R. Darolia

NiAl alloys for high-temperature structural applications

If their properties can be improved, nickel aluminide alloys offer significant payoffs in gas turbine engine applications. For these materials, excellent progress has been made toward understanding their mechanical behavior as well as improving their low-temperature ductility and high-temperature strength. For example, recent work shows that room-temperature ductility can be improved dramatically by microalloying with iron, gallium or molybdenum. The next challenge is to develop an alloy which has the required balance of ductility, toughness and strength. Development of design and test methodologies for components made out of low-ductility, anisotropic materials will also be required. While significant challenges remain, the continuing developments suggest that the prognosis for using NiAl alloys as high-temperature structural materials is good.

The effect of iron, gallium and molybdenum on the room temperature tensile ductility of NiAl

The low density and high thermal conductivity of NiAl compared to nickel-base superalloys make NiAl alloys attractive materials for turbine airfoils. The lack of ductility at lower temperatures has been one of the barriers limiting the use of NiAl alloys. The authors identify microalloying additions to NiAl which have demonstrated very significant improvements in the room temperature tensile ductility. The purpose of this paper is to provide details of the experimental data and preliminary information on the potential mechanisms for the ductility improvement.

Investigation of techniques for measuring lattice mismatch in a rhenium containing nickel base superalloy

Abstract Three techniques for measuring γ/γ′ lattice mismatch have been examined in a Ni-base superalloy subjected to two different aging heat treatments. The techniques used for measuring lattice mismatch were X-ray diffraction, convergent beam electron diffraction (CBED) and interface dislocation analysis. Additionally, a scanning transmission electron microscope equipped with an X-ray energy dispersive spectroscopy (EDS) system has been used to examine phase compositions. From this study, it has been determined that the X-ray diffraction and CBED yield similar results for room temperature lattice mismatch, although care must be taken in applying the CBED technique due to the complex strain fields present in high volume %γ′ alloys. The dislocation analysis technique gives larger negative values of mismatch. It is believed that these latter values represent those which exist at the aging temperature.

Slip systems in 〈001〉 oriented NiAl single crystals

Abstract An investigation of the change in slip behavior in NiAl with temperature has been conducted, with special emphasis on the 〈001〉 “hard” orientation. Single crystal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations and extensive dislocation analysis performed in the TEM on the 〈001〉 oriented specimens. It was found that, although 〈111〉 slip can occur in RT compression of 〈001〉 oriented specimens, the increased tensile ductility observed at higher temperatures is due to the glide of b = 〈110〉 dislocations. The debris left behind by these dislocations consists of b = 〈100〉 dislocations, making identification of the operative Burgers vector difficult after any appreciable plastic strain. A mechanism for the formation of the b = 〈100〉 debris is presented.

The effect of alloying on slip systems in 〈001〉 oriented NiAl single crystals

Abstract An investigation of the effect of alloying on slip behavior in NiAl as a function of temperature has been conducted. Single Cyrstal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations. It was found that the addition of Cr and other alloying additions promote the activation of 〈111〉 slip over deformation by kinking in NiAl based alloys. This is believed to result from differential proportional hardening of the 〈100〉 vs 〈111〉 slip systems. No increase in RT tensile elongation is observed in these alloys. Increased tensile ductility observed at higher temperatures is due to the movement of b = 〈110〉 dislocations.

