High Ductility in a fully martensitic microstructure: a paradox in a Ti alloy produced by selective laser melting

ABSTRACT A fully martensitic Ti alloy consisting entirely of hexagonal-close-packed α′ was produced by selective laser melting (SLM), exhibiting an impressive combination of high strength and excellent ductility. A small quantity of body-centred-cubic β formed at some large primary α′ plates oriented at ∼45° to the tensile direction was found responsible for early fracture owing to α′/β strain incompatibility. Properly selected SLM parameters resulted in a β-free microstructure containing α′/α′ interfaces only, preventing premature fracture. Consequently, large plastic deformation was enabled, creating randomly oriented nano-α′ crystals. The investigation dispels the myth that α′ is inherently brittle. GRAPHICAL ABSTRACT Impact Statement A fully martensitic α′ Ti-6Al-4V with an impressive combination of high ductility and strength was produced using selective laser melting, dispelling the myth that α′ is inherently brittle.


Introduction
Microstructures of the α/β alloy Ti-6Al-4V, one of the most widely used, can be easily manipulated through thermomechanical treatment thanks to a versatile combination of four phases: the equilibrium phases of Al-rich hcp α and V-rich bcc β, and the metastable martensitic phases of orthorhombic α and acicular α which has the same hcp structure as α but is supersaturated with V [1][2][3]. Recently, particular attention has been paid to Ti-6Al-4V processed by additive manufacturing (AM) [4][5][6][7][8][9][10][11]. The high cooling rates ( ∼ 10 6 K/s) involved in AM processes such as selective laser melting (SLM) and electron beam melting (EBM) result in a microstructure dominated by α with tensile elongations of < 8% [4][5][6][7][8][9][10][11]. The low ductility is generally attributed to the inherent brittleness of α , and consequently, it is common practice to decompose it into α and β by post-AM heat treatments More importantly perhaps, the martensitic transformation would generate acicular α in intersecting directions with a broad size distribution [2]. This unique structure is different from the typical structure for α which forms parallel plates upon slower cooling. Although it is possible to produce a fully α Ti-6Al-4V by quenching, the volume is necessarily limited in order to achieve sufficiently high cooling rates. Moreover, the β grains prior to quenching are usually coarse at several hundreds of microns. By contrast, SLM can generate smaller prior β grains and ultrahigh cooling rates throughout the built volume, ensuring an ultrafine, full α microstructure.
Here, we report, for the first time, the fabrication of a fully martensitic α Ti-6Al-4V using SLM, achieving a combination of excellent tensile elongation of 14-15% and yield strength of 1150 MPa, comparable to those obtained in dual-phase α/β Ti-6Al-4V in wrought form or following ageing at low temperatures [3,[18][19][20].

Experimental material and procedures
A gas atomised Ti-6Al-4V powder (15-45 µm) was used. The Renishaw AM250 SLM system was employed for printing rods of 12 mm in diameter and 80 mm in height using stripe scanning strategy with layer thickness of 30 μm on 4 mm tall supports and a titanium substrate preheated to 170°C in an atmosphere containing < 100 ppm oxygen. AM250 uses a pulsed laser, and the scanning speed (v) is determined by the point distance (PD) between two consecutive laser spots and the laser exposure time (t) in v = PD/t [21]. Table 1 lists the printing parameters for the two samples used, coded PD-45 (PD = 45 µm) and PD-55 (55 µm), respectively. Nearly full densities were achieved in both, as indicated by the high relative density of ≥ 99.5% (to the density of 4.43 g cm -3 for bulk Ti-6Al-4V [1]) in Table 1.
Tensile specimens with a gauge section of φ 6 × 30 mm were machined from the rods printed along the build direction. Tensile tests were conducted at an initial strain rate of 2.5 × 10 −4 s −1 according to ASTM E8/E8M-13. Nearly identical stress-strain curves were obtained from three repeated tests on each type of materials, showing excellent repeatability.

