Measuring tensile properties of deeply-undercooled and rapidly-solidified miniature samples of nickel aluminides
Explore the source record for details and available documents.
Carro, G.↗
SEARCH · Search NASA
Results for “aluminide”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Mechanisms of elevated-temperature deformation in the B2 aluminides NiAl and CoAl
A strain rate change technique, developed previously for distinguishing between pure-metal and alloy-type creep behavior, was used to study the elevated-temperature deformation behavior of the intermetallic compounds NiAl and CoAl. Tests on NiAl were conducted at temperatures between 1100 and 1300 K while tests on CoAl were performed at temperatures ranging from 1200 to 1400 K. NiAl exhibits pure-metal type behavior over the entire temperature range studied. CoAl, however, undergoes a transition from pure-metal to alloy-type deformation behavior as the temperature is decreased from 1400 to 1200 K. Slip appears to be inherently more difficult in CoAl than in NiAl, with lattice friction effects limiting the mobility of dislocations at a much higher tmeperature in CoAl than in NiAl. The superior strength of CoAl at elevated temperatures may, therefore, be related to a greater lattice friction strengthening effect in CoAl than in NiAl.
Yaney, D. L.↗
Dispersoids in rapidly solidified B2 nickel aluminides
Rapid solidification processing has been successfully used to create a uniform distribution of 30-nm TiB2 and HfC precipitates in a B2-ordered NiAl matrix, in order to enhance the matrix material's high-temperature strength and creep behavior. Attention is drawn to the question of the stability of carbide and boride dispersoids in an NiAl matrix during high-temperature exposures. The present alloys were consolidated at temperatures above 1422 K; the small size of precipitates in materials processed at such high temperatures gives an indication of their thermal stability, and further coarsening of the precipitates at elevated service temperatures is rendered unlikely by their low solubility in the matrix.
Jha, S. C.↗
Observations on the brittle to ductile transition temperatures of B2 nickel aluminides with and without zirconium
The effect of a zirconium addition (0.05 at. pct) to a stoichiometric NiAl alloy on the brittle-to-ductile transition temperature (BDTT) of this alloy was investigated. Constant velocity tensile tests were conducted to fracture between 300 and 1100 K under initial strain rate 0.00014/sec, and the true stress and true strain values were determined from plots of load vs time after subtracting the elastic strain. The inelastic strain was measured under a traveling microscope. Microstructural characterization of as-extruded and fractured specimens was carried out by SEM and TEM. It was found that, while the addition of 0.05 at. pct Zr strengthened the NiAl alloy, it increased its BDTT; this shift in the BDTT could not be attributed either to variations in grain size or to impurity contents. Little or no room-temperature ductility was observed for either alloy.
Raj, S. V.↗
Preferred orientations in extruded nickel and iron aluminides
A plotting of inverse pole figures is used to characterize the preferred orientations in both powder-extruded and cast-and-extruded NiAl, FeAl, and Ni3Al; the preferred 111 orientation has been noted along the extrusion direction in both powder-extruded and cast-and-extruded NiAl. While powder-extruded FeAl also had a 111 preferred orientation in the as-extruded condition, 110 was observed upon casting and extrusion; this was in turn replaced by a 211 orientation preference upon annealing, although annealing did not change the preferred orientation in either NiAl or in powder-extruded FeAl.
Khadkikar, P. S.↗
Hydrogen environment effects on beryllium and titanium aluminides
The NASP program has intensively studied the hydrogen-environment embrittlement (HEE) susceptibility of Be, alpha(2)-Ti3Al, and gamma-TiAl. Tensile tests are being conducted on candidate aerospace structure and propulsion candidate materials from each of the three groups, at temperatures in the -130 to +204 C range, in both 0.1 MPa and 13.8 MPa He and H2 environments, using SEM and TEM to discern the details of environmental effects. It has been established that while alpha(2) Ti3Al is HEE-susceptible even in the room-temperature and 13.8 MPa environment, neither Be nor gamma-TiAl are susceptible, even at the highest temperature tested.
Fritzemeier, L. G.↗
Continuous fiber-reinforced titanium aluminide composites
An account is given of the fabrication techniques, microstructural characteristics, and mechanical behavior of a lightweight, high service temperature SiC-reinforced alpha-2 Ti-14Al-21Nb intermetallic-matrix composite. Fabrication techniques under investigation to improve the low-temperature ductility and environmental resistance of this material system, while reducing manufacturing costs to competitive levels, encompass powder-cloth processing, foil-fiber-foil processing, and thermal-spray processing. Attention is given to composite microstructure problems associated with fiber distribution and fiber-matrix interfaces, as well as with mismatches of thermal-expansion coefficient; major improvements are noted to be required in tensile properties, thermal cycling effects, mechanical damage, creep, and environmental effects.
