Characterization of the α'-Martensite phase and its decomposition in Ti-6Al-4V additively manufactured by selective laser melting

Cho, J 2018, Characterization of the α'-Martensite phase and its decomposition in Ti-6Al-4V additively manufactured by selective laser melting, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Characterization of the α'-Martensite phase and its decomposition in Ti-6Al-4V additively manufactured by selective laser melting
Author(s) Cho, J
Year 2018
Abstract This thesis has focused on characterising the α′-martensite phase in Ti-6Al-4V additively manufactured by selective laser melting (SLM) and understanding its decomposition behaviours under different conditions. The α'-martensite phase is a predominant phase in SLM-fabricated Ti-6Al-4V alloy under typical SLM processing conditions. It can be categorized into four types, namely primary, secondary, tertiary and quaternary, in accordance with their size and location in prior-β grains. Some α'-martensite phases are heavily twinned while other α'-martensite phases contain very few twins. It is shown that SLM-fabricated Ti-6Al-4V alloy that has smaller prior-β grain sizes and a larger amount of twinned martensite plates exhibit better tensile strength (1034 MPa) and ductility (11.1 %) than other SLM-fabricated Ti-6Al-4V.

The slice-and-view technique with focused ion beam / scanning electron microscope (FIB/SEM) dual beam system has been used to further investigate the three dimensional (3D) morphology of the α'-martensite phase. Although different morphological features of the α'-martensite phase have been observed in 2D micrographs, it is confirmed that the 3D morphology of the α'-martensite phase in SLM-fabricated Ti-6Al-4V is all plate- or patch-like, irrespective of their size. The α'-martensite phase forms in sequence from primary to quaternary martensite in a manner of gradually filling the space of the retained β-phase between the martensite plates that are formed earlier than those to form.

To acquire a profound understanding of the in-situ decomposition of the α′-2 martensite phase during the SLM process, SLM-fabricated Ti-6Al-4V and water-quenched mill-annealed (MA) Ti-6Al-4V specimens were annealed under a variety of conditions.
Twinned α′-laths in SLM-fabricated Ti-6Al-4V are still visible after being annealed at 410 °C for 2 h and at 600 °C for 0.5 h but they disappeared at higher annealing temperatures or longer annealing times. The β-phase began to be observed in both SLM-fabricated Ti-6Al-4V and water-quenched mill-annealed (MA) Ti-6Al-4V when annealed at temperatures higher than 600°C for longer than 2 h. The morphology of the β-phase changed from discrete particle-like to thin fibre-like with increasing annealing temperature above 600 °C for a fixed annealing time of 2 h.

The thickness of the α-phase decomposed from the α′-phase in SLM-fabricated and water-quenched MA Ti-6Al-4V grew with increasing annealing time and/or temperature. The growth rate of the α-lath thickness in water-quenched MA Ti-6Al-4V was faster than that in SLM-fabricated Ti-6Al-4V with increasing annealing temperature for a fixed annealing time of 2 h. In contrast, their growth rates are similar when annealed at 800 °C with respect to different times. The aspect ratio of the α-phase in both alloys gradually decreased with increasing annealing temperature and time. The α′-phase was retained in both alloys when annealed at temperatures below or at 600 °C. For example, no significant change in the aspect ratio and thickness of the α'-phase in both SLM-fabricated and water-quenched MA Ti-6Al4V was observed after annealing at 600 °C for various annealing times. The α′-phase gradually decomposed into the α-phase and -phase at temperatures above 600 °C. However, the α-phase in the SLM-fabricated Ti-6Al-4V showed lower aspect ratios than that in waterquenched MA Ti-6Al-4V after annealing at temperatures from 700 °C to 930 °C for a constant hold of 2 h or at 800 °C for 0.5 h to 20 h. The different growth rates and changes in 3 aspect ratio of the α-phase in both alloys are attributed to different microstructures such as the size of the α′-phase and prior-β grains in those alloys before annealing.

High energy synchrotron X-ray diffraction was employed to investigate the decomposition of the α′-martensite phase in SLM-fabricated and water-quenched MA Ti-6Al4V. The α′-martensite phase and a small amount of β phase were found in both alloys. No α″-martensite and ω-phase were found during in-situ heating. However, the α2-phase (Ti3Al) appeared at temperatures higher than 500 °C in both alloys. With increasing temperature, the micro-strain level applied to the HCP lattice (α- or α′-phase) was reduced while the c/a ratio of the lattice was increased. The volume fraction of the β-phase kept increasing at temperatures higher than 700 °C. The lattice volume of the α- or α′-phase in water-quenched MA Ti-6Al-4V gradually increased while that in SLM-fabricated Ti-6Al-4V fluctuated. The twinned α′-phase in the SLM-fabricated Ti-6Al-4V showed different decomposition
behaviours relative to the α′-phase in water-quenched MA Ti-6Al-4V.

Scanning electron microscopy and micro X-ray diffraction (μ-XRD) were performed to investigate microstructural inhomogeneity in SLM-fabricated Ti-6Al-4V cylindrical specimens of 12 mm in diameter and 20 mm in height. μ-XRD (beam size: 500 μm) was applied to 40 uniformly spaced regions at different build heights, assisted with microscopic examination. The thickness of α- or α′-phase gradually decreased from the bottom to the top of the SLM-fabricated Ti-6Al-4V specimen. The distribution of lattice
parameters for each phase was mapped out based on a detailed analysis of the μ-XRD twodimensional (2D) diffraction data (ring patterns). The α-phase and β-phase were observed in the bottom half of the specimen while the region above the mid-height is comprised of predominant α'-martensite with the top 3.5-mm thick region being fully martensitic. In 4 relation to this, the elastic modulus was found to decrease almost linearly from the midheight to the top. It was further shown that μ-XRD 2D ring patterns offered an improved approach to characterising the β-phase in SLM Ti-6Al-4V.

The research findings made from this project contribute to an improved understanding of the α′-martensite phase in SLM-fabricated Ti-6Al-4V.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Metals and Alloy Materials
Keyword(s) Additive manufacturing
Selective laser melting
Laser powder bed fusion
Decomposition of martensite
3D microstructure
In-situ observation by synchrotron X-ray source
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Created: Thu, 28 Mar 2019, 10:35:56 EST by Adam Rivett
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