Results
The optical microstructures of PD-55 and PD-45 are shown in Figure 1(a,b), respectively. The first-generation martensitic α plates (arrowed) were oriented at ∼ 45°t o the build direction and extended across columns of prior β grains (dashed-line) formed through epitaxial growth during solidification [22], and such a texture was consistent throughout the entire build volume. The average length of the plates (over > 100 measurements) decreased by nearly 40% from ∼ 80 µm in PD-45 to ∼ 50 µm in PD-55. Much finer later-generation α plates formed between them (Figure 1(c,d)). The microstructure appeared to consist entirely of these intersecting generations of α , as confirmed by XRD in Figure 1(e). Indeed, TEM on all the sections of PD-55, including the grip and gauge as well as the necking area of the tensile specimen, detected no β. However, TEM on PD-45 (section AA in Figure 1(d)) revealed very thin layers of β between primary α plates at isolated locations, as identified by SAEDP (Figure 1(f)) and DF (Figure 1(g)). Also, careful SEM over the entire build length found no AM defects such as lack of fusion or significant pores in both materials. Figure 2(a) shows tensile stress-strain curves for PD-45 and PD-55, revealing yield stresses (σ 0.2 ) of 1150 MPa for both, and fracture strains (ε f ) of 5-6 and 14-15%, respectively. The good ductility of PD-55 is confirmed by observations of necking as well as cup and cone ( Figure 2 Figure 3(a,b) show SEM of PD-55 from the as-built and necking areas, respectively. The typical acicular α (Figure 3(a)) was significantly deformed and refined so that its acicular morphology became barely recognisable ( Figure 3(b)). This is confirmed by BF TEM in the necking area (Figure 3(c,d)), showing α crystals of the order   of 100 nm. The corresponding SAEDP in Figure 3(e) shows rings for α , implying random orientations. Figure 4(a) shows a low magnification SEM of the profile view of the fractured PD-45. The fracture facets (dashed-lines) are oriented at ∼ 40 ± 5°to the tensile direction. A close-up (Figure 4(b)) of the circled area in Figure 4(a) reveals that the facet is exactly parallel to the first-generation α plates (arrowed) in a prior β grain delineated by dotted-lines. At the same time, dimples can be seen in regions of fine, later-generation α plates (Figure 4(b,c)). TEM in the dimpled area along section BB in Figure 4(c) shows severely deformed α plates with entangled dislocations and dislocation cells (Figure 4(d,e)). The corresponding SAEDP in Figure 4(f) is similar to that for PD-55 (Figure 3(e)), indicating a randomly oriented α structure.

