The effect of Al on the formation and stability of a BCC – B2 microstructure in a refractory metal high entropy superalloy system

Whitfield, Tamsin; Pickering, Ed; Owen, Lewis; Jones, C.N.; Stone, Howard J.; Jones, Nicholas Blurton

doi:10.1016/j.mtla.2020.100858

Cited by 40 publications

(16 citation statements)

References 43 publications

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“…In some regions, the precipitates formed interpenetrating networks with lamellar morphologies. The size of these regions and the lamellae within them were largest near the grain boundaries and, where coarse, the dark contrast phase appeared to be encased in a layer of the bright contrast phase, a morphology reported in other RSAs [ 20 , 21 ]. Away from these regions, the bright precipitates had a rounded, rod-like morphology.…”

Section: Resultsmentioning

confidence: 58%

“…However, such compositional changes will also influence the structure of Phase A and the likelihood of forming the nanoscale two-phase microstructure shown in Figure 1 b. The enrichment in refractory metal elements and reduction in Al is likely to stabilise a bcc structure [ 21 ]. In addition, since the formation of the nanoscale microstructure is primarily driven by the miscibility gap between Zr and refractory metals, the very low Zr content of the matrix phase is unlikely to result in such a phase separation [ 10 , 22 , 23 ].…”

Section: Resultsmentioning

confidence: 99%

“…At 1200 °C, only the bcc and hexagonal intermetallic phases were present and, therefore, heat treatments above this temperature reside in a two-phase, bcc + hexagonal intermetallic phase field. Consequently, the precursor to the nanoscale B2 + bcc microstructure, which forms on quenching from homogenisation at temperatures >1200 °C, is confirmed to have a bcc structure with the B2 phase forming through a spinodal decomposition plus ordering mechanism [ 15 , 21 ].…”

Section: Resultsmentioning

confidence: 99%

“…Hence, it is believed that following cooling to room temperature the composition of the matrix phase in AlMo 0.5 NbTa 0.5 TiZr meant that it maintained a bcc structure, unlike the matrix phase of AlMo 0.5 NbTa 0.5 TiZr 0.5, which was shown to have a B2 structure following cooling from the exposure temperature. The formation of the B2 phase also indicates that sufficient Al concentration must have been retained within the matrix of AlMo 0.5 NbTa 0.5 TiZr 0.5 to facilitate ordering [ 6 , 8 , 9 , 21 , 23 , 25 ]. The final, important, difference between these two studies was present within the microstructures following exposure at 800 °C.…”

Section: Resultsmentioning

confidence: 99%

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Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures

Whitfield

Stone

Jones

et al. 2021

Entropy

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Refractory metal high-entropy superalloys (RSA), which possess a nanoscale microstructure of B2 and bcc phases, have been developed to offer high temperature capabilities beyond conventional Ni-based alloys. Despite showing a number of excellent attributes, to date there has been little consideration of their microstructural stability, which is an essential feature of any material employed in high temperature service. Here, the stability of the exemplar RSA AlMo0.5NbTa0.5TiZr is studied following 1000 h exposures at 1200, 1000 and 800 °C. Crucially, the initial nanoscale cuboidal B2 + bcc microstructure was found to be unstable following the thermal exposures. Extensive intragranular precipitation of a hexagonal Al-Zr-rich intermetallic occurred at all temperatures and, where present, the bcc and B2 phases had coarsened and changed morphology. This microstructural evolution will concomitantly change both the mechanical and environmental properties and is likely to be detrimental to the in-service performance of the alloy.

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Section: Resultsmentioning

confidence: 58%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

See 2 more Smart Citations

Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures

Whitfield

Stone

Jones

et al. 2021

Entropy

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“…The nano-scale microstructures in RSAs are believed to form via a spinodal decomposition during cooling to produce two coherent bcc phases, one of which undergoes an ordering transition to form the B2 phase [5,8,10,13,14]. Spinodal decomposition occurs when the second derivative of the Gibbs energy curve with composition within a miscibility gap is negative.…”

Section: Introductionmentioning

confidence: 99%

An Investigation of the Miscibility Gap Controlling Phase Formation in Refractory Metal High Entropy Superalloys via the Ti-Nb-Zr Constituent System

Whitfield

Wise

Pickering

et al. 2021

Metals

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Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

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Fundamental Effects of Al and Ta on Microstructure and Phase Transformations in the Al–Cr–Mo–Ta–Ti Refractory Complex Concentrated Alloy System

Mann

Newkirk

2023

Adv Eng Mater

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The effect of aluminum and tantalum concentrations on a refractory metal complex concentrated alloy is reported, particularly with respect to their effect on microstructure and phase composition of the alloy in cast and annealed form. Alloys with an equiatomic composition, (AlCrMoTaTi), an aluminum‐lean composition (Al0.75CrMoTaTi), and a tantalum‐lean composition (AlCrMoTa0.75Ti) are produced via arc melting. The alloys exhibit multiphase structures, confirmed by X‐ray diffraction, microstructural characterization, and thermal analysis. The minor off‐equiatomic adjustments of aluminum and tantalum in this alloy system did not drastically alter the prevalence of the Cr–Ta‐based Laves phase. Correlations between thermodynamic predictions and observed phase transformations via thermal analysis are improved upon refinement of calculations removing impractical intermediate phases. Experimental findings provide information for the refinement of thermodynamic modeling and deliver additional insight into the optimization of alloy compositions within this five‐component system.

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The effect of Al on the formation and stability of a BCC – B2 microstructure in a refractory metal high entropy superalloy system

Cited by 40 publications

References 43 publications

Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures

Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures

An Investigation of the Miscibility Gap Controlling Phase Formation in Refractory Metal High Entropy Superalloys via the Ti-Nb-Zr Constituent System

Fundamental Effects of Al and Ta on Microstructure and Phase Transformations in the Al–Cr–Mo–Ta–Ti Refractory Complex Concentrated Alloy System

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