Polycrystalline MoSi2 has been processed via spex milling and the notched fracture behavior has been determined at 800 -1400°C. Elemental Mo and Si powders were mechanically alloyed to produce MoSi2 and were subsequently vacuum hot pressed to fully dense MoSi2 compacts. Microstructures were analyzed and both hardness and notched fracture tests were conducted at high temperatures. The material toughness was constant at 4.5 MPa m½between 800 and 1000 °C, with significant plasticity exhibited at 1200°C. The results will be compared to recent work on the effects of C on high temperature properties of MoSi2.

Vacuum plasma spray is a melt spray process capable of producing coatings or near-net shaped deposits with ultra-fine grained structures. Mechanofusion is a novel technique for processing powdered materials. In this study, mixtures of elemental Ni and Al powders, either blended or mechanofused, were vacuum plasma sprayed, yielding a series of Ni-Al intermetallic phases. Mechanofusion resulted in significant effects on the phase distribution in subsequently as-sprayed deposits. After heat treatment at 1100 °C, single phase NiAl was obtained for both the mechanofused and blended samples. The microhardness of the NiAl is comparable to that produced by other methods. These preliminary experiments indicate that mechanofusion and vacuum plasma spraying can be employed to produce various compounds using mixed elemental powders.

Synthesis of the Ni-25% at. Al intermetallic compound by the thermal explosion mode subject to the application of compressive stress was systematically investigated. The stress was applied prior to the reaction. It was proved that the compressive stress had a profound effect on the reaction, as well as the densification. At a relative low stress, a full reaction could be completed and the major phase of the final product is Ni3Al. At higher stresses, a partial reaction was resulted and the final product was primary the intermediate phases, e.g. Ni and NiAl, etc. Three distinct reaction products were obtained in this study. When the stress was below approximately 5 MPa, a fully reacted product was obtained. Between 5 and 15 MPa, a fully and partially reacted zone coexisted. Above 15 MPa, only a partial reaction prevailed. In this experiment, a final product of 99.9% relative density was obtained at the high stress level.

The effect of processing on the microstructures and properties of powder metallurgy processed Ti-48Al-2Nb-2Cr alloys was examined. Both gas atomization (GA) and plasma rotating electrode process (PREP) techniques were used to produce pre-alloyed powder of the desired composition. The powders were then consolidated by either HIPing or extrusion. The effects of HIP temperature (1090°-1300°C) and HIP pressure (103MPa and 172MPa) were examined during HIP consolidation of GA powders. In addition, some of the PREP and GA HIPed materials were subsequently hot worked by isothermal forging. The tensile properties of these materials were determined in air in the temperature range 25°-1000°C. These results were then compared with previous data for I/M materials. The inter-relationship of processing, microstructure and properties was examined.

The kinetics of intermetallic Nb5Si3 compound formation via the mechanical alloying process was investigated. Interrupted milling process, X-ray diffraction, SEM examination and TEM imaging and diffraction were utilized to characterize changes in the milled powders, while DTA analyses were used to determine the critical and onset temperatures of reaction as a function of milling time. On the basis of experimental results, a kinetic model was proposed for formation of Nb5Si3 via the interrupted MA process. It is suggested that precipitation of Nb5Si3 particles during cooling in the interrupted milling process is responsible for the exothermic reaction after resumption of milling.

In order to increase the creep strength, small dispersoids were introduced into the intermetallic compound NiAl by mechanical alloying. Under favorable conditions, the resulting specific creep rates approach those of superalloys. To accompany these experimental effects, a new model has been developed which predicts tendencies for the success of dispersion strengthening in ordered alloys. It is shown that the dissociation distance of the partial dislocations, relative to the dispersoid particle size, has an important effect on the strengthening achievable.

The forging and rolling behavior of binary Ti-48Al and ternary Ti-48Al-2Cr (at-%) on laboratory and industrial scale is investigated. Arc-melted and cast ingots are used for forging operations. In order to enable the determination of optimized forging and rolling conditions the plastic flow behavior was examined for low (< 10−2 s−1) and high (1 s−1) deformation rates.

Forging under near-isothermal conditions was first performed on laboratory scale and upscaled as non-isothermal forging to pancakes of about 50 kg. Rolling tests on forged material are described. This paper presents results of microstructural examinations of forged ingots, rolled and annealed sheets and mechanical properties of sheet material under tensile test conditions.

For the powder metallurgical production of Ti-Al alloys, elemental powder blends and prealloyed powders were mechanically alloyed in a planetary ball mill. Thereby metastable solid solutions or amorphous phases are formed depending on the overall composition and the milling conditions. The crystalline phases exhibit an extremely small crystallite size of about 10 - 30 nm. This favors the consolidation of the powders which was performed by hot isostatic pressing in the temperature range between 500 °C and 800 °C. The results show that fully dense Ti-48 at.% Al alloys can be obtained only for a temperature of 800 °C. The crystallite sizes increase during compaction, but remain below about 0.15 µm. This allows the preparation of test specimens to investigate the mechanical properties of Ti-Al alloys with submicron crystallite sizes.

