Characterization of Heat Treated Samples

The microstructures, tensile properties, hardness and densities of the heat treated samples were determined using the relevant techniques. Sintered samples were not subjected to tensile testing because their shape could not allow this.

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3.7.1 Metallographical Preparation and Optical Microscopy

The as-cast, sintered, as well as heat treated samples were sectioned by a LECO MSX 205A machine, with a 20S30 silicon carbide cutting blade. To avoid burning during cutting, generous amounts of water-based coolant were used, together with a fast wheel speed (3500 rpm) and slow feed rate (0.5 mm.s-1_{). The appropriately cut samples were mounted and}

metallographically prepared for optical microscopy examination. Cast samples required sectioning of thin discs from their heads, as shown in Figure 3.3.

The sectioned pieces were hot mounted using Polyfast phenolic resin on a Struers hot mounting machine, while sintered samples were mounted without prior sectioning. The samples were ground on silicon carbide paper, and finally polished to a mirror finish using a Struers TegraForce-5 automatic polishing machine. The grinding and polishing of the samples were problematic because the samples scratched very easily. The roughest grit was 200, and the grit was increased in increments of 200 until 1200. The grinding and polishing were automatic, with the force per sample maintained at 25N and a running time of 5 minutes per stage. For both grinding and polishing, the wheel speed was maintained at 150 rpm, counter clockwise to the movement of the sample. For polishing, a 1µm diamond suspension and colloidal silica (OP-S) on an MD-Chem paper were used, with the same force and wheel speed.

After polishing, the samples were dried using compressed air, and chemically etched using Kroll’s reagent (3ml HF, 6ml HNO3 and 100ml deionized water). The cast and sintered samples

were immersed into the etchant for 30 seconds, and immediately washed with deionised water to avoid over-etching. The washed samples were dried using compressed air and stored in a desiccator. Microstructures of the all the samples were analysed using an Olympus BIOX B51M optical microscope, with a resolution of 20µm. The microscope was connected to an Olympus Motion Stream image analyser, supported by MATLAB software, which was used to determine grain size by the intercept method.

3.7.2 Determination of Morphology and Composition of Heat Treated Cast Samples by SEM and EDX

The heat treated samples were analysed using a JEOL JSM 5400 Scanning Electron Microscope (SEM), which was coupled to a JSM Energy Dispersive X-ray Spectrum Analyzer (EDX). Data acquisition was through an integrated Vantage data acquisition instrument. The incident electrons were accelerated at a voltage of 20 kV, and analysis of the samples was done in either the back scattered electron (BSE) or the secondary electron (SE) mode. Image

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magnification ranged from 5X to 15000X, and to authenticate the EDX analysis, a certified 99.99 wt% titanium reference was used to calibrate the instrument.

3.7.3 Hardness Measurements

Hardness measurements for both as-cast, sintered and heat treated cast samples were done using a Future-Tech FM-700 Vickers hardness testing instrument, with a 300g force. After unloading, the diagonals of the indentation were automatically measured and converted to a Vickers hardness number, which was related to the indentation depth (d) (average length of the diagonals of the indentation) and load (F) according to Equation 3.2. Each sample was randomly indented on 10 different areas to give an average.

𝐇𝐕 =𝟏𝟖.𝟐𝐅

𝐝𝟐 Equation 3.2

3.7.4 Tensile Testing of Cast Samples

Tensile tests of the castings, in the as-cast condition and after heat treatment, were done using an MTS Criterion Universal electromechanical uniaxial tensile testing machine. All the castings were cast and no machining was necessary. The only additional operation on the samples was grinding off the alpha casing using a Manfredi HF 50 micro-polishing machine. The tensile specimens conformed to the ASTM-E8 standard, with dimensions, in mm, shown in Figure 3.3. The tensile specimens were held using collar grips, and tensioned to failure. The initial separation of the grips (Lo) and the diameter of the gauge were measured using a vernier

calliper, and the diameter of the gauge was used to calculate the cross-sectional area of the specimens (Ao). Load cells on the instrument continuously recorded the force applied to the

sample throughout the tensioning, and the grip separation was continuously measured as a function of the load. The load on the sample was normalized against the sample’s original cross-sectional area to give the engineering stress. The elongation of the specimens was normalized against the gauge length to give the engineering strain. In all tests, the strain rate was fixed at 0.1mm.min-1 as required by the ASTM-E8 standard. Data analysis was done on the instrument’s integrated data acquisition computer using TESTWORKS 4 software to allow for the plotting of the stress-strain curve, elastic limit, yield strength, ultimate tensile strength. Reduction in area was determined by measuring the area at the breakpoint of the specimens and dividing it against the original cross-sectional area.

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The relative amounts and compositions of the equilibrium phases of the test alloy were calculated using the titanium database V3 (TTTI3) in Thermo-Calc. The input was the nominal composition of the test alloy and pressure was fixed at 1 atmosphere, while temperature was specified as 1800o_{C to 25}o_{C. Calculations were also performed per phase or crystal structure}

to determine the phases’ individual variations in chemical composition with changing temperature.

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