Dilip M. Shah

Appraisal of other silicides as structural materials

Abstract With the recognition of oxidation-resistant MoSi 2 as the ideal matrix material for high temperature structural composites, there has been a growing interest in “other” silicides. This paper is an attempt to place such interest in proper perspective with a comprehensive review of recent activities. The “other” silicides are appraised in terms of diverse strategies for the development of structural materials, which may be broadly grouped as monolithic, in situ composites and artificially reinforced composites. With alloying as an underlying common theme, it is argued that a broader and a more fundamental understanding of the silicides is necessary. The formation of silicides is reviewed with the focus on the disilicides, 5-3 silicides and monosilicides, as the three principal useful groups. For the refractory metal based disilicides, the relationship between the crystal structures C11 b , C40, C49 and C54 is examined in terms of stacking sequences and contrasted in relation to the structures of aluminides. The role of interstitial elements and the in situ composite approaches are emphasized for the complex 5-3 silicides and the monosilicides as being the most adjacent phases to refractory metal solid solutions. For most silicides with a non-cubic crystal structure, the effect of associated anisotropy of the coefficient of thermal expansion (CTE) on the mechanical integrity through processing and application of the material, is brought forth as a potentially critical issue. A qualitative model is proposed to rationalize the pronounced occurrence of “pest” disintegration in terms of the anisotropy of CTE, the nature of the grain structure and the ductile-brittle transition temperature of intermetallics.

In-situ refractory intermetallic-based composites

Abstract With the ultimate objective of exploiting refractory intermetallics for high-temperature structural materials, several binary and ternary two-phase intermetallics/refractory-metal solid solutions were explored. The ductile solid solution is used to toughen the composite microstructure via in-situ phase separation. While the viability of ductile phase separation in solid state was briefly considered for systems such as Nb 3 Al/Nb, much of the work focused on processing eutectic systems such as Cr 2 Nb/Nb and (Nb,Mo) 5 Si 3 /Nb,Mo). This paper describes results obtained via containerless directional solidification of these high-melting eutectic alloys using an optical float-zone furnace. The observations are explained on the basis of solidification theory and parameters unique to the optical float-zone furnace. It is demonstrated that, by this technique, casting-defect- and macrosegregation-free material, with well-aligned microstructure, can be readily produced. Moreover, the potential to approach sub-micron laminate spacing at high growth rate in alloys with very high melting eutectics has also been established. Room-temperature bend test evaluation of directionally solidified material is discussed in light of prevailing theories of ductile phase toughening. The results of a preliminary exploration of the NbMoCrSiAl multicomponent system are presented, showing the prevalence of eutectic phase separation and the potential for improving oxidation resistance.

Selecting high-temperature structural intermetallic compounds: The engineering approach

While there are nearly 300 high-melting-temperature intermetallic compounds, numerous factors limit the commercial viability of these materials for structural applications. Once the desirability of a material’s crystal structure has been determined, the engineer must then focus on the compound’s oxidation and corrosion resistance, its cost and any associated environmental hazards that may diminish its practical value. The final engineering limitations are imposed by process capabilities, which must be improved if high-temperature intermetallics are to one day emerge from the laboratory.

Evaluation of refractory intermetallics with A15 structure for high temperature structural applications

Abstract With the advent of ceramic superconductors, while interest in the A15 intermetallics such as Nb 3 Al is declining, their potential for high temperature structural applications appears promising. The A15 compounds, though considered to have the simplest of the topologically closed packed structures, form a distinct class of intermetallics with a cubic structure and are adjacent phases to the refractory metal solid solutions. This report summarizes the evaluation of microstructures, mechanical properties and cyclic oxidation resistance of Nb 3 Al and Cr 3 Si and their ternary alloys, and to a limited extent binary V 3 Si. The mechanical properties of binary compounds, elastic modulus, ultimate strength and creep behavior, are compared with known data for other A15 compounds. Solubility limits, melting point suppression and enhancement in creep resistance of several ternary alloys based on Nb 3 Al and Cr 3 Si are summarized. The mechanical properties and microstructural observations are discussed from the perspective of developing high temperature structural materials based on A15 intemetallics.

