2.1 FeMnSi based shape memory alloys

Fe-Mn-Si based alloys discovered as a single crystal of Fe-30Mn-1Si during 1980s. They show a pronounced shape memory effect(SME) even though mainly in oneway. They have encouraging potential to be used as tighteners(pipe coupling) because of their low cost and fine formability. Special concern is given to the fcc(γ) →hcp(ε) transformation mechanism featured by low stacking fault energy and the approaches aiming to the enhancement of SME. The thermodynamic considerations regarding the critical driving force, interfacial energy and stacking fault energy, the prediction of M_s temperature, the crystallography and the transformation mechanism are the main topics for the study of martensitic transformation in Fe-Mn-Si based alloys⁹⁾ It has been revealed by the research that the shape memory capacity of Fe- Mn-Si alloy depends on the annealing temperature. It can be further demonstrated as the effect of annealing on γ → ε transformation. As the annealing condition greatly influences the defects and nature of austenite, therefore a high annealing temperature results in a low density of stacking faults, causing a low nucleation rate during stress induced γ →ε transformation giving rise to the growth of ε martensite plates instead of new formation further resulting into the local slip deformation and finally leading to the deprivation of shape memory effect. Annealing at low temperature also yields to degradation of shape memory effect. Since shape memory effect is dependent of γ → ε transformation therefore pre-strain, annealing treatment, thermomechanical training and deformation temperature influences the shape memory effect of Fe-Mn-Si SMA as these factors impart effect on the γ → ε transformation¹⁰⁾ FeMnSi-alloys exist in different forms depending upon their use and required characteristics. These types are summarized in Table 1.

2.2 FeNiCo based shape memory alloys

Recently, the martensite formation in quaternary FeNi- CoX alloy is ongoing interest in research as it has not yet been unrevealed. So, Transmission electron microscope (TEM), Mössbauer spectrometer, Scanning electron microscope( SEM) and differential scanning calorimeter(DSC) have been utilized to observe the morphological, kinetic, crystallographic, magnetic and thermal characteristics of thermally induced martensite in FeNiCoX alloy and the kinetics of transformation was observed to be athermal. While observing through the microscope, lenticular martensite morphology existed. Besides, observation by differential scanning calorimeter showed the martensite start temperature(M_s) to be −63 °C and the paramagnetic character for the austenite phase and a ferromagnetic character for thermally induced martensite phase was determined by Mössbauer spectra⁵⁰⁾ FeNiCo-alloys exist in different types depending upon their use and required characteristics. These types are collected in Table 2.

2.3 FeMnAl based shape memory alloys

Shape memory alloys have revolutionized the material engineering sciences as they exhibit exclusive features i.e. (SME) Shape memory effect and Super-elasticity. More research has revealed the effectiveness of Fe-based shape memory alloys comparing with Nitinol(high processing cost and low cold workability). These Fe-based shape memory alloys have been used because of the low cost and high cold workability. Further, some of the Febased shape memory alloys i.e. Fe-Pd, Fe-Pt show superelasticity because of thermo-elastic martensitic transformation. Therefore, the Fe-SMAs which bear non-thermoelastic martensitic transformations are changed to thermoelastic martensitic by precipitating nano-sized coherent particles with ordered structure. Typically, Kainuma et al. established the Fe_40.95Ni₂₈Co₁₇Al_11.5Ta_2.5B_0.05 and Fe_43.5-Mn₃₄Al₁₅Ni_7.5 SMAs with thermo-elastic martensitic transformations by precipitating nano-sized coherent γ and B2 phases respectively. Fe_40.95Ni₂₈Co₁₇Al_11.5Ta_2.5B_0.05 exhibits a huge super-elastic strain of 13.5 %, and Fe_43.5Mn₃₄-Al₁₅Ni_7.5 also has the super-elastic strain of over 5 %. Fe_43.5Mn₃₄Al₁₅Ni_7.5 SMA has vast application vision over the other Fe-based SMAs because it shows super-elasticity for large temperature range i.e. from 196 °C to 240 °C with minor super-elastic stress temperature dependence. Hence, FeMnAlNi shape memory alloys have a wider application prospect comparing with other shape memory alloys in the field related to the super-elasticity.⁸⁾ FeMnAl-alloys exist in different types depending upon their use and required characteristics. These types are collected in Table 3.

