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Instruments / Technical Area	Publication Details	Published
I12-JEEP: Joint Engineering, Environmental and Processing	Microstructural stability and evolution in a new polycrystalline Ni-base superalloy G. J. Wise , N. L. Church , C. E. P. Talbot , P. M. Mignanelli , M. C. Hardy , N. G. Jones , H. J. Stone Metallurgical And Materials Transactions A 64 DOI: 10.1007/s11661-023-07211-9 Diamond Proposal Number(s): [30411, 31965] Open Access Show Abstract ▼ Abstract: Polycrystalline Ni-base superalloys that offer equivalent or superior performance to current commercial alloys at lower overall cost are of widespread industrial interest. In this work, a new polycrystalline Ni-base superalloy with low elemental cost has been characterized and compared to current commercially available alternatives. Through a combination of scanning electron microscopy, thermal analysis, synchrotron X-ray diffraction, and hardness testing, a broad preliminary investigation of fundamental alloy properties has been performed, identifying the key areas for further alloy development opportunities.	Nov 2023
I11-High Resolution Powder Diffraction	Delayed parabolic oxidation via transient thermal exposures on a polycrystalline nickel-based superalloy J. W. X. Wo , D. M. Collins , M. P. Taylor , M. C. Hardy , H. J. Stone Corrosion Science 47 DOI: 10.1016/j.corsci.2023.111675 Diamond Proposal Number(s): [18972] Open Access Show Abstract ▼ Abstract: Multiple oxides were observed during the transient oxidation stage of the polycrystalline Ni-based superalloy, RR1000, before a protective Cr2O3 scale formed. Thermogravimetric analysis, synchrotron grazing incidence X-ray diffraction, and electron microscopy were performed on samples subjected to isothermal exposures at 800 °C for up to 100 hours. Transient effects governed the early stages up to 40 hours. NiO, spinels (NiCr2O4, (Ni,Co)(Cr,Co)2O4), Cr2O3/(Cr0.88Ti0.12)2O3, NiTiO3, and CrTaO4 formed during the initial stage with pseudo-linear kinetics. At the onset of parabolic kinetics, extensive Cr2O3 and TiO2 growth dominated scale formation with the former emerging as the major passivating oxide.	Nov 2023
	On the role of internal stresses on the superelastic behaviour of Ti-24Nb (at.%) E. M. Hildyard , L. D. Connor , N. L. Church , T. E. Whitfield , N. Martin , D. Rugg , H. J. Stone , N. G. Jones Acta Materialia 273–275 DOI: 10.1016/j.actamat.2022.118161 Show Abstract ▼ Abstract: Ti-Nb-based alloys undergo a martensitic transformation that gives rise to superelastic and shape memory behaviours over a wide range of temperatures. The design of such alloys for industrial applications is commonly based around the reported martensite start temperature for a given composition. However, there are significant variations in these values within the literature that remain unexplained using current thermally driven transformation theories. Recently, a marked difference in the transformation behaviour of Ti-24Nb (at.%) in the cold rolled and solution treated conditions has been reported and a stress based mechanism has been postulated to rationalise these results. Here, the veracity of this stress based theory has been investigated through in situ studies of the transformation behaviour of Ti-24Nb (at.%) in the cold rolled condition using synchrotron X-ray diffraction. Upon heating to 350˚C, all αʺ initially present reverted to the β phase by ∼ 300˚C. When cooled from 350˚C, thermally induced martensite was observed below ∼ 270˚C and the intensity of the αʺ peaks increased as the temperature decreased. When loaded at temperatures between -150 and 30˚C, further transformation was observed to occur immediately upon the application of a stress. However, critically, αʺ under load was always found to be reversible upon unloading, despite being up to 420˚C below the observed martensite start temperature. This behaviour cannot be accounted for through conventional thermally driven transformation theories for these alloys. Comparison of the stress-strain curves for cold rolled and solution heat treated condition material indicated that their behaviours were similar but offset by a value equivalent to the difference in the critical transformation stress. This offset is readily explained by the total stress-based mechanism proposed and provides further evidence as to its fidelity.	Jul 2022
