Page 87 - MSAM-2-4

Materials Science in Additive Manufacturing Laser DED-produced Ti-6Mn-4Mo alloy

unmelted Mo in the resultant alloy in the present study SEM observations. Hardness of α phase is typically lower
weakened the β stabilizing effect and lead to higher than that of β phase [46,47] .
modulus. In addition, lower concentration of Mo element
might contribute to formation of ω phase, which also 3.6. Fracture surface analysis
increases Young’s modulus. The microstructures of the To gain a deep understanding of tensile fracture
Ti-Mn-Mo alloys produced by MIM and CCLM consist mechanism, fracture surfaces of both as-built and heat-
of equiaxed grains, which vary in size from 37 μm to treated samples were analyzed using SEM and EDS,
165 – 216 μm, respectively [5,8,11] . The average grains in as shown in Figure 14. Figure 14A indicates that the
the current study are much less equiaxed and highly fracture surface of the as-built material is dominated by
elongated, and can reach as long as 500 μm (Figure 6). large areas with smooth surface and large cleavage-like
It is believed that this grain morphology contributes to facets. The higher-magnification SEM image reveals
the low UTS compared to that of Ti-Mn-Mo produced that the smooth surface is covered with a river pattern
by MIM and CCLM reported in the literature. (Figure 14C). Such fracture morphology suggests a brittle
3.5.2. Hardness analysis fracture, which agrees with the observed low elongation
of the as-built sample. The fracture surface of the heat-
Figure 13 shows the Vickers hardness obtained on the treated samples contains large cleavage-like facets as well.
vertical cross-sections of the AB and HT Ti-6Mn-4Mo However, its area contains more dimples compared to
samples. The hardness of as-built sample is 421.1±8.8 the fracture surface of the as-built sample. This explains
HV . Compared with the wrought Ti-6Al-4V (325 HV) , the slight improvement of ductility after heat treatment
[8]
0.1
the as-built Ti-6Mn-4Mo is significantly harder. Hardness for Ti-6Mn-4Mo. However, the elongation value of 1.6
of other Ti-Mn-Mo alloy compositions produced by % is still low in comparison to other Ti-Mn-Mo alloys
CCLM and MIM methods vary from 312 to 392 HV [8,11] . reported in the literature. This can be explained by
In comparison, the hardness of the as-built Ti-6Mn-4Mo the lower concentration of Mo element and potential
in the present study is significantly higher. Hardness presence of embrittling ω phase in Ti-6Mn-4Mo samples.
in Ti-Mn-Mo alloys depends on a few factors, such as The third factor might be the presence of intermetallic
presence of carbides, pores, ω phase, and solid solution phases formed during solidification. An example of such
hardening effect of alloying elements. Hardness tends to intermetallic phase is shown in the inset of Figure 14B.
decrease in Ti alloys with lower amount of ω phase . It shows a large, fractured particle of about 100 μm with
[45]
Consequently, as mentioned earlier, unmelted Mo smooth flat facets. EDS analysis revealed that it consists
particles in the present study could lead to higher amount of only Ti and Mn. The presence of Ti-Mo compound
of ω phase, which, in turn, increases the hardness. The (T Mo) in the heat-treated sample was also confirmed
unusually high hardness of as-built Ti-6Mn-4Mo provides by XRD in this study. Similarly, intermetallic compound
3
potential benefits in biomedical applications where of T Mo was previously observed in Ti–Zr–Hf–Mo high
3
wear resistance is desired. On the other hand, the heat entropy alloy .
[48]
treatment resulted in a noticeable decrease in hardness to
348.14±9.0 HV . The lower hardness value compared to A B
0.1
that of the as-built condition can be explained by higher
amount of α phase, as was demonstrated by XRD and

C D

Figure 14. Scanning electron microscopy fracture surface images of
Figure 13. Vickers hardness of as-built and heat-treated Ti-6Mn-4Mo (A and C) as-built and (B and D) heat-treated Ti-6Mn-4Mo. Yellow boxes
samples, and indentation images. in (A) and (B) are the areas to be magnified, with the higher-magnification
Abbreviations: AB: As-built; HT: Heat-treated. images shown in (C) and (D), respectively.

Volume 2 Issue 4 (2023) 10 https://doi.org/10.36922/msam.2180