2.4 Ferromagnetic shape memory alloys

Unique properties of ferromagnetic shape memory alloys such as magnetic field induced strain(MFIS) and quick response make them useful for diverse applications e.g. sensors and actuators. This exclusive feature, magnetic field induced strain(MFIS) was first observed by Ullakko. Moreover, in order to get excessive MFIS e.g. (twin boundary mobility, high magnetic an-isotropy energy and low volume change as a result of transformation) some specific magnetic and micromechanical requirements must be met69) Ferromagnetic shape memory alloys exist in different types depending upon their use and required characteristics. These types are collected in Table 4.

3.1 Recovery stresses

It has been observed that material properties such as recovery stresses and stacking fault energy are associated not only with the microstructural features of materials i.e. distribution and size of second-phase particles and the grain size, but also on alloy composition. These features affect transformation temperatures and stacking fault energy. Though, alloy configuration is also responsible for recovery stress property. Table 6 shows the results of recovery stresses with different procedures and Fe-Mn-Si configurations.⁷⁾

3.2 Corrosion resistance

Examining the corrosion resistance property for different configurations of Fe-Mn-Si shape memory alloys against the harsh atmosphere of NaCl and H₂SO₄ solutions,^39,75-82) It has been revealed that the good corrosion resistance is due to the high quantity of Si in Fe-based shape memory alloys. It is evident from the fact that the Fe-Mn-Si-Cr- Ni-(Co) alloy containing 8.8 wt% to 12.80 wt% of Cr against H₂SO₄ environment shows better or same corrosion as stainless steel 304. Though Fe-Mn-Si-Cr-Ni-(Co) contains less amount of Cr as that of stainless steel 304.^79,81-82)

3.3 Weldability

Experiments have been conducted to study the welding properties of Fe-Mn-Si alloys using TIG(tungsten inert gas) welding, LB(laser beam welding) and EB(electron beam welding)^83,36) Recently a manufacturing technique for shaft and pipe coupling using welding and forming of Fe-15Mn-5Si-9Cr-5Ni SMA has been suggested. The results for the experiments revealed that welding affected the fracture in welded zone and lowering of the degree of shape recovery by 15 %. Though, the technique can provide appropriate coupling force therefore this manufacturing practice needs to be more explored.¹⁷⁾

3.4 Production

Since the most common method used for the production of Fe-Mn-Si based SMA is thermo-mechanical process involving melting and casting in high vacuum. However, for the mass production electric furnace cannot be implemented for melting of any type of shape memory alloy containing large quantity of M_n elements and having high heat capacitance. As proposed by Maruyama and Kubo for SMA containing more than 20 % M_n, that the impurities in M_n raw materials results into fatal faults in the mechanical characteristics of shape memory alloys. Therefore, lowering the amount of M_n will result in improving mass production and it is cost-effective as well. Another technique for the production of Fe-Mn-Si SMAs uses solid state reactions among powders because of high energy collisions for alloying. While the alloying in technique is followed by sintering.⁷⁾

3.5 Workability at room temperature

As the workability of Fe-Mn-Si SMA’s has not yet been explored much but according to Sato et al Fe-Mn-Si SMAs are identical to TRIP/TWIP steels in the manner that they possess good workability but it should also be taken into consideration that the workability is affected by carbide and nitride produced as a result of adding C or N to prevent SMAs from fracture due to heavy distortion.⁷⁾

