The mechanical properties of binary Ni76.6Al23.4 single crystalline specimens have been studied in compression test over a wide range of temperature (293–1100K). The resolved proof stress (τ0.2%) and the corresponding work-hardening rate (θ0.2%) have been measured as a function of temperature. In addition, a technique of repeated stress relaxations has been used to investigate the related microscopic activation volume (V0.2%) and to characterize the variation in the density of mobile dislocations that occurs during such transient tests. It is observed that not only τ0.2% exhibits an anomalous behavior with temperature but also θ0.2%, while the stress dependence of V0.2% may be considered as normal, i.e. V0.2% is a monotonie decreasing function of stress.

Polycrystals of stoichiometric Ni3Ai are brittle in air at room temperature, at least at strain rates from 10-4 s-1 to 7 s-1. This is indicative of intrinsic brittle behavior. Nickel-rich (Ni24A1) polycrystals, on the other hand, undergo a brittle-to-ductile transition upon raising the strain rate to above about 10-2 s-1. It is argued that the transition reflects the suppression of environmental embrittlement, not of the intermetallic per se but of a Ni-rich solid solution that decorates the grain boundaries of the off-stoichiometric alloy.

High temperature creep behavior of a nickel-rich Ni3(Al,Ta) with the L12 structure is investigated in order to clarify the influence of crystallographic orientation with respect to the stress axis. The single crystals with four different orientations are deformed in compressive creep at temperatures ranging from ‘M 23 to 1273 K under a constant load, initial shear stress being 35 to 120 MPa for (111)[101] slip system. The results show a distinct orientation dependence of creep strength, although shape of creep curves, stress exponent and the activation energy seem to be independent of the orientation. It is shown, however, the internal stress, being measured by strain transient dip tests, is found to be orientation dependent and the creep behavior is independent on orientation if it is interpreted using the effective stress instead of the applied shear stress.

This study examines the time-dependent deformation of Ni3(Al, Ta) at room temperature. Tension creep experiments have been performed on single crystals with one {111}<101> slip system active at the start of the test, where the applied stress ranged from 66.4 MPa (the measured 0.01% flow stress) to 143 MPa (which produced approximately 9% plastic strain). All creep curves displayed primary creep leading to eventual exhaustion, where the measured creep strain declined at a rate faster than predicted for logarithmic creep. However, no correlation between the applied stress and the form of the declining creep rate can be made at this rime. Many creep curves can be obtained from one sample, as the creep curves from both virgin samples and samples with prior deformation history (at the same test stress) were indistinguishable. At the beginning of an incremental creep test, where the stress is increased by a small amount to reinitiate plastic flow in an exhausted sample, a significant retardation of the plastic response of the sample occurred when the stress increment was below 4 MPa. Preliminary TEM studies of a sample strained to 6% suggest that room temperature creep tests may not be ideal for examining the flow of Anti-Phase-Boundary (APB) dissociated dislocations.

The development of more efficient engines and power plants for future supersonic transports depend on the advancement of new high-temperature materials with temperature capabilities exceeding those of Ni-based superalloys. Having theoretical modelling techniques to aid in the design of these alloys would greatly facilitate this development. The present paper discusses a successful attempt to correlate theoretical predictions of alloy properties with experimental confirmation for ternary NiAl-Ti alloys. The B.F.S. (Bozzolo- Ferrante-Smith) method for alloys is used to predict the solubility limit and site preference energies for Ti additions of 1 to 25 at. % to NiAl. The results show the solubility limit to be around 5 % Ti, above which the formation of Heusler precipitates is favored. These results were confirmed by transmission electron microscopy performed on a series of NiAl-Ti alloys.

The quasi-binary eutectic NiAl-9% Mo with faceted molybdenum fibers was subjected to both thermal annealing conditions and to annealing under thermal cycling conditions to determine the microstructural stability of this alloy. The static temperature tests were run at 0.85TM - 0.97TM in an argon gas atmosphere. The thermal cycling tests were performed between temperatures of 700°C and 1200°C by induction heating disk-shaped specimens in an argon gas atmosphere using time-temperature heating and cooling profiles to approximate potential engine applications. To quantify microstructural changes, the fiber size and size distribution and number of fibers per unit area were measured as a function of time at temperature. The overall results demonstrate that the directionally solidified eutectic NiAl-9Mo subjected to thermal fatigue conditions exhibits cell boundary coarsening and large shape changes, whereas the microstructure under static stress-free annealing is stable.