Microstructural studies have been performed on two superplastically deformed Ti3Al-based alloys, regular α2 and super α2. Dynamic grain growth, formation of phase stringers, and changes in the apparent volume fraction of the phases occurred in both alloys. The lath-like α2 phase in the super α2 alloy broke up during deformation. All these features are examined from the viewpoint of cooperative grain boundary sliding (CGBS). Cooperative grain boundary migration (CGBM), which is coupled with CGBS, is proposed to be the mechanism for strain induced grain growth and changes in the phase volume fraction.

A new strategy of microstructure design for improvement of the mechanical properties of Fe3Al based aluminides was developed. This new approach emphasizes microstructure control on the basis of ordered B2 phase structure matrix instead of the conventional D03 structure. This approach is characterized by the improvement of ductility without lose of strength. The observed data from various Fe3Al based alloys illustrated that the flattened pancake-shaped grains with ordered B2 phase structure increased both yield strength and elongation of sheet specimens compared to the equiaxed grains. The three dimensional configuration of the flattened grains and their annealing temperature dependence as well as fractography were studied.

A theoretical examination of the FeAl/Al2O3 fiber intermetallic matrix composite (IMC) system was coupled with experimental results to determine optimum processing conditions for maximum FeAl adhesion to A12O3 fibers and minimum attraction of FeAl and A12O3 fibers to themselves. Optimizing the processing conditions leads to a more uniform green composite without matrix or reinforcement rich zones. The theoretical examination consisted of applying a model developed using traditional colloidal approaches to suspensions while accounting for the complexities of a multicomponent composite system. The model uses the suspension properties of the individual materials such as size and surface potential along with the processing conditions for the system as the basis for calculation. This model describes the interaction potentials between components of the suspension and the stability of and between various components of the system in terms of a stability ratio, W. The optimum processing conditions were found by determining the conditions under which the calculated values of W are ideal. Experimental results utilizing the model predictions have been examined and include verification of the FeAl particle adhesion to the A12O3 fiber and preliminary consolidation studies.

The deformation and recrystallization structure as well as the evolution of crystallographic textures of heavily deformed, partially and fully recrystallized Ni3Al, ductilized with minor additions of boron, were investigated. The macroscopic orientation distribution was determined by X-ray pole-figure measurements and subsequent computation of the orientation distribution function (ODF). Convergent electron beam diffraction (CEBD) was employed to obtain information on the orientation topography and the orientation relationship of volume fractions adjacent to deformation inhomogeneities. Single grain orientation measurements by electron back scattering patterns (EBSP) were carried out to study the orientation distribution of new grains in the recrystallized microstructure. From this data the micro texture and the misorientation distribution function were calculated and the grain boundary character distribution was analysed.

The results are discussed with regard to the mechanisms which govern the evolution of the conspicuously inhomogeneous deformation microstructure and deformation textures. A model for the interpretation of the observed microstructure and texture evolution is proposed.

Net shape forming processes are under development to allow affordable production of intermetallic components. Powder injection molding (PIM) may be employed for the production of complex-shaped intermetallic geometries. Proper choice of powder parameters and processing conditions can lead to the formation of fully dense structures through pressure-less sintering. In this study, Ni3Al with 0.04 wt.-% boron has been successfully injection molded and sintered to full density. A yield strength of 340 MPa, ultimate tensile strength (UTS) of 591 MPa, and 8 % elongation were attained for injection molded and sintered tensile bars. Powder characteristics and sintering behavior are given for the nickel aluminide employed in this study to highlight the powder attributes needed for injection molding. Molding parameters, debinding and sintering schedules, along with mechanical properties are presented to indicate the viability of PIM for intermetallics. This approach based on the understanding of key powder characteristics and use of the reactive synthesis powder process may be extended to the successful injection molding of other intermetallic systems.

The use of x-ray diffraction has been used to determine phases present after reaction synthesis of Ni3Al powder. The complex diffraction spectra produced by the powder prompted the development of a simulator. The simulator uses nonlinear regression to determine the weight percent of the phases present. The simulator also determines the broadening of each peak in the spectrum. The phases present in Ni3Al powder produced by reaction synthesis has been determined with the simulator. The simulator has been used to monitor the progress of phase transformation during various thermal treatments of Ni3Al powder. A thermal cycle of 1200°C for two hours has been shown to produce a phase-pure product. The activation energy for the intcrdiffusion of nickel and aluminum has been determined to be 260 ± 35 k.J/mole.

Solidification conditions of a 0.64 cm × 2.5 cm × 15.2 cm slab and a 1.3 cm ɸ × 15.2 cm bar were simulated for two NiAl alloys. Simulation results were correlated with experimental inspection results to establish a relationship between dendrite arm spacing and local solidification time. The results were also used to construct a defects map for NiAl single crystal castings. Results have been compared to a similar model for single crystal nickel-base superalloys.

