The influence of Al: Nb ratio on the microstructure and mechanical response of quaternary Ni-Cr-Al-Nb alloys

The influence of Al:Nb ratio on the microstructure and properties of Ni–Cr–Al–Nb alloys has been investigated following long-term exposure at elevated temperatures. The γ′ volume fraction, size and lattice misfit were seen to increase with a larger Al:Nb ratio, although these changes resulted in reduced hardness. The change in the critical resolved shear stress (CRSS) associated with strong dislocation coupling was determined to be the dominant strengthening mechanism and increased with decreasing Al:Nb ratio. A distribution of tertiary γ′ was observed to be necessary in maximising the mechanical properties of these alloys.This work was supported by the EPSRC/Rolls-Royce Strategic Partnership (EP/H022309/1 and EP/H500375/1).This is the final published version, which can also be found on the Elsevier website at: http://www.sciencedirect.com/science/article/pii/S0921509314007369

Phase evolution in an Al<inf>0.5</inf>CrFeCoNiCu High Entropy Alloy

The phase evolution of an Al₀.₅CrFeCoNiCu High Entropy Alloy has been characterised following isothermal exposures between 0.1 and 1000 hours at temperatures of 700, 800 and 900˚C. The NiAl based B2 phase formed extremely quickly, within 0.1 hours at the higher exposure temperatures, whilst the Cr-rich σ phase formed more slowly. The solvus temperatures of these two phases were found to be ~ 975 and ~ 875˚C respectively. Compilation of the data presented here with results previously reported in the literature enabled the production of a time-temperature-transformation diagram, which clearly indicates that the diffusion kinetics of this material cannot be considered sluggish.The authors would like to thank K. Roberts and S. Rhodes for their assistance, and the EPSRC/Rolls-Royce Strategic Partnership (EP/M005607/1 and EP/H022309/1) for funding.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.intermet.2015.12.00

Microstructural evolution of a delta containing nickel-base superalloy during heat treatment and isothermal forging

The next generation of aerospace gas turbine engines need to operate at higher temperatures and stresses to improve their efficiency and reduce emissions. These operating conditions are beyond the capability of existing nickel-base superalloys requiring the development of new high temperature materials. Controlling the microstructures of these new materials is key to obtaining the required properties and therefore, it is critical to understand how these alloys respond to processing and heat treatment. Here, the microstructural evolution of V207M, a new δ containing, nickel-base superalloy, has been investigated following heat treatment and forging. The solvus temperatures of the γ′ and δ phases, determined by differential scanning calorimetry and microscopy, were found to be ~ 985 and ~ 1060 ˚C respectively. Isothermal forging of the alloy was conducted at 1000, 1050 and 1100 ˚C, corresponding to different volume fractions of retained δ. Considerable softening was observed prior to steady state flow when forging at 1000 ˚C, whilst only steady state flow occurred at 1050 and 1100 ˚C. The steady state flow process was believed to be dominated by dynamic recovery in the γ phase, with an activation energy of 407 kJ.mol-1. Samples that exhibited flow softening also showed a significant change in the orientation of the δ precipitates, preferentially aligning normal to the forging axis, and this reorientation was thought to be the cause of the observed flow softening.The authors would like to acknowledge M. Shakib for assistance with the forging and the EPSRC/Rolls-Royce Strategic Partnership for supporting this work through EP/H022309/1 and EP/H500375/1.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0921509314013252#

In situ study of sigma phase formation in Cr-Co-Ni ternary alloys at 800°C using the long duration experiment facility at Diamond Light Source.

The new long duration experiment facility on beamline I11 at Diamond Light Source has been used to study the kinetics of sigma phase formation in three Cr-Co-Ni alloys. Diffraction data acquired during in situ exposure at 800°C for 50 d showed progressive increases in the sigma fraction. This was accompanied by changes in the proportions of the other phases, which differed markedly between the alloys studied. These results demonstrate the capabilities of the long duration facility for the study of metallurgical phenomena over periods of months to years, a capability not previously available at a synchrotron source