I12-JEEP: Joint Engineering, Environmental and Processing	The nature of the β→α″ transformation in Ti-Nb alloys E. M. Hildyard , L. D. Connor , N. Martin , D. Rugg , H. J. Stone , N. G. Jones Matec Web Of Conferences 321 The 14th World Conference on Titanium DOI: 10.1051/matecconf/202032111052 Open Access Show Abstract ▼ Abstract: The martensitic transformation in Ti-Nb alloys can result in superelastic and shape memory behaviour but significant discrepancies exist between the transformation conditions reported for a given composition. To elucidate the reasons for these variations, in situ synchrotron diffraction experiments have been performed on Ti-24Nb (at.%) in two different microstructural conditions. Markedly different transformation behaviour was observed between these two conditions, including fully reversible superelastic behaviour below the apparent martensitic start temperature. These results could not be rationalised using thermally based transformation descriptions. As such, a new approach is introduced based upon the total stress level in a material.	Oct 2020
I12-JEEP: Joint Engineering, Environmental and Processing	The influence of microstructural condition on the phase transformations in Ti-24Nb (at.%) E. M. Hildyard , L. D. Connor , L. Owen , D. Rugg , N. Martin , H. J. Stone , N. G. Jones Acta Materialia DOI: 10.1016/j.actamat.2020.08.004 Diamond Proposal Number(s): [16210] Show Abstract ▼ Abstract: Ti-Nb based alloys exhibit superelastic and shape memory properties over a wide range of temperatures, making them attractive for industrial applications. However, within the literature there are significant variations in the reported transformation behaviour for a given composition that remain unexplained. These variations are problematic to engineering design and limit the industrial uptake of these alloys. To investigate the source of these discrepancies, the transformation behaviour of Ti-24Nb (at.%) in two significantly different microstructural conditions, following cold rolling (CR) and following a subsequent solution heat treatment (ST), has been studied using in situ synchrotron diffraction data whilst undergoing a three step thermal cycle. When cooling from 350 to -196˚C, the CR condition material underwent both the β to αʺ and β to ω transformations, whilst only the β to ω transformation was observed in the ST condition material. These results show that the transformation behaviour of Ti-Nb alloys, in particular the β to αʺ transformation, is significantly influenced by prior processing and that theories of mutual exclusivity between αʺ and ω phase formation need revisiting. In addition, several features within the datasets were inconsistent with a solely thermally driven martensite formation mechanism and an alternative description that includes other factors is discussed.	Aug 2020
	In situ observations of continuous cooling transformations in low alloy steels E. J. Pickering , J. Collins , A. Stark , L. D. Connor , A. A. Kiely , H. J. Stone Materials Characterization DOI: 10.1016/j.matchar.2020.110355 Open Access Show Abstract ▼ Abstract: A typical heat treatment for a low alloy steel will often involve a quenching heat treatment step, in which the steel is cooled from high temperatures to trigger austenite decomposition. The particular cooling rate during the quenching step can have a marked influence on the phase transformations taking place, and the resulting steel microstructure and mechanical properties. Although methods such as dilatometry have been available for many decades to characterise continuous-cooling transformation (CCT) behaviour in steels, the use of in situ synchrotron X-ray diffraction (SXRD) to elucidate CCT behaviour in a systematic way has not been reported. In this work, we measure the CCT behaviours of two pressure vessel steels in situ using simultaneous dilatometry and SXRD. Both steels are subject to austenitisation followed by quenching at a range of cooling rates. On comparing results from SXRD and dilatometry, it is found that recorded starts of transformations appear to be in good agreement. However, calculations of phase fractions derived from dilatometry data significantly overestimate the fraction of ferrite that forms in comparison to SXRD when the formation involves the partitioning of carbon. This happens for two reasons: first, because the method to extract ferrite volume fractions from dilatometry data generally ignores the presence of any retained austenite at low temperatures, and second, because analyses of dilatometry data do not account for the expansion of the austenite during transformation due to enrichment in carbon. This enrichment leads to an increase in strain, and the standard analysis method falsely attributes this increase to ferrite formation, thereby overestimating it. The results highlight that caution must be exercised when interpreting the results of dilatometry, since levels of ferrite (especially diffusively-formed) and retained austenite are important quantities for the prediction of mechanical behaviour, and they are not readily quantified by the analysis of dilatometry data alone.	May 2020

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