Creep of Mechanically-Alloyed NiAl Containing 13 Vol % Mo or Mo 2 C Dispersoids
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KK9.2.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
EXPERIMENT Two materials (referred to as NiAl-Mo and NiAl-Mo 2 C in the following) were synthesized by reactive MA performed for 20 h at room temperature in a vibratory mill (SPEX mixer/miller 8000) under argon. Individual batches of 30 g mixed powder were loaded in a cylindrical steel container with five steel balls weighing 8.35 g each. Pure elemental powders of Ni, Al and Mo were used, together with small amounts of methanol to avoid cold-welding. For NiA1-Mo 2C, pure graphite powder was added after MA. Mechanically-alloyed powders were compacted into steel containers and consolidated by HIP (4 h, 1 120'C, 104 MPa). Some specimens were further annealed for 22 h at 1340 'C under hydrogen. A control NiA1 material without molybdenum or carbon additions was also produced under nominally identical conditions.
Table I. Materials Characteristics. NiA1-Mo 2C Phases (vol.%) NiAl-Mo NiAI-Mo2 C Elements (wt %) NiAI-Mo Matrix Matrix 87.1 87.3 55.6 NiAl Ni 55.9 26.4 Metallic disp. Al 26.3 0.9 MO 8.5 Nio50 2 AI 49.8 Ni 49.3A150.7 stoichiometry Ceramic disp. Dispersoids 2.3 0.4 16.6 A120 3 MO 16.6 2.1 11.4 0.18 0.95 Mo 2C C 4.4 11.8 0.64 0.12 total 0 from chemical analysis and after taking into account formation of alumina.
Creep specimens (4.5 mm x 4.5 mm x 10 mm) were cut by electrical discharge machining.
Compression tests were performed in air between 700 and 900 'C and for stresses between 12 and 300 MPa. Transmission Electron Microscopy (TEM) specimens were prepared using standard procedures More details con(grinding-dimpling-ion milling) and were observed in a JEOL 200CX microscope. 5 cerning materials processing and experimental procedures are given elsewhere.
RESULTS Microstructure Table I gives the chemical composition of the samples. Elevated carbon and oxygen levels were
observed in NiAl-Mo due to several sources (decomposition of methanol, residual oxygen in the argon atmosphere, surface oxide on aluminum powders, surface oxidation after MA). This lead to the formation of small amounts of Mo2C, as confirmed by X-ray diffraction (XRD) and A120 3, as indicated
by Auger analysis. In NiAl-Mo 2C, almost complete reaction of the Mo with graphite to form Mo 2C was achieved during HIP. In both materials, XRD of HIP specimens confirmed complete reaction between nickel and aluminum to form nearly stoichiometric P-NiAl. Table I gives the phase volume fractions calculated on the basis of chemical analysis. Both materials contain a total of about 13 vol.% of second phases. The dispersoids are mainly in metallic form in NiAl-Mo and almost completely in
carbide form in NiAl-Mo 2C. The very fine grain size of the MA powder increased to an average of about 0.7 Pim after HIP, as determined from TEM observations. A typical micrograph is shown in Fig. 1 for the NiAl-Mo material; NiAl-Mo 2 C shows a similar microstructure. Statistical analysis of the dispersoid diameters on the basis of TEM prints for a total number of more than 800 dispersoids indic
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