Gamma-gamma prime-gamma double prime dual-superlattice superalloys

Improving the efficiency of gas turbine engines requires the development of new materials capable of operating at higher temperatures and stresses. Here, we report on a new polycrystalline nickel-base superalloy that has exceptional strength and thermal stability. These properties have been achieved through a four-element composition that can form both gamma prime and gamma double prime precipitates in comparable volume fractions, creating an unusual dual-superlattice microstructure. Alloying studies have shown that further property improvements can be achieved, and that with development such alloys may be suitable for future engine applications

Research data supporting 'On the time-temperature-transformation behaviour of a new dual-superlattice nickel-base superalloy'

This dataset contains images of the alloy's microstructure acquired using scanning electron microscopy; X-ray diffraction data; differential scanning calorimetry data; and, hardness values

On the prediction and the formation of the sigma phase in CrMnCoFeNi<inf>x</inf> high entropy alloys

The almost limitless variations in potential compositions of high entropy alloys necessitates the use of computational methods when attempting to optimise for any given application. However, the accuracy of the current thermodynamic approaches commonly being used for this purpose remains under debate, as relatively few validatory studies have been performed. Within the CrMnFeCoNi family of alloys, the formation of the σ phase and how it is influenced by compositional variations is of particular interest for elevated temperature structural applications. Here, the role of Ni on the formation of the σ phase has been studied through a systematic series of CrMnFeCoNix alloys, 0 ≤ x ≤ 1.5, following 1000 hour exposures at temperatures typically found to promote σ formation. Ni was found to have a significant effect on the phase stability of these alloys, suppressing the σ phase such that a single solid solution phase was the only stable phase in the CrMnFeCoNi1.5 alloy, whilst the CrMnFeCo alloy formed the σ phase during solidification. The corresponding thermodynamic predictions varied dramatically from the experimentally observed microstructures, indicating that the underlying databases require further optimisation. Interestingly, it was found that a relatively simple electronic structure based approach, New PhaComp, provided much more accurate predictions of the observed σ phase formation in the CrMnFeCoNix and CrMnxFeCoNi systems and could be manipulated to obtain σ formation temperatures. As such, this method could be extremely useful to those wanting to design CrMnFeCoNi high entropy alloys that are free from the σ phase

Phase stability of the Al<inf>x</inf>CrFeCoNi alloy system

The addition of Al to the A1 CrFeCoNi alloy has been shown to promote the formation of intermetallic phases, offering possibilities for the development of alloys with advantageous mechanical properties. However, despite numerous experimental investigations, there remain significant uncertainties as to the phase equilibria in this system particularly at temperatures below 1000˚C. The present study makes a systematic assessment of the literature data pertaining to the equilibrium phases in alloys of the AlxCrFeCoNi system. Two alloys, with atomic ratios, x = 0.5 and 1.0, are then selected for further experimental investigation, following homogenisation (1200˚C/100 h) and subsequent long-duration (1000 h) heat-treatments at 1000, 850 and 700˚C. The Al0.5 alloy was found to be dual-phase A1 + B2 in the homogenised condition and following exposure at 1000˚C but D8b phase precipitates developed following heat-treatment at the lower temperatures. In the Al1.0 alloy, B2, A2 and A1 phases were identified in the homogenised condition and at 1000˚C. At 850 and 750˚C, the A2 phase was replaced by the D8b phase. These experimental observations were used alongside literature data to assess the veracity of CALPHAD predictions made using the TCHEA4 thermodynamic database

The effect of manganese and silicon additions on the corrosion resistance of a polycrystalline nickel-based superalloy

The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750 °C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1 wt% Mn and one containing 0.5 wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protection, reducing the sulfidation depth by 2/3 and the hot corrosion depth by ½

Research data for "Phase stability of the AlxCrFeCoNi alloy system"

Data generated from investigations of phase stability in a series of alloy specimens subjected to long-duration heat-treatments at 1000, 850 and 700°C. This includes: SEM images, DSC traces, XRD patterns, Excel spreadsheet of phase quantification data from point EDX spectra. In certain files, the alloy specimens are refered to using the following names: HEA_16 - Al0.5CrMnCoNi, HEA_17 - AlCrFeCoNi. '1200', '1000', '850' and '700' indicate the the exposure temperatures in degrees celsius. 'Hom' refers to material in the 1200°C/72 h homogenised condition