Titanium aluminide powder of composition Ti-48A1-2W (at%) was consolidated by hot isostatic pressing (HIP) at 200 MPa and 1050°C, 1100°C, 1150°C and 1250°C, respectively. The as-HIP'ed microstructure and substructure were characterized by optical microscopy and transmission electron microscopy (TEM). The densified powder alloy possesses an inhomogeneous microstructure consisting of fine-grained prior dendrites and coarse-grained interdendritic regions. The former contain the , 2 and phases. The HIP processing induced substantial deformation in the latter regions where dislocation segments and tangles, deformation twins and dislocation sub-boundaries were formed, indicative of plastic yielding and creep deformation. Both 1/2<110] unit dislocations and superdislocations are activated during HIP, but the former are the predominant dislocations in the sub-boundaries. With increasing HIP temperature, dynamic recrystallization occurs to a greater extent, resulting in a relatively uniform microstructure. The microstructure can be divided into hard and soft regions in which the densification is dominated by different mechanisms. Methods of optimizing the HIP cycle parameters to produce a more homogeneous microstructure are discussed.

Textural and microstructural changes in super α2 titanium aluminide (Ti-14Al-20Nb-3.2V-2Mo in wt%) subjected to hot-rolling process were investigated. X-ray pole figure measurements on sheet specimens were conducted to quantify the crystallographic texture development during the hot-rolling process. Results show strong texture development in both the α2and β phases. High temperature compression tests at various strain rates were conducted on specimens taken from hot-rolled plates, and the microstructural changes induced by the deformation were examined using transmission electron microscopy (TEM). Results indicated that deformation at low strain rates retains the hot rolled microstructure with elongated and strongly aligned α2 phase. At higher strain rates, the characteristic α2 phase appears to break down into equiaxed and/or recrystallized grains during deformation. Furthermore, results are correlated between the macroscopic orientations of texture components and the local orientations of α2 phase.

The low room-temperature ductility of iron-rich B2-structured FeAl has been attributed to atomic hydrogen arising from a reaction between water vapor and aluminum atoms at crack tips [C.T. Liu and C.G. McKamey, in “High Temperature Aluminides and Intermetallics”, eds. S.H. Whang et al (TMS, Warrendale, PA, 1990), p. 133]. Since hydrogen diffusion is time-dependent, the ductility might be expected to improve with increasing strain rate. Such behavior has been confirmed by tensile tests on Fe-45A1 and Fe-40Al-5Cr in air at strain rates ranging from 1 × 10−6 s−1 to 1 s−1. A brittle-to-ductile transition was observed at a strain rate of ∼10−2 s−1. The material with Cr appeared to be slightly less susceptible to environmental embrittlement, as reflected by the shift of the brittle-to-ductile transition to a lower strain rate. Fracture toughness tests on Fe-45A1 in air demonstrated a similar strain rate effect.

This paper summarizes results of our recent work on moisture-induced environmental embrittlement in Ni3Al. We took the unconventional approach of starting with single crystals of B-free Ni3Al, which were cold rolled and recrystallized to produce crack-free polycrystalline material. Our results show that the intrinsic ductility (∼16%) of Ni3Al (23.4 at.% Al) is considerably higher than previously thought; however, it is severely embrittled by moisture in air (ductility dropping from a high of ∼16% when tested in oxygen to a low of ∼3% in air). Since B-doped Ni3Al does not show such embrittlement (i.e., ductilities high in both air and oxygen), we conclude that a significant part of the beneficial effect of B must be related to suppression of this environmental effect. However, B must also improve grain boundary (GB) cohesion in Ni3Al, since our B-free alloy fractures intergranularly whereas B-doped alloys in general fracture transgranularly. Addition of a small amount (0.26 at.%) of Zr to Ni3Al significantly improves its ductility: to 11-13% in air, and 48-51% in oxygen. The ductilities observed in oxygen are comparable to the highest ever ductility observed in B-doped Ni3Al, indicating that the GBs in this Zr-doped alloy are not intrinsically brittle (rather, environmental embrittlement is the main reason for its brittleness). Zr dramatically increases the resistance of Ni3Al to GB fracture— perhaps by increasing GB cohesion. However, Auger analysis shows little or no Zr segregation on the GBs of Ni3Al, making it unclear how it might actually affect GB cohesion. Zr does not significantly increase the resistance of Ni3Al to environmental embrittlement; nor does it suppress intergranular fracture. In both these respects Zr behaves differently than B.

The effects of hydrogen on titanium aluminide alloys based on Ti3Al or α2 are now beginning to be understood. It has been established that large amounts of hydrogen are readily absorbed into these alloys, and on cooling to room temperature, virtually all this hydrogen is precipitated as a hydride phase or phases. These hydrides in turn affect mechanical properties much as in other hydride-forming materials. Extensive data now exist on these effects in the alloys based on α2. Fracture in particular is now being studied in detail, including work on micromechanisms of hydrogen fracture, which can be quantitatively compared to experiment. Needs for additional work are identified.