82 83 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Using pressure and X-ray diffraction to reduce uncertainty in themodelling of Ti-6Al-4V Related publication : MacLeod, S. G., Errandonea, D., Cox, G. A., Cynn, H., Daisenberger, D., Finnegan, S. E., McMahon, M. I., Munro, K. A., Popescu, C., & Storm, C. v. The phase diagramof Ti-6Al-4V at high-pressures and high-temperatures. Journal of Physics: CondensedMatter , 33 154001 (2021). DOI: 10.1088/1361-648X/abdffa Publication keywords: Ti-6Al-4V; X-ray diffraction; High-pressure; High-temperature; Phase transition; Equation-of-state T i-6Al-4V is a titanium alloy used in a wide range of commercial and industrial applications. Its superior strength-to-weight ratio, resistance to corrosion and ease ofmachinability are desirablematerial properties. Ti-6Al-4V is particularly attractive to the aerospace, automotive and defence sectors. Design engineers in these industries require accurate experimental data to improve their materials models to meet the constant demand to improve performance. Researchers wanted to study the crystal structure of Ti-6Al-4V at high pressures and temperatures, to generate a pressure-temperature phase diagram for Ti-6Al-4V. They used the Extreme Conditions beamline (I15). This beamline is dedicated to high-pressure research and provides all the technical expertise and ancillary equipment needed for a successful outcome. The X-ray micro-focus of 20 microns is essential for studying materials properties up toMbar pressures in diamond anvil cells (DACs). The research teamused the experience gained performing DAC experiments at I15 over many years to help develop their resistive heating capability, working closely with the beamline staff. As a result, the team was able to study Ti-6Al-4V up to 93 GPa and ~850 K using DACs, X-ray diffraction and resistive heating. From the diffraction patterns collected and analysed, they produced an experimental pressure-temperature phase diagram for Ti-6Al-4V and used the data to constrain their theoretical model for Ti-6Al-4V. The a-phase of titanium is hexagonal-close-packed at ambient conditions (see Fig. 1). Ti-6Al-4V (90% by weight titanium, 6% by weight aluminium, and 4% by weight vanadium) is an alloy created to stabilise the a -phase character of titanium. Ti-6Al-4V crystallises predominantly in the a phase, with a small amount of β-phase, or body-centred-cubic structure, around the a grain boundaries. Alloying aluminium and vanadium with titanium, in the presence of impurities oxygen, carbon and iron, produces a metal possessing a greater hardness, yield strength and tensile strength than pure titanium. Since the mechanical properties are strongly influenced by the underlying crystal structures, defects, impurities, and grain boundaries, design engineers require models that accurately represent all of these properties. However, without quality experimental data to constrain and validate these models, design uncertainties will not improve. Surprisingly, the crystal structures of Ti-6Al-4V have rarely been studied at the extreme pressures and temperatures commercial and industrial designs may be exposed to during operation, for example aerospace vehicles during the various stages of flight. This paucity of data impedes the development of a truly predictive Ti-6Al-4V model. The structural pathway for pure titanium and Ti-6Al-4V is shown in Fig. 1. At room temperature, and under compression, a -titanium transitions to a hexagonal but more brittle w -phase structure at 3-11 GPa 1 . The a -β- w triple point, where all 3 phases can coexist at equilibrium conditions, is estimated to occur at 7.5 GPa and 913 K 2 . Since Ti-6Al-4V is strengthened in the more desirable a -phase, the aim of this work was to investigate how much the structural pathway of Ti-6Al-4V diverged away from titanium. The diamond anvil cell (DAC) is a device for studying materials at static high pressures. It comprises two back-to-back diamonds with a metal gasket between them to form a pressure chamber. Driving the diamonds together reduces the chamber volume and increases the pressure on the material in the chamber. DACs are frequently used to study the equations-of-state (EoS) of materials. The EoS of a material can be described as the variation of its pressure as a function of atomic volume and temperature and is best studied at a dedicated high-pressure synchrotron beamline. Ti-6Al-4V samples were loaded into several DACs, alongside materials known as pressure gauges, whose thermal EoSs have been previously characterised 3 . Resistive heaters wrapped around the DACs ensured samples could be heated to ~900 K. K-type thermocouples monitored the temperature. Under