The possibility of producing novel microstructures in MoSi2 materials was examined by HIP reacting MoSi2 and Ti powders at 1400°C/207MPa to produce a two-phase MoSi2-based alloy. The HIP reaction produced MoSi2 + hexagonal (Mo0.42, Ti0.58)5Si3- The (Mo0,42.Ti0.58)5Si3 was composed of 0.5µm diam., equiaxed grains, and the average MoSi2 grain size was reduced by the reaction. Preliminary compression testing showed no loss of strength compared to single-phase MoSi2, and two-phase specimens were compressible to 24% strain without any cracking. The (Mo0.42,Ti058)5Si3 phase appeared to be plastically undeformable even at 1400°C.

The fracture toughness at room temperature of a brittle nickel silicide containing the ductile phases Ni(Si) and Ni3Si has been investigated. The microstructure was comprised of a brittle nickel silicide matrix, ductile Ni(Si) particles distributed within the matrix, and Ni3Si rims around the particles. It was obtained by annealing hot-extruded Ni-23 at.%Si (with and without 0.19 at.% boron) at 1100°C and then air cooling. The fracture toughness values were in the range of 13 MPa m1/2 to 22 MPa m1/2. Systematic effects of annealing time and of boron were not apparent. It would appear that the incorporation of the ductile particles has a beneficial effect on the fracture toughness of an otherwise brittle silicide.

NiAl/Nb and NiAl/Cr composite materials were prepared by a powder metallurgical approach. The content of the second phases varied between 5 and 10 weigth percent. The thermodynamic stability of Nb and Cr particles in the NiAl matrix was studied at 1473K (1200°C) by means of SEM, XRD and microhardness measurements. In the case of Nb the formation of a Laves phase (NbNiAl) was observed whereas the Cr particles remained stable but were hardened to a remarkable extent due to in-diffusion of Ni and Al. This paper presents the results of three-point bending tests performed on as-HIPed and annealed material at room temperature and elevated temperatures. The yielding behavior at room temperature and the creep behavior at 1200K (927°C) were investigated by compression tests.

The fracture behavior of V-V3Si in-situ composite alloys has been evaluated for intermetallic phase fractions of 30, 50, and 70 volume percent. In addition, the mechanical properties of the ductile constituent in these composites have been evaluated from two alloys containing silicon at its solubility limit in vanadium at the eutectic temperature and at 1400°C. The fracture toughness was found to increase monotonically with increasing volume fraction of the ductile phase. Fractography has shown evidence of crack bridging by the vanadium solid solution phase and increased ductility within the composite relative to bulk measurements. The toughness results are compared with predictions based on an existing model of ductile-phase toughening using our measurements of component properties.

We have studied the mechanical behavior of two fine grained MoSi2 alloys containing 0.61 at% (0.19 wt%) and 0.29 at% (0.09 wt%) oxygen, respectively. By preparing these alloys in almost identical fashion, their only difference was their oxygen content. The mechanical behavior was studied by four-point flexure tests in unnotched specimens between 800°C and 1400°C. We interpret the mechanical behavior data in terms of Davidenkov diagrams which describe the dependence of the apparent ductile-to-brittle transition temperature on the SiO2 content in the alloys.

The Al3Ti-based Ll2 intermetallic alloy Al66Fe9Ti25 has shown low density, high oxidation resistance, good strength and appreciable compression ductility at room temperature. To explore the approach of combined toughening and strengthening of this alloy, composites are being studied. The interface reactions between Al66Fe9Ti25 matrix and SiC reinforcement material during different fabrication processes were investigated using SEM, EDS, EPA and X-ray diffraction analyses. The extent of reaction was determined and the reaction products were identified. It was found that SiC is chemically incompatible with the Al66Fe9Ti25 matrix, strong interfacial reactions occurred during fabrication and the formation of ternary compounds leads to the deterioration of the matrix alloy.

The primary objective of this investigation was to confirm previous results on AIN particulate strengthened NiAI (J. Mater. Res 5, 271(1990)) produced by cryomilling and to determine if cryomilling modified to produce NiAI containing fine Al2O3 particles would also yield a material with equally attractive strength levels. Compression tests conducted on NiAI-AIN at a strain rate of 1.2 × 10−4 s−1 between 300-1300 K and as a function of strain rate at 1300 K indicated that its strength was comparable to previously produced material. While the NiAl-Al2O3 material was stronger than unreinforced NiAI at lower temperatures, at 1300 K it exhibited strength levels commensurate with fine grain sized NiAI. At all test temperatures and strain rates NiAI-AIN exhibited higher strengths compared to either NiAI-AI2O3 or unreinforced NiAI.