Discussion
It is an outstanding achievement that the high ε f of 14-15% and σ 0.2 of 1150 MPa are obtained in a fully martensitic α Ti-6Al-4V alloy (PD-55). To put this in perspective, Table 2 compares with other Ti-6Al-4V alloys produced by different processing methods. All the alloys dominated by acicular α other than PD-55 exhibit much smaller ε f of < 6% with comparable yield strength. Although, the coarse lamellar α/β alloys processed by EBM possess comparable ductility, their strengths are considerably lower by more than 200 MPa. Only those with ultrafine α/β lamellar structures are similar, although their ductility is still lower. It should be noted that the high strength and ductility achieved here are equivalent to those displayed by the best of commercial wrought Ti-6Al-4V with yield strength of 900-1100 MPa and tensile elongation of 10-18% [3].
The high strength in both PD-45 and PD-55 can be derived from the high solute contents in the supersaturated α [13], fine crystallite sizes, as well as substructural dislocations and twins [2,[24][25][26]. The much better ductility in PD-55, however, needs explanation. There are two major differences between the two materials. First, very thin β lamellae can be found, albeit at only isolated locations, in PD-45 (Figure 1(f,g)) while there is a total absence of β in PD-55. This is attributed to the reduction in v by ∼ 18% with decreasing PD from 55 to 45 µm (Table 1), resulting in ∼ 22% increase in the average energy input (E) in PD-45 since E ∝1/v [21]. This extra E is apparently sufficient to raise temperatures at some small number of locations in the previously deposited layers to form thin β, but not enough to cause substantial α decomposition as PD-45 is still dominated by α (Figure 1(e)). Owing to different crystal structures and chemical compositions of α and β, strain incompatibility and stress concentration at the α/β interface are expected. Indeed, FEA demonstrates that both strain and stress are localised at the α/β interfaces [27], contributing to stress concentration at them. When the thin β plates formed in PD-45 (Figure 1(g)) are associated with the first-generation α plates oriented at ∼ 45°to the tensile direction (Figures 1(b) and 4(b)), the α /β interfaces coincide with the plane of maximum shear stress. A high stress concentration is expected, causing crack initiation and propagation along the interface, as evidenced by the facets in Figure 4(a,b). Such a detrimental effect of β  on ductility is also observed in an α dominated Ti-6Al-4V processed by conventional methods [17]. Even if β lamellae could form at later generation α , they would be less damaging as their orientations are off the maximum shear plane and sizes much smaller. By contrast, the total absence of β in PD-55 means a microstructure containing α /α interfaces only, resulting in no strain incompatibility and significantly higher ductility. It should be noted that the same level of ductility ( ≥ 11-12%) at the similar strength level ( ∼ 1100-1200 MPa) can also be obtained in fully lamellar α/β, as shown in Table 2 (the ST + aged with fine lamellar structure, and the SLM with ultrafine lamellar structure). This is because the stress distribution in the alternating lamellar α/β structure is much more even, compared to that at the presence of an isolated β lamella as in the case of PD-45, thanks to the overlapping stress fields from neighbouring plates. As the stress variation decreases with decreasing lamellar thickness, the stress concentration becomes insignificant in the fine and ultrafine lamellar structured materials, leading to improved ductility.
Although the large ( > 80 µm) first-generation α plates oriented at the maximum shear stress appear to cause brittle fracture along the α /β interface in PD-45, the areas between them are β free (Figure 4(f)) and contain much finer α plates of later generations where the fracture is locally ductile with dimple formation (Figure 4(b,c)). Indeed, much refined, randomly oriented α of 100-200 nm containing a large number of entangled dislocations and dislocation cells (Figure 4(d-f)) is observed there, suggesting substantial dislocation slip which cuts and reorients α . Such refinement of α is also observed after cold rolling by ∼ 90% of several α structured Ti alloys, producing nano-scaled dislocation cells [14][15][16]. Such local ductile fracture in a fully martensitic Ti-6Al-4V processed by SLM was also reported elsewhere [9].
The second, although less significant, factor influencing ductility is the sizes of the α plates. In the case of PD-55, not only there is no β anywhere, but also the first generation α plates are considerably smaller (by ∼ 40%, comparing Figure 1(a,b)). For continuous laser, the prior β grain size (Lβ-referring here to the width of the β columnar grain) can be related to the scanning speed (v) and solidification cooling rate (Ṫ) byṪ = 2.07 × 10 4 v 1.2 [28] and L β = 3.1 × 10 6 ·Ṫ −0.93 for Ti-6Al-4V [29], i.e. Lβ would increase with decreasing v. Although such a quantitative relationship has not been established for pulsed laser, the smaller v used for PD-45 (Table 1) indicates a larger prior β width and thus longer first generation α . Since the sizes of the α plates, in particular the first-generation ones bounded by β grain boundaries, increase with the sizes of the prior β grains [22,29], the various generations of α in PD-55 are considerably smaller. Since twinning becomes more difficult with decreasing grain size in a variety of alloys including hcp structured Ti [30][31][32][33], twinning is likely to be more restricted in finer α . Indeed, although quite a number of twins were spotted in the coarser, early generation α in PD-45, there were hardly any twins in PD-55. The absence of twins is expected to enhance dislocation activity by reducing the number of barriers to dislocation movement, leading to enhanced ductility. In addition, a high amount of Al in α promotes basal slip [13,34,35], and a finer α structure can enable activation of a wider range of orientations for the basal slip systems, providing more compatible straining.
In summary, a combination of high yield strength of 1150 MPa and excellent tensile elongation of 14-15% is achieved in a fully martensitic α Ti-6Al-4V alloy by SLM, which is comparable with the best of commercial wrought α/β Ti-6Al-4V. It is critical to produce pure intersecting ultrafine α plates free of β by optimising SLM parameters in order to avoid stress concentration at the α /β interface and easy crack propagation along it. Plastic deformation would then proceed by dislocation slip which cuts and reorients α into randomly oriented nano-scaled grains until final ductile fracture with necking and dimple formation. The usually observed poor ductility associated with α is thus attributable to the difficulty in preventing the formation of β and in producing ultrafine α by conventional processing, rather than to the myth that α is inherently brittle. The present study demonstrates the potential for development of fully martensitic α/β Ti alloys, especially by AM.