High temperature evaluation of topologically close packed intermetallics

Abstract This report summarizes the high temperature mechanical properties evaluation, oxidation resistance and ternary alloying studies conducted on a category of compounds referred to as the topologically close packed (TCP) structures. Those compounds evaluated to the greatest extent were Cr 2 Nb, Co 2 Nb and Nb 2 Al, while Fe 2 Nb, Fe 2 Zr, Co 2 Zr, Co 2 Zr, Cr 2 Hf, Mo 2 Hf and W 2 Hf were investigated to a lesser degree. The zirconium containing compounds resulted in pyrophoric powders thus limiting their utility. Elevated temperature bend, creep and cyclic oxidation tests were conducted on a number of these compounds to establish baseline properties. Ductile-brittle transition temperatures are reported as well as ultimate strengths at temperature, minimum creep rates and creep activation energies, and oxidation rates and oxide products.

Critical Plane Fatigue Modeling and Characterization of Single Crystal Nickel Superalloys

Single crystal nickel-base superalloys deform by shearing along (111) planes, sometimes referred to as octahedral slip planes. Under fatigue loading, cyclic stress produces alternating slip reversals on the critical slip systems which eventually results in fatigue crack initiation along the critical octahedral planes. A critical plane fatigue modeling approach was developed in the present study to analyze high cycle fatigue (HCF) failures in single crystal materials. This approach accounted for the effects of crystal orientation and the micromechanics of the deformation and slip mechanisms observed in single crystal materials. Three-dimensional stress and strain transformation equations were developed to determine stresses and strains along the crystallographic octahedral planes and corresponding slip systems. These stresses and strains were then used to calculate several multiaxial critical plane parameters to determine the amount of fatigue damage and also the critical planes along which HCF failures would initiate. The computed fatigue damage parameters were used along with experimentally measured fatigue lives, at 1100°F, to correlate the data for different loading orientations. Microscopic observations of the fracture surfaces were used to determine the actual octahedral plane (or facet) on which fatigue initiation occurred. X-ray diffraction measurements were then used to uniquely identify this damage initiation facet with respect to the crystal orientation in each specimen. These experimentally determined HCF initiation planes were compared with the analytically predicted critical planes.

High Tempeature Ordered Compounds for Advanced AERO-Propulsion Applications

Advanced aero-propulsion engine designs now being considered for implementation require structural materials with high temperature strength and creep properties above 1300°C, in excess of the range where nickel base superalloys are now being used. A number of ordered single phase compounds having melting points above 1500°C have been identified which both hold promise for engine applications and are representative of a number of different crystal structures such as B2, C15 (Laves), A15, C1 and DO 19 . Elevated temperature characterization has been conducted on these compounds which includes ductile/brittle transition temperature determination, minimum creep rate analysis, elastic modulus, tensile strength and cyclic oxidation testing. The results of these tests have led to insight in the processing methods required for these compounds and selection of compounds based on elemental constituents and crystal structure.

Ternary Alloying of Refractory Intermetallics

One of the most attractive attributes of intermetallics is their potential for alloying to achieve the desired balance of engineering properties via the creative use of in-situ composite processing. Following our initial investigation of 20 binary refractory intermetallics, selected primarily on the basis of structure type; seven systems were down selected for ternary alloying, based on a balanced consideration of room temperature toughness, high temperature creep strength, oxidation resistance and ultimate strength. These systems were Nb 3 Al and Cr 3 Si with A15 structure, CO 2 Nb and Cr 2 Nb Laves phases with C14/C15 structure, MoSi 2 with C11 b structure, and Nb 2 Al and Mo 5 Si 3 sigma phases. The alloying potential of each of these binary systems is evaluated with respect to solubility limits, phase relationships, melting point suppression with alloying, and potential for enhancement of creep resistance. The results show that, with alloyed intermetallics, realistic creep strength is attainable and several opportunities exist for in-situ processing of intermetallic based composite systems to improve room temperature toughness.

Feasibility Study of Intermetallic Composites

The feasibility of developing high-temperature intermetallic composites for use as gas turbine engine components is assessed for a wide range of high temperature intermetallic matrices including aluminides, silicides, Laves and Sigma phases along with Al 2 O 3 , SiC, TiC, Si 3 N 4 and Y 2 O 3 as well as ductile refractory metals as either reinforcing phases or coatings. Preliminary evaluations of fabricability and observations of matrix/reinforcing phase compatibility are presented and discussed in terms of various factors, including interstitial impurities, equilibrium phase relationships, kinetics, and physical and mechanical properties of both matrix and reinforcing phases.