Effect of Heat Treatments on Microstructure and Mechanical Properties of Fe-Mn-Ni-Al-Gd Shape Memory Alloy

There are significant scientific and industrial efforts to develop and optimize Iron-based shape memory alloys (SMA) such as FeMnNiAl for cost-sensitive applications. This alloy system shows shape memory and superelastic properties across a large temperature range. However, many studies have pointed out the need for rather complex thermo-mechanical treatments for the optimization of the SMA properties. In addition, works considering the effects of alloying on the development of microstructures that are more conducive to pseudo-elasticity in this system remain limited. Hence, systematic studies aiming at the investigation of the microstructural evolution of the FeMnNiAl(Gd) system are of great interest. In this study, solution heat treatment is done to tune the microstructure for optimum mechanical properties. The effect of phase distribution on mechanical properties is investigated at different heat treatments. Whereas cyclic heat treatment induced abnormal grain growth (AGG) in all samples, so large grains were obtained. The phase variation and elemental composition are analyzed by X-ray diffraction and Energy Dispersive Spectroscopy, respectively. The microstructure and phase distribution are observed using Scanning Electron Microscope and then related to the microhardness results. The microstructure has a good correlation with mechanical properties where the fine distribution of phases results in a higher hardness number.

Currently, the Ni-Ti system dominates the SMA market however, there is a collaborative worldwide effort on iron-based shape memory alloys because they are cost-effective and offer larger compositional and temperate tuneability of the functional mechanical properties along with some electronic and magnetic functionalities.Reversible martensitic transition in Fe-based shape memory alloys can be grouped into four systems: (i) Fe-noble metal-based alloys, such as Fe-Pt [12] and Fe-Pd [13], (ii) The Fe-Ni-Co-based, (iii) Fe-Ni-Co-based alloys, such as Fe-Ni-Co-Al-Ta-B [14], Fe-Ni-Co-Al-Nb-B [15], and Fe-Ni-Co-Al-Ti-B [16][17] [18][19] [20] and (iv) BCC Fe-Mn-based alloys, in particular, can show extraordinary superelasticity.
The Fe-Mn-Al-Ni shape memory alloy exhibits a wide temperature range of superelasticity owing to the negligible entropy change during the martensitic transformation [21][22][23] [24][25] [26].Certain parameters influence superelastic behavior, including grain size in proportion to sample cross-sectional area [21][27] [28], precipitate size and their volume percentage [29][30] [31], and crystallographic orientation [32][33] [34].In this alloy system [35], Abnormal Grain Growth (AGG) generated by cyclic heat treatment [36] [37] further improves the usefulness of alloys.Subgrain structure in the BCC phase and the sub-boundary energy driving an AGG have been discovered following cyclic heat treatment between the single-phase BCC and the dual phase BCC+FCC regions.By cyclic heat treatment [38], relatively large-sized crystals can be produced which is conducive to superelasticity in this system, avoids prestress losses and tunes the transformation anisotropy [39].
The originally published Fe-34Mn-15Al-7.5Ni(at.%) alloy [21] was tweaked to solve a number of practical issues, such as; the crack development during final quenching, the difficulties of producing significantly bigger single-crystal samples via AGG, and the insufficient superelasticity.Rare earth (RE) elements such as Nd, Gd, Ce, etc., have been shown to improve the microstructure and mechanical characteristics of a wide spectrum of alloy systems [40][41][42] [41].Because each RE element has a unique character, combining additions of various RE elements to examine their interactions is particularly vital.In this context, Xie et al. [42] looked into how RE (Y and Nd) elements affected the AZ81 alloy's microstructures and mechanical characteristics.The findings revealed that the right amount of RE elements could drastically purify the grains, resulting in the formation of the Al2Y and Al2Nd phases.
In this study, the effects of heat treatment on the microstructure and abnormal grain growth of the FeMnAlNiGd alloy are investigated in order to finetune optimum microstructures and their relationship with mechanical properties such as hardness.The dual-phase structure is thoroughly studied both in terms of crystallography and elemental composition.The sample with optimum microstructure and hardness can be a potential candidate to replace existing SMAs.

