Microstructure and Mechanical Properties of Laser-Processed Ni-Al-Mo Base Alloys
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Kear,
B.C.
Giessen,
and
M. Cohen,
523
editors
MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LASER-PROCESSED Ni-Al-Mo BASE ALLOYS
DAVID B. SNOW United Technologies Research Center,
East Hartford,
Connecticut
06108
ABSTRACT The addition of molybdenum to nickel base superalloys which contain a Y' volume fraction of • 0.35 allows them to be consolidated from powder by laser processing at solidification rates of 104°C/s. The as-solidified microstructure of these alloys is dendritic, and consists of a fine dispersion -20nm cuboidal Y' in a supersaturated FCC Y matrix. When annealed at 500-7000C, additional age hardening occurs via the precipitation of :lOnm, Dla-structure Ni 4 Mo, D0 2 2 - structure Ni 3 Mo and/ or Pt 2 Mo-structure Ni 2 Mo. This combined strengthening by very fine y' and NixMo dispersions in laser-processed material makes alloys of this type attractive for potential turbine disk applications. Kowever, the 6-15 at % molybdenum required to prevent cracking during laser consolidation renders many of these alloys prone to a previously unreported, embrittling cellular phase transformation, Y + Y' + NixMo - Y' + orthorhombic Ni 3 Mo, at temperatures of S900C.
INTRODUCTION The fabrication of nickel base superalloy turbine disks by the sequential buildup of rapidly-solidified thin layers (the powder-feed LAYERGLAZETM process) requires the use of alloys which possess a unique combination of properties [1, 2]. The complex state of stress produced by post-solidification cooling after LAYERGLAZE deposition causes cracking in conventional, high Y' nickel base superalloys [11; a phenomenon also observed after other forms of laser processing [3-5]. Empirical correlations between alloy composition and cracking propensity demonstrate that the addition of L6 at% Mo, together with a reduction in Y' content to '35 v/o will permit crack-free laser processing. The lower Y' volume fraction reduces the strength, so that an additional strengthening mechanism must be provided. The structure-property relationships observed in several Ni-Al-Mo base alloys [6-7] suggested that the precipitation of Ni4Mo and/or D0 2 2 -structure Ni 3 Mo from the rapidly solidified Y solid solution might provide this additional strengthening at temperatures below 850'C in alloys to which molybdenum contents of 6-12 at% had already been added to prevent cracking during LAYERGLAZE processing. However, this quantity of molybdenum can also promote the onset of an embrittling cellular phase transformation (described subsequently) after exposure to 150C'C for L30 hours. This report describes both the as-rapidly solidified microstructure and the complex sequence of phase transformations, together with associated mechanical properties, observed in two such alloys developed for LAYERGLAZE consolidation [1],
524
in which both NixMo dispersion strengthening and embrittlement by a cellular transformation occur. EJUERIMENTAL PROCEDURE The two laser-fabricable nickel base superalloys described herein are identified as 8-12-3 (Ni-8atZKAl-12atMo-3at%Ta
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