In situ experiments have been conducted in NiAl single crystals in the soft orientation, at room temperature, in order to investigate the glide process of [100] dislocations. The results show that dislocations have a kinked shape in agreement with what is expected from anisotropie elasticity. Intensive cross-slip is observed, leading to slip in non-crystallographic planes and dipole formation. The ductility of the alloy is then discussed.

The superplastic behavior of a NiAI and Ni3Al duplex alloy was investigated. It was found that the alloy exhibits superplastic behavior over a narrow temperature range, from 975 °C to 1025°C at the strain rate of 1.52 × 10-4 s-1. A maximum tensile elongation of 149% was obtained at 1000°C with the strain rate sensitivity up to 0.375. The superplastic deformation of the duplex alloy can be approximately described by an empirical equation of the form: ε = Ao2.67 exp(-303,000 / RT). Optical microstructure and TEM observation show that the superplastic behavior mechanism of the investigated alloy is a process of continuous recovery and recrystallization during deformation.

In an effort to understand dislocation mobility in stoichiometric NiAl single crystals, in-situ tensile deformation experiments have been performed in a transmission electron microscope. Commercially pure and high purity single crystals with <001> and <110> orientations have been examined. Two different thermal treatments were adopted in order to effect the mechanical response. Dislocation motion was observed in all samples. Pre-existing dislocations, either isolated or tangled, were not observed to move at any point leading up to sample failure. Cross-slip of the mobile dislocations was observed in some cases. In commercially pure single crystals, dislocations were found to move at a much slower rate and uniform manner in contrast to motion in high purity single crystals which occurs by rapid jumps.

A comparison of published creep data on binary NiAl showed that there is a discrepancy in the reported magnitudes of the stress exponents, n, which usually vary between about 4.5 and 6.5. In general, a close examination of the data suggested that n ≈ 4.5 for cast materials arid 6.5 for powder-metallurgy extruded NiAl. Constant load compression creep tests were conducted on a cast and extruded binary NiAl between 800 and 1200 K over a wide range of initial applied stresses varying between 4.0 and 200 MPa. The microstructures were characterized by transmission electron microscopy. The observed variations in the creep behavior of the extruded cast and powder-metallurgy NiAl appeared to be due to a grain size effect. Despite similarities in the values of n, no significant substructure was observed in most of the grains in the cast and extruded specimens at 1100 and 1200 K in contrast to the PM-extruded alloy, which revealed a wide range of substructural features in the power-law creep region. However, extensive subgrain formation and dislocations were widely observed at lower temperatures and higher stresses in the cast and extruded material.

We have investigated the high-temperature deformation behavior of the solid-solution strengthened alloy Ni-47.5Al-2.5Ti. Single crystals were deformed in compression in the “hard” <001> and “soft” <111> orientations, at temperatures between 900°C and 1200°C. The results show that Ti has a very powerful solute strengthening effect in NiAl. The creep rates for the solid-solution alloy were observed to be three to four orders of magnitude lower than for the stoichiometric material. We discuss our efforts to understand this solid-solution strengthening effect. We have studied high-temperature deformation transients in an effort to determine whether solute drag effects contribute to the creep resistance of this solid solution. In addition, we have examined the solute size effect of Ti as it replaces Al on the Al sub-lattice. We discuss the probable mechanism of creep of this alloy in light of TEM observations of the dislocation structures in creep-deformed crystals.

In an effort to superimpose two different elevated temperature strengthening mechanisms in NiAl, one lot of an oxide dispersion strengthened (ODS) NiAl powder has been milled in liquid nitrogen (cryomilled) to introduce A1N particles at the grain boundaries, and a second lot of ODS powder was simply roasted in gaseous nitrogen as an alternative means to produce AIN reinforced grain boundaries. Powder from both of these lots as well as the starting material have been consolidated by hot extrusion and tested at 1300 K. Both nitrogen roasting and cryomilling produced AIN within the ODS NiAl matrix which strengthened the alloy; however, based on the AIN content, cryomilling is more effective.

Dislocation substructure in NiAI single crystals oriented for single slip and deformed at ambient temperature has been studied using weak-beam transmission electron microscopy. Deformation is localized in bands that consists mostly of near-edge dislocations, with an interspersion of a high density of elongated prismatic loops. Pure screw dislocations are not observed, but dislocations having “zigzag” configurations that are near-screw in orientation are present. A high density of jogs is observed on both near-edge and zigzag dislocation segments. The mechanisms for the development of this substructure are discussed, emphasizing the role of double cross slip and resulting glissile and sessile jogs of varying heights.