Materials and Methods
Fe40.02Mn32.65Al5.90Ni7.69Gd13.74alloy in wt.% was fabricated from high-purity metals by induction melting process under an inert atmosphere.The cast ingot was then homogenized at 1200 o C for 1440 min in an inert Ar atmosphere.Following homogenization, 4 mm thick slices were cut from the ingot using a water jet.Rectangular specimens with a 4×4 mm 2 cross-section and 8 mm length were machined from the 4 mm strips.Specimens were subjected to various heat treatments including solution heat treatment (SHT) and cyclic heat treatments.To prevent oxidation during such high-temperature treatments, all specimens were encapsulated in quartz tubes, evacuated, and backfilled with Ar.
(  1. Sample classification with their respective heat treatment parameters. The conducted solution heat treatment conditions are summarized in Table 1.As noted above, all treatments were done following homogenization.The full details of the treatment conditions (i.e., temperature setpoints, heating rates, holding times, and quenching temperature) are explained schematically in Fig. 1 for the solution treatment and in Fig. 2 for the cyclic heat treatments.In all cases, the heating rates were fixed to 3 o C/min for the entire heat treatment.Before testing, a low-temperature aging treatment was conducted at 200 o C for 240 min.The heat-treated samples were ground and polished using a metallurgical-grade grinding setup for the examination.Crystal structure, phase purity, and chemical homogeneity of the studied material were examined by Panalytical X ′ pert 3 powder X-ray diffraction (XRD) and TESCAN VEGA-3 Scanning Electron Microscope (SEM) which was supported by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments).The target compositions were confirmed by EDX analysis within a 1% error margin.Micro Vickers hardness tests were performed on the polished surfaces for the hardness analysis to deduce a correlation between microstructure and mechanical properties.A force of 1.96 N was applied for a dwell time of 10 sec.

Results and Discussions
SEM analysis shows a dual-phase structure (α-BCC and -FCC phases) in all the samples.Both phases have a unique crystallographic and elemental composition which is further confirmed by XRD and EDS analysis.The SEM microstructures were taken at 500x (Fig. 3) for all the samples.The homogenized sample contains a dual-phase structure which is more like a dendritic structure.The second phase is in the form of sharp platelets and is evenly distributed.The 1000-20 sample is almost like the homogenized sample in terms of microstructure, but the size of the second phase increased.In general, the size of the cluster/second phase grains (-FCC) increased with higher treatment temperatures and/or longer treatment times.On the other hand, the 1300-120 sample exhibits dendritic like structures which are mostly formed of α-phase along with relatively flat and uniform -FCC phases.With the cyclic heat treatment, abnormal grain growth is favored, resulting in more coarse grains of both phases instead of matrix phase segregation at the grain boundaries.During cyclic heat treatment, the smaller and equally distributed second phase can also be seen within the matrix.Representative EDS results from two specimens (homogenized and the cyclic-80 samples) are shown in Figs.4-5.In the homogenized conditions, the α-phase (BCC) consists of all the constituents initially present in the alloy, whereas the second phase (show) has only Fe, Mn, Al, and Ni with the absence of Gd.The composition of the other constituent elements also differs between the two phases, which will be discussed later in this section.
The composition in each of the observed phases for a representative case is presented in Fig. 6.Spectrum 1 was obtained from the Gd-free phase (FCC) while Spectrum 2 was collected from a BCC region.For the reported case in Fig. 6, the FCC phase had a composition of Fe53.2Mn36.6Ni4.2Al5.9Gd0.0wt.% while the BCC region had a composition of Fe24.1Mn29.6Ni12.8Al7.8Gd25.7.Note that both the Fe and Mn have intrinsic BCC structures adverse to Ni and Al that have FCC structures.Hence, we believe Gd segregation could be a result of competition between two structural phases.More specifically, BCC elements are consuming Gd (HCP) phase and form the BCC structure in the considered alloy.It is also observed that the BCC phase is comparatively Fe deficient (24.1 wt.%) while the FCC phase is Fe dominant phase (53.2 wt.%), whereas Ni is behaving the other way around.

