R.D. Field

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.

Microscopy and tensile behavior of melt-spun Ni-Al-Fe alloys

Microscopy and room-temperature tensile tests were performed on as-spun and annealed ribbons of Ni-20 (at. pet) Al-Fe alloys containing 20 to 40Fe. The ribbons had the duplex structures consisting of grains of ordered bec β-NiAl and grains of disordered fee γ-Ni, which contains precipitates of γ′-Ni3Al. The 25 to 30Fe alloys exhibited high ductility (∼10 pet elongation) in both the as-spun and annealed conditions. These results indicate that rapid solidification-induced effects, such as the suppression of ordering, do not enhance ductility as previously reported. The ductile alloys were found to contain high dislocation densities in both they and β grains, with no evidence of stress-induced martensite formation in the β phase. Dislocation analysis revealed that the vast majority of dislocations in theβ had ≤100≥Burgers vectors; however, ≤111≥ dislocations were also observed. Additionally, slip bands were frequently observed meeting at γ-β grain boundaries. Since they tend to align across the interphase grain boundary, deformation transfer between γ and β is inferred. The deformation transfer was found to be facilitated by a specific orientation relationship between the grains. The unusual deformation of ββby ≤111≥ slip and by deformation transfer from neighboring grains may be responsible for the high ductility.

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.

The Effect of Strain Rate on the Mechanical Properties of Single Crystal NiAl

In this study, the strain rate sensitivity of single crystal NiAl has been investigated by performing tensile tests as a function of temperature and two strain rates. Three crystallographic orientations, [100], [110], and [111] were studied. The tensile test results investigated include yield strength, work hardening rate and plastic strain to failure. The data are discussed in terms of deformation mechanisms in NiAl.

Evidence of Inherent Ductility in Single Crystal NiAl

The ductility and fracture toughness of single crystal NiAl have been studied as functions of thermal treatments at moderate and high temperatures. The data indicate that fast cooling through the temperature range 400°C - 20°C results in a material with a tensile elongation of 7% and a fracture toughness in the range of 13 -17 MPam 1/2 . It is concluded that prior reports of brittle behavior in single crystal NiAl may be a result of strain-age embrittlement, similar to that observed in mild steels. The data strongly suggest that ductility and toughness in NiAl are more strongly dependent upon mobile dislocation density rather than on the inherent mobility of dislocations in the ordered lattice. Similar behavior may also be possible in other intermetallic compounds.

A Mechanistic Study of the Microalloying Effect in NiAl Base Alloys

Alloys based on the B2 compound NiAl have significant potential for applications in hot sections of aircraft engines due to their low density, high melting point, and high thermal conductivity. A major disadvantage of this class of materials is low ductility at ambient temperatures. Recently, it was discovered that small levels of certain elements (eg. Fe, Ga, Mo) result in dramatic improvements in room temperature ductility. In this paper, results are presented from a mechanistic investigation of the “microalloying” effect. Tensile and compression testing as a function of temperature and orientation has been performed on both the binary compound and microalloyed material. Data on ductile to brittle transition temperatures, critical resolved shear stress values as a function of temperature on the different slip systems, and dislocation structures from TEM analysis of the tested specimens are presented. These data are discussed in terms of possible mechanisms for the microalloying effect in NiAl alloys.