Fig. 1. (a) SEM image, with inset highlighting the surface morphology, and (b) the particle size distribution of gas-atomized TA15 alloy powders.

Fig. 2. (a) Schematic illustration of the scan strategy and measurement locations for microstructural observations and XRD analysis, and (b) sample of a 20 × 20 × 5.5 mm3 dimension fabricated by EBSM.

Fig. 3. Phase constitution of the EBSM-built TA15 samples in different regions: (a) XRD patterns of S1, S2, S3, S4, and S5, and (b) enlarged XRD patterns highlighting α(100), and (c) α(002) and α(101) peaks, indicating the existence of martensite α′ in S5.

Fig. 4. Microstructure of EBSM-built TA15 sample: (a) OM images showing the whole cross-sectional image of the as-built samples along the building direction, SEM images of (b) S1; (c) S2; (d) S3; (e) S4; (f) S5 samples, and (g) the average width of adjacent β laths in S1-S5.

Fig. 5. Microstructure and elements distribution near the bottom region: (a) SEM micrographs showing the near-equiaxed to columnar transition of β grains, and (b) magnified image of near-equiaxed grains; and element distribution along the building direction: (c) Ti; (d) Al; (e) V; (f) Mo; and (g) Fe.

Fig. 6. (a) TEM bright-field images showing the microstructural features in S5 of the EBSM-built TA15 sample and (b) HRTEM image of β phase; (c) high density dislocations and (d) twins in martensite α′; (e) magnified image of red ellipse in (d); HRTEM image of red rectangle area in (e), showing shear transformation process during martensitic transformation.

Fig. 7. (a) TEM bright-field images showing the microstructural features in S1 of the EBSM-built TA15 sample; (b) the high density dislocations in α phase; (c) α phase and β phase followed by Burgers orientation relationship; (d) α phase and adjacent β phase; (e-f) filtered HRTEM image taken from the red cycle 1 and 2 in. (d), showing the different orientation relationship in the adjacent α and β phases.

Fig. 8. Vickers hardness along building direction and corresponding indentation morphology (inset).

Fig. 9. (a)Temperature measured by a thermocouple attached to the bottom of the substrate. The insert graph is the enlarged view of point 2 to illustrate the local temperature change during the process; (b) Schematic diagram of heat transfer in EBSM.

Fig. 10. Schematic diagram illustrating the different thermal cycle types of EBSM. Phase analysis indicates the existence time of each phase in different thermal cycles.

Fig. 11. Schematic illustration of microstructural evolution during different thermal cycles (prior β and α′ are in cyan and faint yellow, α in deep yellow and β in brown). (a) Liquid phase and prior β grains as the peak temperature exceeds the liquidus temperature; (b-e) Martensitic transformation in different thermal cycles and a final microstructure consists of different morphologies; (f) martensite α′ decomposes to α and β under the annealing-like effect with the peak temperature under MS; and β precipitates nucleated heterogeneously inside and at plate-plate interfaces of martensite α′.