Figure 7. XRD analysis of solution heat-treated samples
In line with the SEM analysis, XRD peaks show a dual phase structure confirming the presence of both FCC and BCC phases as shown in Fig. 7. FCC () peaks appeared at 2 values of around 43 o , 50 o and 73 o whereas for BCC () peaks are at 38 o , 45 o , 65 o and 88 o [43] [6].With the addition of Gd, BCC () phase became the dominant phase in the structure without heat treatment.However, the stability of BCC () changes for different SHT processes.The presence of FCC phase compared to BCC is suppressed in the homogenized sample but with the increase in temperature, the clusters have grown, resulting in the higher peak intensity of -phase.Comparable  and  coexistence is observed in the cyclic-80 sample.This -phase stability is associated with the increment in the Gd-rich phase in SEM microstructures.
Micro Vickers hardness tests were performed to analyze the mechanical properties in relation to the various microstructures introduced through the different SHT processes.The hardness results are shown in Fig. 8, indicating a clear relation between the microstructural evolution and hardness variation.In general, reduced hardness magnitudes are obtained with higher SHT temperatures.This is attributed to the coarser grain structure obtained under such conditions.The minimum hardness of 250.12 HV was achieved in the 1300-120 sample while the maximum hardness of 392.03HV was obtained for the 1000-20 sample.The cyclic sample behaves differently because of abnormal grain growth in both phases and because of finer second-phase distribution in the matrix phase.This finer distribution is more prominent in the cyclic-120 sample which resulted in a higher hardness value of 310.15 HV than 285.35HV in the cyclic-80 sample.The lower hardness value of cyclic-80 is because the time to form a uniformly distributed phase inside the matrix is less due to the lesser holding time.

Conclusions
The microstructural evolution of FeMnAlNiGd alloy is studied at different solution heat treatment cycles.FeMnAlNiGd alloy exhibits a dual-phase structure having BCC and FCC phases forming the matrix and the secondary phase, respectively.Both phases have unique chemical compositions with a prominent difference in the presence of Gd, which exists only in the matrix phase.The secondary phase () is Fe and Mn-rich phase whereas more Ni and Al are found with Gd in the matrix phase ().Large grains were more easily formed, and AGG was induced in all samples via cyclic heat treatment.Microstructure evolution at different SHT parameters correlates well with mechanical properties such as hardness.The fine distribution of phases results in a higher Vickers number.1000-20 sample is showing the finest microstructure amongst all the samples, yet suitable for high strength applications.But the cyclic-120 sample is showing optimum properties due to AGG and significant hardness values, which is potentially suitable for shape memory and superelastic behavior.

Future Perspective
FeMnNiAl is a shape memory material that shows superelastic properties so heat treatment can be further optimized for the FeMnNiAlGd system to get shape memory effects.Also, the magnetic properties of (other than the 1225-60 sample) can be investigated in detail with electric transport properties.Other mechanical properties including compression and tribology can be further investigated for the self-lubricating steels.

Figure 1 .
Figure 1.Schematic diagram of the solution heat treatment process

Figure 2 .
Figure 2. Schematic diagram of the cyclic heat treatment process

Figure 6 .
Figure 6.EDS point analysis of 1225-60 sample, (a) microstructure of sample showing selected regions for Spectrum 1 and Spectrum 2, (b) elemental peak intensity versus wavelength and elemental distribution in wt.%.

Figure 8 .
Figure 8. Histogram of Micro Vickers Hardness of heat-treated samples