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Page 2 of 25 Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33

INTRODUCTION
Extensive research has been carried out on Fe-Mn-Si-based shape memory alloys (SMAs) since their first
[1]
development in the 1980s . Similarities with steels in terms of their compositions and production routes
provide confidence for researchers in the quest for commercial applications ; these alloys are now finding
[2]
their way into structural applications. Being more inexpensive than NiTi, the Fe-based and Cu-based SMAs
were identified as viable options for applications requiring shape memory and pseudoelasticity . For
[3]
implant applications, the Fe-based SMAs, in particular, the Fe-Mn-Si alloy, are widely considered because it
consists of essential and non-toxic elements [4-10] , and was even reported to be biocompatible and
noncytotoxic in vivo [11,12] . Therefore, there is a continuing investigation into its shape memory and
biodegradable behaviour for implant applications [13-18] .

Biodegradable implants have attractive properties because they can safely degrade to their elemental
constituents over time, thus eliminating post-surgery removal. With this function in mind, the alloy
composition would then be limited to biocompatible elements. A recent review of biodegradable SMAs
[19]
identified Mg-Sc, Fe-Mn-Si, Fe-Pd, and Fe-Pt alloys as potential candidates , but the Fe-Mn-Si system is
advantageous because of its widely available raw materials.

In contrast to the copious literature on conventionally processed Fe-based SMAs, research on the additive
manufacturing of this alloy system is in its infancy [20-27] . To the best of the authors’ knowledge, alloy
compositions of Fe-36Mn-7Al-9Ni (wt.%), Fe-17Mn-10Cr-5Si-4Ni (wt.%), and Fe-34Mn-8Al-7Ni (at.%)
have been LPBF fabricated to date. In Fe-36Mn-7Al-9Ni alloy, a columnar and highly textured
microstructure was noted in 0.5 mm sized parts built on a 200 °C preheated substrate , but the
[26]
microstructure changed to equiaxed and columnar grains with a weak texture when the substrate was
heated to 500 °C . The conflicting trend in microstructural features was associated with the difference in
[21]
substrate temperature that altered the temperature gradient and solidification rate . Both Ferretto et al.
[21]
and Kim et al. investigated the Fe-17Mn-10Cr-5Si-4Ni alloy and reported a change in microstructure as the
laser power was varied [22,23,28] . A fully austenitic and equiaxed grain structure exhibiting a weak
crystallographic texture was achieved at high laser power, but this changed to a highly elongated, weakly
textured and δ-ferrite dominated structure at lower laser power. The authors suggested that the nucleation
of the austenite grains from the δ-ferrite was possible at high laser power because of the low cooling rate in
this setting. Lastly, Patriarca et al. fabricated a bulk and micro-lattice structured Fe-34Mn-8Al-7Ni alloys
[27]
and heat treated the alloys to achieve a microstructure desirable for the pseudoelastic property .
The limited source of pre-alloyed powder may have restricted the research progress on the adaptability of
the LPBF technique to Fe-based SMAs. Most of the studies on the additive manufacturing of Fe-based
SMAs used pre-alloyed precursors. It is worth noting that Niendorf et al. and Wiesener et al. fabricated a
Fe-based alloy with Ag for biomedical applications by mixing Ag powder with pre-alloyed high-manganese
TWIP steel powder and Fe-Mn powder, respectively [29,30] . These studies achieved a microstructure with well-
dispersed Ag particles that accelerated the corrosion rate of the Fe-Mn alloy. Mixing of metallic powder
would therefore enhance the potential of the technique. The LPBF of homogenised powder is however
challenging due to the difference in the thermal and optical properties between the powders and chemical
inhomogeneity in the product , and this warrants the careful selection of processing parameters.
[31]
This study demonstrates that a Fe-Mn-Si SMA, a potentially biodegradable alloy, can be prepared from a
blended metallic powder and processed using the LPBF technique. The influence of laser power, scan speed,
and laser re-scanning on the solidified microstructure of the built product was examined. Then the
solidification mechanisms were explained based on the knowledge gained from both the microstructure and