Mechanical alloying followed by hot extrusion has been used to produce fully dense, crack free, very fine grained NiAl-based alloys containing a bimodal distribution of aluminum oxide dispersoids. The unique microstructure provides the materials with high strength and good compressive ductility at ambient and elevated temperatures. The emphasis of the paper is on the importance of the dispersion phase in controlling grain size, texture, deformation mechanisms and ultimately mechanical properties of the mechanically alloyed NiAl-based materials.

Plasma-sprayed microstructures of MoSi2 have been studied by electron microscopy. The as-deposited microstructures are metastable and inhomogenous. Two new dissociations of [001] and 1/2[331] dislocations have been observed for the first time. The 1/2[331] is dissociated on the (lTO) plane bounding a superlattice intrinsic stacking fault (SISF). The reaction is given as: 1/2[331] = 1/4[331] + SISF + [331]. The [001] is also dissociated on (110) plane into two symmetrical components, with the reaction being given as [001] = 1/2[001] + 1/2[001].

The high temperature deformation behavior of directionally solidified (DS) MoSi2-Mo5Si3 eutectics was studied and compared to powder processed MoSi2-Mo5Si3 composites having the same volume fraction of Mo5Si3. Decremental step strain rate tests were performed in the temperature range of 1100-1300°C and at strain rates between 10−4 to 10−6/s. A considerable increase in the flow stress was observed for the directionally solidified material. At 1200°C and a strain rate of 10−6/s the flow stress of the DS eutectic was 255 MPa as compared to 20 MPa for the powder processed composite. The high temperature strength of the DS eutectic was unaffected by changes in the scale of the lamellar micro-structure and these results were modeled using a constitutive relation for power law creep. A stress exponent of 4.5 and an activation energy of 300 kJ/mol was determined for the DS eutectic. Evidence of dislocation glide and climb was observed in the MoSi2 lamellae whereas the dislocation density was small in the Mo5Si3 phase. The improved creep strength of the eutectic is believed to be a result of both the fibrous morphology and a stronger interface structure as compared to the powder processed composite.

A study has been conducted to determine the feasibility of using Er2Mo3Si4 as a reinforcement phase to improve the high temperature strength of MoSi2. The melting temperature of Er2Mo3Si4 was determined to be 1930 ± 20°C whereas the eutectic of Er2Mo3Si4-39 vol% MoSi2 melted at 1790 ± 10°C. Elevated temperature microhardness tests show that Er2Mo3Si4 has significantly higher hardness than MoSi2 above 1000°C, e.g. approximately 5.8 GPa versus 1.5 GPa at 1300°C, respectively. A MoSi2/ Er2Mo3Si4/20p composite was produced by ball milling and hot pressing arc-melted MoSi2-20 vol% Er2Mo3Si4 materials. At 1300°C the MoSi2/ Er2Mo3Si4/20p composite and a directionally solidified Er2Mo3Si4-MoSi2 eutectic exhibited hardnesses of 2.4 GPa and 4 GPa, respectively. Preliminary results from compressive decremental step strain rate tests at 1300°C indicate that the creep strength of the MoSi2/ Er2Mo3Si4/20p composite is comparable to that of a MoSi2/SiC/20w composite. The creep stress exponent was determined to be 3.3 at 1200°C and 3.7 at 1300°C.

Decremental step strain rate tests in compression were performed on MoSi2 and MoSi2-10 vol % TiC at 1150°C and 1200°C. The composite exhibits a lower stress exponent than MoSi2 between 10-4 s-1 and 10-5 s-1 TEM observations reveal the deformation of the carbide phase, as well as the efficient generation of dislocations into the matrix from the carbide-matrix interfaces. The substructures of both the matrix of the composite and monolithic MoSi 2appear to consist of mixed dislocations of predominantly edge character which occur on the same slip systems.

AlN particulate strengthened NiAl containing about 10vol% AlN was investigated in stress change experiments above, below, and through a transition stress near 80 MPa. All creep transients were normal, and incubation periods were observed in all stress drops near or below the transition stress. Stress change experiments indicated that the microstructure is very stable, and provided similar values for steady state strain rate as the monotonic tests. Analysis of the stress change data reconfirmed the existence of several deformation mechanisms in this material.

Creep deformation of as-forged and heat-treated titanium aluminides, Ti-48Al with 5 and 10% volume fraction of TiB2, has been studied under tension in the temperature range of 649°-871°C to determine the rate-controlling mechanism. Microstructure of these alloys correspond to a mixture of equiaxed grains of γ and lamellar regions containing eutectoid γ+α2.