Dynamic strain aging has been investigated in polycrystalline NiAl alloys containing combined additions of interstitial and substitutional impurities. The results indicate that strain aging can be enhanced in polycrystalline alloys containing concentrations of silicon or iron in excess of 0.15 at.% in addition to conventional levels of carbon. This co-doping leads to very dramatic strain aging events, comparable to those typically observed in single crystals. This effect will be discussed with respect to conventional theories of strain aging.

Compression tests have been performed on hard-oriented Ni-44A1 single crystals for several temperatures and strain levels. There is a slip transition from a<111> type slip to non-a<111> type slip which corresponds with the observation of yield points. Transmission electron microscopy studies provide evidence for decomposition of a<111> dislocations into a<101> and a<010> dislocations near the “knee” of the yield strength curve. The mechanism of this slip transition and the nature of the dislocation processes both below and above the “knee” are described.

The compressive ductilities of Ni3Al-Zr-B base alloys with sole addition of magnesium (0.02∼0.06wt.%) and combined addition of magnesium(0.02wt.%) and silicon(0.54∼1.08wt.%) respectively responding to strain rate rising from 10-4sec-1 to 10-1 sec-1 have been studied in a high temperature range of 1073∼1273K. The results show that the compressive strains at rupture(CSR) of the alloys have been greatly improved by sole addition of magnesium and the alloys with combined addition of magnesium and silicon reveal even higher CSR values, furthermore, at temperatures of 1073K and 1273K, the strain rate dependence of CSR reveals to be anomalous, i.e, the CSR value increases as the strain rate rises, and then declines until it surpasses the peak value, which is corresponded to the strain rate of 10-3 sec-1 and 10-3 sec-1 respectively.

The beneficial effect of magnesium and silicon exists in their competence of reducing strain rate sensitivity exponent values. The mechanisms of the anomalous ductilizing behavior in the Ni3Al as affected by ternary elements are discussed.

From our previous work, Ni3Al polycrystals with combined addition of magnesium and silicon kept high values of compressive strain at rupture (CSR) when the strain rate rose. In order to further improve hot workability of Ni3Al, 7.9wt.% Cr was added. The compressive tests showed that 30%∼40% CSR values of the alloy had been kept in a wide temperature range of 1173K-1373K at strain rates of 10-2sec-1 and 10-2sec-1 in contrast with 15%∼25% CSR values of the alloy without Cr addition.

In practical hot rolling process, at initial deformation temperature of 1373K, strain rate of 1.0 sec-1 and by controlling reduction within 10∼15% each rolling pass, Ni3Al ingots were successfully hot-rolled into polycrystals with different deformations, the maximum of which was 55%. The deformed alloys had manifestly enhanced mechanical properties shown by tensile tests.

The dislocation configurations of deformed alloys have been investigated by using transmission electron microscope(TEM).

Based on the superkink model, an explanation is given in this paper for the small strain-rate sensitivity of the anomalous yield stress of Ni3Al. The yield stress is proposed to be the stress needed to destabilise the average superkinks and it varies inversely with superkink height. The yield stress is insensitive to the strain-rate because the superkink height is controlled by the rate at which the superkink widens and is not directly related with the superkink propagation speed.

Single crystalline specimens of Ni74.8Al21.9Hf3.3 were subjected to compression tests at different temperatures. Thin foils for transmission electron microscopy observations were prepared from several specimens deformed within and above the yield stress anomaly domain. The dislocation microstructure was studied. The weak beam imaging and image simulation techniques followed by anisotropie elasticity calculations were used for the determination of antiphase boundary energies in both cube and octahedral planes, resulting in values of 237 mJm-2 and 252 mJm-2 respectively. The comparison of the present results with data taken from literature shows the influence of Hf on mechanical properties, dislocation microstructures and APB energies.

Ni3Al single crystals are known to exhibit a flow stress anomaly between 200 and 800K. The purpose of our work is to examine such an anomaly by means of a simulation of the dislocation dynamics at a mesoscopic scale. The simulation basic rules are: i) the dislocation glide in {111} octahedral planes, ii) the conditions at which screw lines are locked and unlocked by the formation of Kear-Wilsdorf locks, iii) the mobility of jogs in the {100} cube plane. Our results suggest that two different temperature regimes occur in the domain of the anomaly. At low temperatures, the plastic flow is governed by kink bow-out, itself a function of the kink length. At high temperatures, the plastic flow is governed by the unlocking of the weakest Kear-Wilsdorf locks in the microstructure. These outcomes of the simulation are discussed in relation with the existing theoretical models of the flow stress anomaly.