Cleavage fracture in B2 aluminides

Abstract Cleavage fracture of B2 aluminide single crystals, including FeAl, NiAl and CoAl, has been investigated at temperatures below their ductile-brittle transition temperatures. Single-edge notched bend specimens oriented along specific crystalline directions were tested by 4-point bending. The fracture resistance was highly anisotropic because of the existence of a preferred cleavage plane in these B2 aluminide crystals. With a deep through notch NiAl and CoAl crystals that have high ordering energies generally cleave on {110} planes, while substoichiometric FeAl having a low ordering energy shows {100} cleavage as do most b.c.c. metals. In the case of NiAl, a transition fracture region, composed of fracture facets on {511} transient planes, appears at the initial cracking stage, followed by final cleavage on {110}. Different stoichiometric effects on the fracture toughness of B2 aluminides are observed when the A1 concentration is reduced. A general discussion on different mechanistic models has been used to explain the preferred cleavage planes in B2 structures. The intrinsic fracture toughness of an aluminide crystal can be determined by an ideal fracture test in which the cleavage plane is parallel to the notch plane and is normal to the applied stress. Because of geometric constraints an increased fracture resistance is obtained when the natural cleavage plane is not parallel to the notch plane, and the anisotropy of fracture toughness can be explained by a simple approach of resolved normal stress intensity.

Ductility and fracture toughness issues related to implementation of NiAl for gas turbine applications

Abstract Progress made in improving properties of NiAl alloys to compete with single crystal Ni-base superalloys is described. NiAl turbine airfoils with complex design features were fabricated by a combination of casting and machining processes. Such airfoils were successfully component and engine tested. Key issues related to poor ductility and fracture toughness limiting the application of intermetallics are discussed. Progress made in resolving some of the issues is discussed. Some of the production implementation issues are also discussed.

Microstructure and phase stability of single crystal NiAl alloyed with Hf and Zr

Six near stoichiometric, NiAI single-crystal alloys, with 0.05-1.5 at.% of Hf and Zr additions plus Si impurities, were microstructurally analyzed in the as-cast, homogenized, and aged conditions. Hafnium-rich interdendritic regions, containing the Heusler phase (Ni2AIHf), were found in all the as-cast alloys containing Hf. Homogenization heat treatments partially reduced these interdendritic segregated regions. Transmission electron microscopy (TEM) observations of the as-cast and homogenized microstructures revealed the presence of a high density of fine Hf (or Zr) and Si-rich precipitates. These were identified as G-phase, Nil6X6Si7, or as an orthorhombic NiXSi phase, where X is Hf or Zr. Under these conditions the expected Heusler phase (/3 _) was almost completely absent. The Si responsible for the formation of the G and NiHfSi phases is the result of molten metal reacting with the Si-containing crucible used during the casting process. Varying the cooling rates after homogenization resulted in the refinement or complete suppression of the G and NiHfSi phases. In some of the alloys studied, long-term aging heat treatments resulted in the formation of Heusler precipitates, which were more stable at the aging temperature and coarsened at the expense of the G-phase. In other alloys, long-term aging resulted in the formation of the NiXSi phase. The stability of the Heusler or NiXSi phases can be traced to the reactive element (Hf or Zr) to silicon ratio. If the ratio is high, then the Heusler phase appears stable after long time aging. If the ratio is low, then the NiHfSi phase appears to be the stable phase.

Strain aging embrittlement of the ordered intermetallic compound NiAl

Abstract The deformation behavior and fracture toughness of single crystals of the ordered intermetallic compound NiAl were investigated as functions of relatively low temperature thermal treatments. A strain aging embrittlement phenonenon, similar to that observed in mild steels, was identified. In the non-embrittled condition, tensile ductilities on the order of 7%–8% and fracture toughness values of 15–17 MPa m 1 2 were obtained for crystals with a 〈110〉 axis tested at room temperature. Additional observations of serrated yielding during compression testing at temperatures between 100 and 200 °C are consistent with strain aging induced by the low temperature diffusion of interstitial impurities or constitutional vacancies to dislocations, thus rendering them immobile at room temperature.

Overview of NiAl Alloys for High Temperature Structural Applications

NiAl alloys offer significant payoffs in gas turbine applications. Excellent progress has been made in understanding their mechanical behavior and improving low temperature ductility and high temperature strength. Significant improvements in mechanical properties have been obtained with microalloying. The next challenge is to develop an alloy which has the required balance of ductility, toughness, strength and other properties such as fatigue and impact resistance. Development of design, processing and test methodology for components made out of low ductility and anisotropic materials will also be required. While significant challenges remain, the prognosis for using NiAl alloys as high temperature structural materials is good.