Mackay, R. A.↗
Hydride structures in Ti-aluminides subjected to high temperature and hydrogen pressure charging conditions
The distribution and chemistry of hydrides produced in single and dual phase alloys with a composition near TiAl have been investigated by using a combination of TEM and X-ray diffraction techniques. The alloys were exposed at 650 C to 13.8 MPa of gaseous H2 for 100 h. In the single-phase gamma alloy, large hydrides preferentially nucleated on the grain boundaries and matrix dislocations and a population of small hydrides was distributed throughout the matrix. X-ray and electron diffraction patterns from these hydrides indicated that they have an fcc structure with a lattice parameter of 0.45 nm. EDAX analysis of the hydrides showed that they were enriched in Ti. The hydrides were mostly removed by vacuum annealing at 800 C for 24 h. On dissolution of the hydrides, the chemistry of hydride-free regions of the grain boundary returned to the matrix composition, suggesting that Ti segregation accompanied the hydride formation rather than Ti enrichment causing the formation of the hydride.
Legzdina, D.↗
Evaluation of a hydrogen resistant titanium aluminide alloy
The Ti-24Al-11Nb (Ti-24-11) alloy heat treated to the fine basketweave microstructure was shown previously to be hydrogen tolerant. In order to assess its limit of hydrogen tolerance, the tensile, creep, fracture toughness, and sustained load crack growth behaviors of this alloy were studied as a function of hydrogen content. All test specimens were thermally charged with internal hydrogen and tested at 25 and 600 C. Coupon specimens were used for developing the hydrogen charging procedures and for studying compatibility of the alloy with high temperature, high pressure gaseous hydrogen. The mechanical test results indicated that the fine basketweave microstructure was tolerant to hydride embrittlement for hydrogen contents up to approximately 1500 wt. ppm, providing that the hydride formed was of the TiH2 type. On the other hand, hydrogen charging experiments indicated that the Ti-24-11 alloy was severely cracked and pulverized under zero load when the hydrogen content exceeded 3000 wt. ppm. X-ray diffraction results revealed that the dichotomous behaviors might be due to the formation of TiH(1.924) type hydrides at higher hydrogen contents. Thus, hydrogen embrittlement in the Ti-24-11 alloy with the fine basketweave microstructure depends on hydrogen content and the nature of the hydrides formed.
Chan, K. S.↗
The influence of chromium on structure and mechanical properties of B2 nickel aluminide alloys
Major obstacles to the use of NiAl-based alloys and composites are low ductility and toughness. These shortcomings result in part from a lack of sufficient slip systems to accommodate plastic deformation of polycrystalline material (von Mises Criterion). It has been reported that minor additions of chromium to polycrystalline NiAl cause the predominant slip system to shift from the usual. If true, then a major step toward increasing ductility in this compound may be realized. The purpose of the present study was to verify this phenomenon, characterize it with respect to chromium level and Ni to Al ratio, and correlate any change in slip system with microstructure and mechanical properties. Compression and tensile specimens were prepared from alloys containing 0 to 5 percent chromium and 45 to 55 percent aluminum. Following about one percent strain, transmission electron microscopy foils were produced and the slip systems determined using the g x b = 0 invisibility criterion. Contrary to previous results, chromium was found to have no effect on the preferred slip system of any of the alloys studied. Possible reasons for the inconsistency of the current results with previous work are considered. Composition-structure-property relationships are discerned for the alloys, and good correlation are demonstrated in terms of conventional strengthening models for metallic systems.
Cotton, James Dean↗
Residual thermal strains and stresses in nickel aluminide matrix composites
Thermally induced residual strains and stresses developed during postfabrication cooling in Saphikon/NiAl and tungsten/NiAl high-temperature composites are investigated through three-dimensional elastoplastic finite-element analyses. Average axial and transverse strains in the matrix are found to be tensile and compressive, respectively, and similar for both Saphikon and W-fiber-reinforced NiAl composites. It is suggested that the residual matrix stresses and strains are controlled more by the low-matrix yield stress than by the fiber/matrix expansion mismatch. Residual thermal strains in the matrix of these composites are measured by using a neutron-diffraction technique; the measured axial and transverse strains in the matrix are found to be in agreement with the computed values.