compression, isotherms were collected at 298 K, 418(2) K, 517(2) K, 586(2) K, 642(3) K, 713(1) K, 844(5) K, and 886(2) K to study the a + w transition. The highest pressure reached was 93 GPa at ~850 K. The waterfall plot of integrated angle-dispersive X-ray diffraction patterns in Fig. 2 shows typical results for the isotherm collected at 642(3) K. The a → w phase transition occurs at ~29 GPa, with both phases coexisting until the transition to the w phase is completed at ~40 GPa. Thermal EoSs were generated for the a and w phases of Ti-6Al-4V. The bulk moduli (or compressibility) were found to be 110(2) GPa for a and 115(8) GPa for w , in good agreement with titanium, which is 114(3) GPa for a and 107(3) GPa for w 4 . In parallel with the DAC study, a large volume press (LVP) experiment, which measured a change in electrical resistivity rather than X-ray diffraction to signify a phase change, determined the (a+ β ) → β transition at high temperatures. Resistivity changes were detected at 1,230(30), 1,180(30), and 1,140(30) K, and at pressures 4.0(2), 8.0(2) and 12.0(2) GPa respectively. Data from the DAC and LVP experiments were combined to produce the pressure-temperature phase diagram shown in Fig. 3. Extrapolating the LVP and DAC data suggests the a -β- w triple point occurs at ~30 GPa and ~910 K. An almost vertical a - w phase boundary, with slope ~550 K GPa -1 , coupled with an ( a + w ) phase coexistence of 8-10 GPa up to 844 K is indicative of a weak temperature dependence on the kinetic barrier to the transition to the w -phase. This data was then used, along with previous experimental data collected at room temperature 5 , to constrain a calculation using the particle swarm optimisation (PSO) method, represented by the grey dashed lines in Fig. 3. Although this work represents the first time the alloy Ti-6Al-4V has been studied at high-pressure and high-temperature using synchrotron X-rays, there is still much work to do in gaining a better understanding of the phase diagram, for example, confirmation of the a -β- w triple point, measuring the w -β phase boundary and the melt curve. Extending our experimental reach should lead to improvements in our modelling capability and have real-world impact. Future DAC experiments planned at I15 will incorporate a new resistive heater capable of reaching 2,000+ K. In addition, the existing laser heating apparatus will be used to study melting. References: 1. Errandonea, D. et al. Pressure-induced transition in titanium metal: a systematic study of the effects of uniaxial stress. Physica B: Condensed Matter , 355 116–125 (2005). DOI: 10.1016/j.physb.2004.10.030 2. Zhang, J. et al . Experimental constraints on the phase diagram of titanium metal. Journal of Physics and Chemistry of Solids , 69 2559–2563 (2008). DOI: 10.1016/j.jpcs.2008.05.016 3. Dorogokupets, P. I. et al . Equations of state of MgO, Au, Pt, NaCl-B1, and NaCl-B2: Internally consistent high-temperature pressure scales. High Pressure Research , 27 431–446 (2007). DOI : 10.1080/08957950701659700 4. Zhang, J. et al . Thermal equations of state for titanium obtained by high pressure—temperature diffraction studies. Physical Review B , 78 054119 (2008). DOI: 10.1103/PhysRevB.78.054119 5. MacLeod, S. G. et al . Experimental and theoretical study of Ti-6Al- 4V to 220 GPa. Physical Review B , 85 224202 (2012). DOI: 10.1103/ PhysRevB.85.224202 Funding acknowledgement: We thank Diamond Light Source for access to beamline I15 (EE8176 and EE9366). Corresponding author: Dr Simon MacLeod, AWE Plc,
[email protected] Crystallography Group Beamline I15 Figure 1: The transformation pathway for titanium and Ti-6Al-4V at high-pressure and high-temperature. Figure 2: A waterfall plot of integrated ADXRD patterns showing the a → w transition in Ti-6Al-4V at 642 K. The a and w phase peaks are all indexed according to their X-ray scattering planes. The arrow at 29.8 GPa points to the emergent dominant w phase (101)/(110) peaks. * represents reflections from NaCl, + from Cu, and † from the gasket material. Figure 3: Phase diagram of Ti-6Al-4V, constructed from DAC isotherms and the LVP data, and showing the range of the desirable a phase. Red circles represent: a; half-red half-white circles, mixed phase ( a+w ); white circles w ; black diamonds, phase changes using Bridgman cell. Data from previous work: half-dark yellow half-white squares mixed phase ( w+b ) 5 ; dark yellow triangles b 5 . Black dashed lines are a visual guide for proposed a-w and ( a+b)-b phase boundaries. Black dashed-dot line indicates complete transition to w from ( a+w ). Grey dashed lines represent solid-solid phase boundaries calculated using particle swarm optimisation.