## 2 Materials and Methods  
Production, Preparation, and Investigation of Ti6 Al4V(ELI) Test Samples with Different Microstructures  
The Ti6 Al4V(ELI) test samples for use in this study were produced using the direct metal laser sintering (DMLS) process, one of the trademarks of LPBF. An EOSINT M280 DMLS machine from Electro Optical Systems (EOS) GmbH, Krailling, Germany, was used for this work. The raw material for the process was gas-atomized spherical Ti6 Al4V(ELI) alloy powder. The elemental chemical composition of this powder can be found in [38]. Cylindrical rods with a length and diameter of 80 mm and 6 mm, respectively, were manufactured. The produced samples while still on the machine-build platform are shown in Figure 1.  
Figure 1: Manufactured LPBF Ti6 Al4V (ELI) cylindrical compression test samples while still on the DMLS machine-build platform.  
The samples shown in Figure 1 were initially stress-relieved to alleviate process-related residual stresses before being subdivided into three groups, designated hereinafter as samples C, D, and E, for further high-temperature heat treatment. It is important to note that sample designations A and B were reserved for as-built and stress-relieved samples, respectively. These two forms of alloy were not exposed to mechanical testing as they are known to exhibit non-equilibrium microstructure and therefore low ductility. [16; 17; 39]. Stress-relieving and high-temperature heat treatment cycles were conducted in an SS12/24-13 MDX Super SeriesTM vacuum furnace system that has a horizontal vacuum chamber. The stress-relieving heat treatment was conducted at a temperature of 650 \({}^{\circ}\)C for a residence time of 3 h, followed by furnace cooling to room temperature. A summary of the high-temperature heat treatment cycles applied to samples C, D, and E is shown in Table 1. Controlled cooling rates were achieved by regulating the rate of flow of a stream of argon gas through the furnace chamber. This was done to ensure that there was sufficient grain growth, while minimising the excessive coarsening of the grains formed during cooling.  
Small pieces of samples with a height of 12 mm were cut from the middle area of the samples shown in Figure 1, and then sectioned into halves across the diameter, for preliminary microstructural investigation. The sectioning was executed using a wire-cutting electrical discharge machine (EDM). The sectioned pieces were then hot-mounted using Bakelite, with their sectioned surfaces exposed, followed by the chemical and mechanical polishing of the free surfaces, cleaning under tap water, and finally blowing with a strong stream of air. The polished surfaces were then etched using [PERSON]'s reagent, which consisted of 92 mL of distilled water, 2 mL of hydrofluoric acid, and 6 mL of nitric acid. The microstructures of the LPBF Ti6 Al4V(ELI) samples were then investigated using a JEOL JSM-7001F scanning electron microscope (JEOL, Tokyo, Japan). The SEM was also equipped with an electron backscatter diffraction (EBSD) detector, which was used to study the grain misorientation in the three forms of the LPBF Ti6 Al4V(ELI) alloy.  
The heat-treated cylindrical rods of samples C, D, and E were cut into small compression test samples with a diameter and length of 6 mm and 9 mm, respectively. The end surfaces of these cut compression specimens were then faced off on a lathe machine to ensure perfect contact with the loading cells of the uniaxial testing frame and Gleeble machine during compression testing.