograins. They are produced by powder met-
B. DeMESTRAL, Research Assistant, and G. EGGELER, Professor, are with the Fakultfit fiir Maschinenbau, Institut ~ Werkstoffe, Ruhr-Universitat Bochum, Institute of Materials, Ruhr-University,D-44780 Bochum, Germany. H.-J. KLAM, Materials Engineer, is with the Materials Technology Department, ABB Powder Generation Ltd., 5401 Baden, Switzerland. This article is based on a presentation made at the "High Temperature Fracture Mechanisms in Advanced Materials" symposium as part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee. METALLURGICALAND MATERIALSTRANSACTIONS A
allurgy. The ODS nickel-base superalloys have high creep strengths at temperatures in excess of 1000 ~ f3J The processing of this class of materials involves (1) mechanical alloying of powders, (2) consolidation of the alloyed powders by hot extrusion, and (3) zone annealing, which increases the grain size by four orders of magnitude and produces a highly elongated grain structure,t41 Zone annealing results in directional recrystaUization and can be considered as the solid state counterpart of directional solidification. The microstructure of the ODS superalloy MA 760 consists of a 3/matrix with 7' precipitates. In addition, MA 760 contains small Y203 oxides which are introduced during mechanical alloying and which provide effective obstacles to dislocation motion at temperatures in excess of 1000 ~ At these temperatures, ODS superalloys are creep brittle and can fail with rupture strains as small as 2 pct. t51 For both DSt61 and ODS alloys,t71 it was shown that the grain morphology is impor
Data Loading...