Saigal, A.↗
Fracture toughness and the effects of stress state on fracture of nickel aluminides
The effects of stress state on the fracture behavior of Ni3Al, Ni3Al + B, and NiAl were determined using either notched or fatigue-precracked bend bars tested to failure at room temperature, in addition to testing specimens in tension under superposed hydrostatic pressure. Although Ni3Al is observed to fail in a macroscopically brittle intergranular manner in tension tests conducted at room temperature, the fracture toughnesses presently obtained on Ni3Al exceeded 20 MPam, and R-curve behavior was exhibited. In situ monitoring of the fracture experiments was utilized to aid in interpreting the source(s) of the high toughness in Ni3Al, while SEM fractography was utilized to determine the operative fracture modes. The superposition by hydrostatic pressure during tensile testing of NiAl specimens was observed to produce increased ductility without changing the fracture mode.
Lewandowski, John J.↗
Compatibility of potential reinforcing ceramics with Ni and Fe aluminides
The compatibility of candidate ceramic reinforcement materials with intermetallic matrices for high temperature composite systems has been evaluated. Powders of FeAl and NiAl were mixed with ceramic powders and consolidated by hot isostatic pressing and subsequent heat treatment. The microstructures of these composites and the nature of the ceramic/matrix interfaces were assessed using a wide variety of electron-beam techniques. The system FeAl/TiB2 was found to be particularly promising. The matrix appears to be bonded to the ceramic particles, which may be the result of diffusion of Fe into the ceramic. The particles stabilized in a previously unreported monoclinic crystal structure, rather than the equilibrium hexagonal form exhibited by the binary compound.
Moser, J. A.↗
Compatibility of potential reinforcing ceramics with Ni and Fe aluminides
There is a great deal of interest in the possible utilization of intermetallic compounds in advanced high temperature gas turbine engines. These compounds exhibit a variety of promising properties, including reasonable strength, high melting points, relatively low densities, and good corrosion resistance. However, in general, they also show limited ductilities and toughness, and less than optimum creep strengths at elevated temperatures. In addition, in applications involving advanced gas turbine engines, it is often necessary for candidate materials to have large elastic moduli. The present study is part of a program whose objective is to identify a high temperature fiber reinforced composite. The approach adopted was to fabricate laboratory samples of the combinations of materials considered by Misra, in order to determine the extent to which the thermodynamic calculations can predict phase stability. As many of the ceramic phases considered are not currently available in fiber form, they were added as particulates to the alloy matrices. The ways in which the materials were produced and evaluated are described.
Clark, William A. T.↗
Creep deformation of B2 aluminides
The creep resistance and elevated temperature deformation mechanisms in CoAl, FeAl, and NiAl are reviewed. The stress and temperature dependencies of the steady state creep rate, the primary creep behavior, the dislocation substructure, and the response during transient tests are used as the main indicators of the deformation processes. In single phase intermetallics, the influence of grain size, stoichiometry, and solid solution hardening have been examined. In addition, the effect of adding dispersoids, precipitates, and other types of reinforcements to improve creep strength are compared.
Nathal, M. V.↗
Chemical stability of titanium diboride reinforcement in nickel aluminide matrices
Chemical stability of TiB2 reinforcement in NiAl (45 at percent Al) and Ni3Al (24 at percent Al) matrices has been theoretically and experimentally investigated. Calculations were made using thermodynamic properties of the systems to predict behavior at temperatures between 1173 and 1573 K. Experimental investigation of hot-press consolidated TiB2 particulate/prealloyed matrix powder blends were conducted using energy dispersive X-ray analysis, XRD, AES, and TEM. The theoretical and experimental analyses suggest that TiB2 is chemically stable in both matrices up to 1573 K; however, TiB2 was found to be less active in NiAl than in Ni3Al due to lower nickel activity in NiAl.
Rigney, J. D.↗
The effect of fiber microstructure on evolution of residual stresses in silicon carbide/titanium aluminide composites
This paper examines the effect of the morphology of the SCS6 silicon carbide fiber on the evolution of residual stresses in SiC/Ti composites. A micromechanics model based on the concentric cylinder concept is presented which is used to calculate residual stresses in a SiC/Ti composite during axisymmetric cooling by a spatially uniform temperature change. The silicon carbide fiber is modeled as a layered material with five distinct transversely isotropic and orthotropic, elastic layers, whereas the titanium matrix is taken to be isotropic, with temperature-dependent elastoplastic properties. The results arc compared with those obtained based on the assumption that the silicon carbide fiber is isotropic and homogeneous.
Pindera, Marek-Jerzy↗
Thin CVD Coating Protects Titanium Aluminide Alloys
Feasibility of using very thin CVD coatings to provide both protection against oxidation and surfaces of low catalytic activity for thin metallic heat-shield materials demonstrated. Use of aluminum in compositions increases emittances of coatings and reduces transport of oxygen through coatings to substrates. Coatings light in weight and applied to foil-gauge materials with minimum weight penalties.
Clark, Ronald↗