Page 159 - MSAM-4-3

Materials Science in Additive Manufacturing L-PBF Ti-10Ta-2Nb-2Zr: Microstructure and Strength

annealing at 900°C for 1 h, followed by furnace cooling. This above the β-transus temperature (862°C), the martensitic
temperature was selected based on the DSC analysis results α’ structure completely transforms to a single-phase β
(Section 3.2), which identified a β-transus finish temperature structure; subsequently, during the slow furnace cooling,
of 862°C, ensuring complete transformation to the β-phase α-phase nucleates preferentially at prior β grain boundaries
during the annealing process. The annealing temperature and grows into the grains as parallel plates along specific
of 900°C was chosen to be approximately 40°C above the crystallographic planes of the parent β-phase.
measured β-transus to guarantee complete transformation
while minimizing grain growth. 3.5.2. Phase composition changes
The microstructural evolution resulting from heat XRD analysis was performed to confirm the phase
treatment is illustrated in Figure 9, which presents SEM transformation resulting from heat treatment. Figure 7
micrographs at different magnifications of the alloy after presents a comparison of the XRD patterns for the as-built
heat treatment. and heat-treated conditions.
The XRD pattern of the heat-treated alloy reveals
As evident in Figure 9A, the heat treatment resulted in
complete recrystallization of the as-built microstructure, significant differences compared to the as-built condition.
While the as-built sample exhibited predominantly α’
with elimination of the directional features and melt pool martensitic phase with peak broadening characteristic
boundaries that were characteristic of the L-PBF process. of high internal stresses, the heat-treated sample shows
The previous fine acicular martensitic structure has been distinct peaks corresponding to both α and β phases. The
transformed into a well-defined lamellar arrangement presence of β-phase peaks, most notably at 2θ ≈ 38 – 39°
of α + β phases. The microstructure exhibits a classical (110) β, confirms the formation of a dual-phase structure
Widmanstätten or “basket-weave” pattern consisting of during heat treatment.
α-plates within prior β grains.
In addition, the α-phase peaks in the heat-treated
At higher magnification (Figure 9B), the structure condition appear sharper and more defined compared to
reveals colonies of parallel α-plates, significantly coarser the as-built condition, indicating reduced internal stresses
than the fine martensitic features observed in the and increased crystallinity resulting from the annealing
as-built condition. The α-plates are arranged in multiple process. The peak positions also show slight shifts,
crystallographic orientations within individual prior β reflecting compositional redistribution between α and β
grains, which exhibit sizes of approximately 50 – 200 μm. phases during the controlled cooling process.
Thin regions of retained β-phase are present between the
α-plates, creating the characteristic lamellar structure. 3.5.3. Effects on mechanical properties
The observed microstructural transformation can be The microstructural evolution resulting from heat treatment
attributed to the following mechanism: during heating had a significant impact on the mechanical properties of

A B

Figure 9. Scanning electron micrographs of Ti-10Ta-2Nb-2Zr alloy after heat treatment (vacuum annealing at 900°C for 1 h, furnace cooled). (A) Low
magnification shows complete recrystallization and elimination of L-PBF-induced directional features, scale bar: 200 μm; magnification: ×500; (B) Higher
magnification revealing characteristic lamellar α + β microstructure, scale bar: 20 μm; magnification: ×5000

Volume 4 Issue 3 (2025) 11 doi: 10.36922/MSAM025220044