Data indicate that with increase in load, stress exponent increases from one to seven. The activation energy for creep varies from 340kJ/mol to 455kJ/mol in the temperature range investigated. Creep data of these alloys were analyzed using several theoretical models.

Titanium aluminide was reinforced by AVCO SCS–6 continuous SiC fibers in unidirectional or 0°/+45°/–45° direction to make SiCf/Ti–A1 composites through hot-pressing of Ti and A1 powder mixture. The toughness of the composites is greatly improved at room temperature. The typical tensile elongation is over 0.8%. The room temperature strength of the composite can be sustained up to about 700°C. The processes developed in this study have two merits: (1) Ti and A1 powders instead of Ti–A1 powder can be directly used as raw materials, and (2) the mechanical properties of composites can be tailored in a similar fashion as those of conventional composites.

Autogeneous gas tungsten arc welding was performed on plates (about 3 mm thick) cut from cast ingot of aNi3 Al alloy (Ni 73.80 Al 15.82 Cr 8.33Mo 1.68 Zr 0.52 B 0.05), known as IC-396M. The alloy was susceptible to cracking in the heat-affected zone (HAZ). The cracks in HAZ were liquation cracks and occurred along dendritic boundaries and around eutectic cells. The fusion zone (FZ) contained many fine eutectic cells (about 5-10 μm in diameter) and there were microcracks (1-5 μm in size) in the cells. Two types of eutectic structures were observed in the base metal, HAZ and FZ of IC-396M. One had web-like structure and the other had lamellar structure, the later having high enrichment with Zr. The role of Zr enrichment in dendritic boundaries and eutectic cells in causing liquation cracks will be discussed.

Conventional fusion welding procedures are not likely to be useful for joining NiAl because of its poor low temperature ductility. Thus, the potential use of this material could be limited. This study was designed to demonstrate the feasibility of using a diffusion brazing process to fabricate NiAl components. A unique feature of this process is the development of self-generated filler metal through elevated temperature vacuum exposure. Butt joints were made in extruded bar and joint quality was evaluated using metallographic techniques. Sound joints, which were produced with complete grain growth across the braze interface, were indistinguishable from the base metal. Thus, the feasibility of diffusion brazing NiAl was established.

Compression tests on Ti-48AI-2Cr were performed in the temperature range of 650-1000°C. The stress vs. strain curves show an initial increase with a decreasing rate due to dynamic recovery. In the second stage, correlated with dynamic recrystallization (DRX), the stress passes through a maximum and decreases to a steady state value. The characteristic steady state stress associated with dynamic recovery exhibits a lower activation energy (269 kJ/mol) than the steady state stress during DRX (400 kJ/mol). Constant-load creep experiments show a corresponding behaviour. After primary creep the creep rate passes through a minimum. The final steady state creep conditions are dominated by DRX as well.

The fracture toughness of a nickel aluminide alloy IC-396M has been measured from room temperature to 800°C. The material was tested in the as-cast condition. Specimens were oriented for crack growth either parallel or perpendicular to the dendritic structure (C-R and R-C orientations, respectively). Both orientations had acceptable levels of toughness from room temperature to 600°C. At 650°C the toughness decreased markedly with some recovery at 800°C. Examination of the fracture surfaces revealed that the fracture mode changed from a ductile tearing mechanism at temperatures up to 600°C to interdendritic fracture at 650°C and higher temperatures. At 650°C the crack grew along the dendrite axis for the C-R specimens, but for the R-C specimens the crack grew out-of-plane in attempting to follow the dendritic structure. It is believed that the presence of the sharp crack in the fracture toughness specimens allowed oxygen to penetrate to the crack tip, where straining exposed metal surfaces that were not protected by an oxide layer. The result was embrittlement of the interdendritic regions and low energy fracture, analogous to intergranular fracture in wrought materials.

Microstructural observations, phase identification, and hardness measurements of several ternary and quaternary A13Nb− and A13Ti-based alloys containing Ni or Fe have been carried out after homogenization at 1100°C. An L12 phase is formed in an (A1,Fe)3Ti-based alloy when a small amount of Nb is added. Conversely, in Al-Nb-Ni alloys whose compositions are based on Al3Nb, no L12 phase has been detected. In this case, alloys are either three-phase(B2, DO22 and Laves) or two-phase(B2 and Laves). The absence of the Ll2 phase formation in AlgNb-based alloys containing Ni is attributed to limited solubility of Ni in the Al3Nb compound. Rapidly solidified and slowly cooled specimens have also been analyzed.