Transformation-induced plasticity in the heterogeneous microstructured Ti-Zr-Nb-Sn alloy via in-situ alloying with directed energy deposition
Graphical Abstract
Introduction
Ti-Zr-Nb-Sn alloys have recently developed to achieve high strength, excellent biocompatibility, and low Young’s modulus for biomedical applications [1], [2], [3], [4], [5]. Such outstanding properties of Ti-Zr-Nb-Sn alloys are associated with a body-centered cubic (BCC) β → orthorhombic α′′ martensitic phase transformation, and this phase transformation usually occurs at the grain boundaries, which have high interfacial strength and relatively weak bonding [6], [7]. This means that the martensitic phase transformation behavior in Ti-Zr-Nb-Sn alloys can be controlled in specific regions if the high interfacial regions are engineered by adding inclusions. Traditionally, it was difficult to control the inclusion addition in a specific area, but recent additive manufacturing (AM) methods allow the design of artificial microstructures by implementing inclusions or other phases in the matrix [8], [9], [10]. Among the metal-based AM technologies, the laser-based directed energy deposition (DED) method has the advantage of designing heterogeneous microstructures owing to its high-energy laser beam and flexible powder feeding, which allows for the creation of new alloy systems called in-situ alloying [11], [12], [13], [14]. Compared with AM using pre-alloyed powders, in situ alloying has several benefits for the development of AM-processed metallic alloys, including a flexible powder usages and reducing difficulties for manufacturing pre-alloyed powders.
Therefore, many researchers have attempted to use an in-situ alloying strategy with laser-based AM [15], [16], [17], [18], [19], [20]. Zhang et al. spatially modulated Ti-6Al-4V alloy with 316L stainless steel using an in-situ design method, and the modulated alloy exhibited progressive transformation-induced plasticity (TRIP) that led to a high tensile strength (~1.3 GPa) with moderate ductility [18]. Arias-González et al. attempted to design Ti-Nb and Ti-Zr-Nb alloys using elemental powders and successively built them without external defects, although some Nb particles remained in the matrix [19]. Polozov et al. also designed Ti-Al-Nb alloys using an in situ alloying strategy and revealed that the post-heat treatment required the removal of unmelted Nb particles from the matrix [20].
The previous in-situ alloying results on Ti-Nb-X alloy systems indicate that Nb particles, which have a high melting temperature (2750 K), are difficult to completely melt during laser-based AM, and many studies have attempted to remove them by conducting post-treatments. On the other viewpoints, however, such unmelted Nb particles can induce interfacial regions with a matrix where contains high interfacial energy. Since the BCC β → orthorhombic α′′ martensitic phase transformation usually occurs in high interfacial energy regions, it might be possible to induce phase transformation near the unmelted particles. However, there has been no specific investigation on the TRIP behavior in heterogeneous microstructure Ti-Nb-X alloy systems. Therefore, in the present work, the TRIP of in situ alloyed Ti-Zr-Nb-Sn alloys was investigated by conducting mechanical tests and microstructural characterization. To fabricate a heterogeneous microstructured Ti-Zr-Nb-Sn sample, a DED machine equipped with a multiple hopper powder feeding system was used with the elemental powders. In addition, scanning electron microscopy (SEM)-based observations were conducted to investigate the phase transformation behavior during tensile deformation. Based on the experimental results, the deformation-induced α′′-martensitic transformation behavior and the strengthening of the in situ alloyed Ti-Zr-Nb-Sn sample will be discussed.
Section snippets
Experimental procedure
The gas-atomized elemental powders for DED were provided by MK Metal (Korea), and Fig. 1 shows the SEM micrographs and particle size distribution diagrams of each elemental powder. The elemental Zr powder was replaced with Zr-2.65Nb pre-alloyed powder (Zircaloy-4, ATI Specialty Material, USA) because of safety issues. The Sn and Zr-2.65Nb powders (Fig. 1(b) and (d)) represent relatively rough surfaces with satellite particles compared with the Ti and Nb powders (Fig. 1(a) and (c)), which may
Results
Fig. 3 shows an optical micrograph of the in situ alloyed Ti-Zr-Nb-Sn specimen. In 170 and 200, several lack-of-fusion sites (yellow arrows) were observed owing to the limited energy density from the low laser power. Such a lack of fusion is reduced as the laser power increases and disappears in the 260 sample, which experiences the largest energy density. Because the melting temperature of Nb particles (2750 K) is significantly higher than that of other elements (Ti: 1961 K, Zr: 2128 K, Sn:
TRIP behavior of the in-situ alloyed Ti-Nb-Zr-Sn sample
Fig. 11 summarizes the deformation-induced BCC → orthorhombic phase transformation behavior in the conventional Ti-Zr-Nb-Sn alloys and the present in-situ alloyed sample. In conventionally processed Ti-Zr-Nb-Sn alloys, the BCC → orthorhombic phase transformation is initiated at the grain boundaries, which have a high interfacial energy and relatively weak bonding [6], [7]. This means that a similar phase transformation can be induced in high interfacial energy regions, such as inclusions and
Conclusions
In the present work, the role of a heterogeneous microstructure on the deformation-induced martensitic transformation behavior of in situ alloyed Ti-Nb-Zr-Sn alloy was investigated by conducting DED and SEM-based microstructure characterization. Based on the experimental results, the following conclusions can be drawn.
- 1)
Both the porosity and the unmelted Nb particle size decreased as the laser power increased during the DED process. Because the laser power was insufficient to melt the Nb
CRediT authorship contribution statement
Yukyeong Lee: Data curation, Formal analysis, Investigation, Visualization, Writing – original draft. Shuanglei Li: Formal analysis, Investigation, Writing – review & editing. Eun Seong Kim: Formal analysis, Investigation. Dong Jun Lee: Formal analysis, Investigation. Jae Bok Seol: Writing – review & editing. Hyokyung Sung: Writing – review & editing. Hyoung Seop Kim: Investigation, Writing – review & editing. Taekyung Lee: Writing – review & editing. Jung Seok Oh: Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) [2020R1A4A3079417]. H.S. acknowledges support from the Ministry of Trade, Industry, and Energy of the Republic of Korea (20214000000480). J.G.K. notices that the present work has been supported by the POSCO Science Fellowship of the POSCO TJ Park Foundation. H.S.K. acknowledges the support of a National Research Foundation of Korea (NRF) grant funded by the Korean government (
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