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Page 16 of 25 Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33

component. For instance, a columnar and textured grain structure is ideal for the pseudoelastic behaviour
seen in Fe-Mn-Al SMAs [101,102] and the unrestricted martensitic phase transformation for shape memory in
[103]
Cu-based SMAs .

Possible factors influencing hardness
Effect of grain size
Figure 8 shows the hardness of the LPBF-built alloy prepared as a function of LED. The two low LEDs (0.25
and 0.29 J/mm) have a close hardness value (278 ± 7.6 and 273 ± 3.9 HV2, respectively). The hardness in the
two high LEDs (0.44 and 0.88 J/mm) is also close (287 ± 5.5 and 292 ± 3.6 HV2, respectively.) Meanwhile,
the reference as-cast alloy had the lowest hardness (226 ± 6.7 HV2). The hardness of the material varies with
grain size according to the classic Hall-Petch relation [104,105] . Also, in Figure 8, the grain size increases with an
increase in LED up to 0.44 J/mm, and then drops when the laser re-scanning step was added to achieve
0.88 J/mm LED. This change was associated with the thermal history of the LPBF alloy [Figure 7]. The
reference alloy has a lower hardness than each of the LPBF-fabricated alloys. This is due to the coarse,
equiaxed grains generated in the reference alloy by hot working and the 14-h homogenisation [17,106,107] . In the
LPBF alloy, the hardness is seen to increase together with the grain size, thereby negating the established
influence of grain size on hardness. This suggests that some other factor affects the hardness of the LPBF-
fabricated alloys.

Effect of phase types
Figure 9 shows the relationship between the volume fraction of phases and hardness as the LED is
increased. This parameter was also found to influence the relative volume fractions of austenite and
ε-martensite in the LPBF alloy, whereby austenite decreases while ε-martensite increases with increasing
LED. Martensite is formed from austenite by either a stress or thermally induced transformation [41,108-110] ,
which results in the observed inverse relationship between the two phases. The effect of LED on the volume
fractions of the phases was associated with the grain size, and that is, fine grains are detrimental to the
formation of the ε-martensite phase [111,112] . The increase in ε-martensite volume fraction may also be caused
[113]
by the decrease in Mn concentration at high LED [Figures 3B and 9] . Hardness as a function of phase
volume fraction is also given in Figure 9, where it appears that hardness directly correlates with the amount
of ε-martensite in the microstructure. This confirms that both the type and volume of phases present in the
LPBF-fabricated alloy have a very strong effect on hardness.

Boundaries exist between the phases in a multi-phased material, and each phase has a distinct
[114]
characteristic . The reference alloy was fully austenitic, whereas the LPBF alloy contains both austenite
and ε-martensite, and other minor phases [Figure 5 and Table 2]. Since austenite is much softer than ε-
[115]
martensite , this resulted in the low hardness of the reference alloy. In comparison, the amount of
austenite and the pre-existing ε-martensite in the LPBF alloy, for example, in 0.44 J/mm LED, were 62% and
31%, respectively. The relationship between hardness and the volume fraction of ε-martensite has also been
reported in a powder metallurgy fabricated Fe-30Mn-6Si alloy . A high hardness was found in the as-
[116]
sintered condition, but it decreased after heat treatment because of the corresponding decrease in ε-
martensite. The addition of 5 wt.% Cr, an austenite stabiliser [117,118] , in an as-cast Fe-30Mn-6Si alloy also
resulted in a soft alloy due to the absence of ε-martensite .
[119]

[120]
Pre-existing ε-martensite has been reported to block plastic flow, which leads to high work hardening .
The impeding action of pre-existing ε plates was observed by Sato et al. using TEM, and they also reported a
hardened Fe-30Mn-1Si alloy . The group likened the ε plate phase boundary to a grain boundary. In the
[1]
Fe-30Mn-6Si reference and LPBF alloys, ε plates may have nucleated and grown in the austenite grains