The eutectic composition Ni-33Al-33Cr-1Mo has been directionally solidified (DS) via a modified Bridgman technique at rates ranging from 7.6 to 508 mm/h to determine if the growth rate affects the ...mechanical properties. A growth rate of 127 mm/h represents the best combination of fast processing and mechanical properties for this system.
The compressive properties of near 〈0
0
1〉 and 〈1
1
1〉-oriented NiAl–2Cr single crystals and near 〈0
1
1〉-oriented NiAl–6Cr samples have been measured between 1100 and 1500
K. The 2Cr addition ...produced significant solid solution strengthening in NiAl, and the 〈1
1
1〉 and 〈0
0
1〉 single crystals possessed similar strengths. The 6Cr crystals were not stronger than the 2Cr versions. At 1100 and 1200
K plastic flow in all three Cr-modified materials was highly dependent on stress with exponents >10. The 〈0
1
1〉-oriented 6Cr alloy exhibited a stress exponent of about 8 at 1400
K and 1500
K; whereas both 〈0
0
1〉 and 〈1
1
1〉 NiAl–2Cr crystals possessed stress exponents near three which is indicative of a viscous dislocation glide creep mechanism. While the Cottrell–Jaswon solute drag model predicted creep rates within a factor of three at 1500
K for 〈0
0
1〉-oriented NiAl–2Cr; this mechanism greatly over predicted creep rates for other orientations and at 1400
K for 〈0
0
1〉 crystals.
The 1100–1500
K slow plastic strain rate compressive properties of 〈0
0
1〉 oriented NiAl–3.6Ti single crystals have been measured, and the results suggests that two deformation processes exist. While ...the intermediate temperature/faster strain rate mechanism is uncertain, plastic flow at elevated temperature/slower strain rates in NiAl–3.6Ti appears to be controlled by solute drag as described by the Cottrell–Jaswon solute drag model for gliding
b=a
0
〈1
0
1〉 dislocations. While the calculated activation energy of deformation is much higher (∼480
kJ/mol) than the activation energy for diffusion (∼290
kJ/mol) used in the Cottrell–Jaswon creep model, a forced temperature-compensated power law fit using the activation energy for diffusion was able to adequately (>90%) predict the observed creep properties. Thus, we conclude that the rejection of a diffusion controlled mechanism cannot be simply based on a large numerical difference between the activation energies for deformation and diffusion.
A study of the 1300 K compressive and tensile creep properties of 001-oriented NiAl–1Hf (D209) single crystals has been undertaken. Neither post homogenization cooling treatment, minor chemical ...variations within an ingot or from ingot-to-ingot, nor testing procedure had a significant effect on mechanical behavior; however a heat treatment which dissolved the initial G-phase precipitates and promoted formation of Heusler particles led to a strength reduction. Little primary creep was found utilizing direct measurement of strain, and a misorientation of 18° from the 001 did not reduce the creep strength. The effects of heat treatments on properties and a comparison of the flow stress–strain rate data to those predicted by the Jaswon–Cottrell solid solution hardening model indicate that the 1300 K strength in NiAl–1Hf single crystals is mainly due to precipitation hardening mechanisms.
In an effort to superimpose two different elevated temperature strengthening mechanisms in NiAl, several lots of oxide dispersion strengthened (ODS) NiAl powder have been cryomilled in liquid ...nitrogen to introduce AlN particles at the grain boundaries. As an alternative to cryomilling, one lot of ODS NiAl was also roasted in nitrogen to produce AlN. Both techniques resulted in hot extruded AlN-strengthened ODS NiAl alloys which were stronger than the base ODS NiAl between 1200 and 1400 K. However neither the cryomilled nor the N
2-roasted ODS NiAl alloys were as strong as cryomilled binary NiAl containing like amounts of AlN. The reason(s) for the relative weakness of cryomilled ODS NiAl is not certain; however the lack of superior strength in N
2-roasted ODS NiAl is probably due to its relatively large AlN particles.
Three directionally solidified (DS'ed) bars of NiAl-14.5(at%)Ta possessing a non-uniform distribution of approximately 10 vol% of dendritic NiAl in a lamellar NiAl and NiAlTa eutectic mixture have ...been tested in air at elevated temperatures. Samples from two bars were subjected to constant velocity and constant load compression testing between 1200 and 1400 K, while specimens cut from the third bar were creep rupture tested in tension at 1200 and 1300 K. Although a number of samples failed on loading at 1200 K, five successful tensile creep tests were conducted with elongations of 10% or more prior to failure. A Larson-Miller analysis indicated that the stress rupture properties of DS'ed NiAl-14.5Ta were equivalent to the Ni-based superalloy René80 and better than those of first generation NiAl-based single crystals and a particulate AlN reinforced NiAl. In spite of the inhomogeneous microstructure, no dependency on either test mode or bar of material was found in the flow stress-strain rate-temperature properties. Comparison of the 1300 and 1400 K creep properties of several directionally solidified NiAl-based alloys indicated that DS'ed NiAl-14.5Ta is one of the strongest materials studied to date; however, its inherent brittleness, as reflected by a low room temperature fracture toughness and high brittle to ductile transition temperature (BDTT), appear to limit its usefulness.
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