4.1 Fe-SMA Types and properties

There are several types of Fe-SMAs. These types are characterized as FeMnSi-based SMA, FeNiCo-based SMA, FeMnAl-based SMA and Ferromagnetic shape memory alloys. The results showing characteristic features of each of these types, have been discussed in detail in subsection 2.1→subsection 2.4. A detailed review of a lot of existing types of each of FeMnSi-SMAs, FeNiCo- SMAs, FeMnAl-SMAs and Ferromagnetic-SMAs, is performed there. All these types have been individually collected in Table 1→Table 4 with proper composition and references. Hence, Table 1→Table 4 also show the results of this work. Fig. 1 shows the analysis performed by observing the data in Table 1→Table 4. It shows that most of the existing types of Fe-SMAs are FeMnSi- SMAs, secondly FeNiCo-SMAs, thirdly FeMnAl-SMAs and fourthly Ferromagnetic-SMAs. Researchers are not only trying to develop the existing types of Fe-SMAs, but they are also trying to find new types of Fe-SMAs. The authors hope that promptly growing development and research on Fe-SMAs will lead towards excellence in the coming near future.

Fig. 1

Different types of ferrous based shape memory alloys.

The results related to different properties of Fe-SMAs have already been discussed in section 3.

4.2 Importance of Fe-SMAs

A literature analysis has been carried out using the “google scholar search engine” with search keywords of “ferrous based shape memory alloys”. An increasing sequence of Publications & US Patents was observed for different year groups mentioned in Fig. 2, during the last 50 years. This increasing sequence was observed during the both cases (i) when keyword is anywhere in the research article (ii) when keyword is in title of research article as shown in Fig. 2.

Fig. 2

Increasing sequence of ferrous based SMA’s publications & US patents during the last 50 years accessed in 2017.

A literature analysis has been carried out using the “google scholar search engine” with search keywords of “FeMnSi based shape memory alloys”. An increasing sequence of Publications and US Patents, was observed for different year groups mentioned in Fig. 3, during the last 50 years. This increasing sequence was observed during both cases (i) when keyword is anywhere in the research article (ii) when keyword is in title of research article as shown in Fig. 3.

Fig. 3

Increasing sequence of Fe-Mn-Si based SMA’s publications & US patents during last 50 years accessed in 2017.

A literature analysis has been carried out using the “google scholar search engine” with search keywords of “FeNiCo based shape memory alloys”. An increasing sequence of Publications and US Patents, was observed for different year groups mentioned in Fig. 4, during the last 50 years. This increasing sequence was observed during both cases (i) when keyword is anywhere in the research article (ii) when keyword is in title of research article as shown in Fig. 4.

Fig. 4

Increasing sequence of Fe-Ni-Co based SMA’s publications & US patents during last 50 years accessed in 2017.

A literature analysis has been carried out using the “google scholar search engine” with search keywords of “FeMnAl based shape memory alloys”. An increasing sequence of Publications and US Patents, was observed for different year groups mentioned in Fig. 5, during the last 50 years. This increasing sequence was observed during both cases (i) when keyword is anywhere in the research article (ii) when keyword is in title of research article as shown in Fig. 5.

Fig. 5

Increasing sequence of Fe-Mn-Al based SMA’s publications & US patents during last 50 years accessed in 2017.

A literature analysis has been carried out using the “google scholar search engine” with search keywords of “ferromagnetic shape memory alloys”. An increasing sequence of Publications and US Patents, was observed for different year groups mentioned in Fig. 6, during the last 50 years. This increasing sequence was observed during both cases i.e. (i) when keyword is anywhere in the Research Article (ii) when keyword is in title of Research Article as shown in Fig. 6.

Fig. 6

Increasing sequence of ferromagnetic SMA’s publications & US patents during last 50 years accessed in 2017.

The above-mentioned analysis in the form of Fig. 2 to Fig. 6 as well as results and discussions on these Fig. 2 to Fig. 6, show that there is an increasing trend in all types of ferrous based shape memory alloys during the last 50 years, In other words, number of publications and US Patents are being increased with the passage of time. Researchers are taking more interest in evaluating the features of Fe-SMAs and trying to enhance the number of applications of such type of cheap and commercially available alloys.