Ni-Mn-In-Co alloys

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magneto-structural properties of Ni-Mn-In-Co alloys, which will be discussed in detail in next chapters.

1.3 Ni-Mn In-Co alloys

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under the magnetic field of 70kOe. Karaca et al. report a large transformation strain of around 6% in Ni45Mn36.5In13.5Co5 [115]. A large reversible MCE has been reported in a Ni49.8Mn33.5In15.5Co1.2 alloy with a 7-layered modulated monoclinic martensite structure with ΔSm= 14.6J\kgK under a magnetic field of 50kOe [116].

The entropy change during the MT can be controlled by long-range atomic order in these alloys [117, 118]. As mentioned above, the degree of atomic order can be modified by subsequent thermal treatments via high temperature quenching or under the appropriate ageing process. In Ni-Mn-In-Co quaternary alloys, the MT temperature decreases and Curie temperature increases with quenching temperature T < 900K, whereas above this temperature, an opposite behavior is observed, contrary to ternary Ni-Mn-In alloys [117, 119]. Thus the entropy change at the MT increases up to T < 900K, similar to ternary alloys, and slightly decreases further. The study suggests that the difference in the behavior of these quaternary and ternary alloys for different quenching temperatures could be related to the retained degree of atomic order at high temperatures where the ordering process is assisted by the higher vacancy concentrations.

Moreover, the neutron studies reveal that the L21 atomic order decreases on heating above 700K. The study also suggests that the appropriate way to tune the atomic order in Ni-Mn-In-Co alloys appears to be an "in-situ" post-quench ageing treatments, where the evolution of the entropy change is similar to the ternary Ni-Mn-In alloys. However, the influence of atomic order on the MT in quaternary alloys is much lower than in the ternary ones subjected to the same thermal treatments [117, 120, 121]. Under the appropriate ageing process, it is possible to control the entropy change of a single alloy from 40 to 5J/kgK. This amplifies the possibilities to tune the functional properties of these alloys for application purpose.

1.3.1 Drawbacks

As previously mentioned, it is possible to modify the transformation temperature, magnetic properties and MT characteristics of Ni-Mn based Heusler alloys through compositional modifications, doping, and heat treatments. The goal is to fine-tune the multi-functional properties by modifying MT and its characteristics. Despite their promising features, Ni-based Heusler alloys possess poor mechanical properties, which include brittleness and fragility, hindering their development for practical devices and applications. Thus, efforts have been made over the last decade to improve the mechanical properties of these alloys, as it is crucial for the development of functional elements based on Ni-based Heusler compounds.

Over the past few years, various alternatives have been explored to overcome the limitations of bulk materials, including the use of ribbons [122], foams [123], and films made of alloys, as

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well as embedding shape memory particles in a polymer matrix [124, 125]. The approach to use fine particles instead of bulk materials as an active material has been proven to control the brittleness and fragility of these materials [126–129]. As a result, utilizing micro-particles and powdered samples as active elements is becoming increasingly popular as an effective method for addressing the defective mechanical properties [127, 130–132] of these materials. The method of production of Ni-Mn based Heusler alloys is of utter importance. The most common used technique, which is simple and cost effective, is mechanical grinding (ball-milling).

However, this technique has shown to induce defects, local stresses, dislocations etc., which affects the martensitic transformation acutely and the associated properties with it [16, 80, 93–

95]. Many studies have been performed in order to examine the features that are affected by ball-milling. The residual stresses induced by the production of fine powders leads to stabilization of the martensite far above the martensitic transformation and the irreversibility of the martensitic transformation [83, 133]. The degradation of the martensitic transformation and larger hysteresis linked to it are also one of the results of induction of local stresses and strain induced by milling process [83, 134]. The long range atomic order that highly influence the transformation temperatures, Curie temperature and other magnetic properties of the Ni-Mn-Sn and Ni-Mn-In alloys [18, 20, 135] remains unaffected by the milling process [32]. However, the variation of atomic-order by quenching or the thermal treatments shifts the transformation temperatures up to 100K [78]. Therefore, by the combined effort of milling, quenching and thermal treatments, it is possible to optimize the properties of these materials. Furthermore, recent studies have suggested that shape memory alloy nano-particles may exhibit non- hysteretic behavior below a certain size [99], making them promising option for simultaneously improving mechanical behavior and related multi-functionalities. Although using powdered Ni- based Heusler alloys offers unique advantages, like controlling defects, local stresses, internal strains, and dislocations, it becomes crucial in this regime since they directly influence the MT [95, 136, 137]. Thus, controlling the microstructure becomes even more essential to improve the multi-functional properties of micro-particles and films. Recent studies have shown that by controlling the microstructure, it is possible to fine-tune the MT and related multi-functional properties [20, 138, 139]. Thus, powdered samples may overcome the main drawback of the bulk samples; however, they introduce other issues that need to be considered in micro and nano-scale systems.

To sum up, when it comes to taking advantage of the versatile characteristics associated with the appearance of MT in Ni-based Heusler alloys, reducing the size of the active components is becoming a successful method or overcoming the undesirable mechanical properties that are present in the bulk alloys. However, as previously mentioned, in the micro/nano-scale,