Toughening the grain boundaries by introducing a small amount of the second phase: Ni-Cu-Mn-Ga shape memory alloys as an example

The grain boundary brittleness featured by intergranular fracture behavior is a typical mechanical characteristic of both structural and functional materials [1], [2], [3], [4], [5] that possess covalent bonds. Although many efforts have been taken to overcome the grain boundary brittleness of materials, especially including the proposed concept of grain boundary engineering [4,[6], [7], [8], [9], [10], [11]], studies on toughening the grain boundaries are still ongoing. More importantly, the functional materials such as biomaterials [12], battery materials [13], sensor materials [14] and shape memory alloys [15], which are key components of various devices have a strong demand to achieve both outstanding functionalities and excellent mechanical properties. Thus, toughening the grain boundaries of these functional materials simultaneously retaining their functionalities are of crucial importance. The Heusler-type Ni-Mn-X (X=Ga, In, Sn, Al, Ti) shape memory alloys belong to such kind of functional materials. Since the discovery of Ni-Mn-Ga alloys in 1984 [16], the severe grain boundary brittleness has been remained as a bottleneck hindering the developments of Ni-Mn-X shape memory alloys as intermetallic compounds.

Compared with traditional shape memory alloys such as Ni-Ti [17,18], Cu-Zn-Al [17], and Fe-Mn-Si alloys [19], the Ni-Mn-X alloys exhibit comprehensive advantages, such as continuously tunable temperatures of the martensitic transformation over wide temperature range, simple phase structures, highly thermal stability of the martensitic transformation and shape memory effect. In some Ni-Mn-X alloys, the martensitic transformation can be triggered by magnetic field giving rise to multiple magnetic functionalities, such as magnetic field-induced strain [20], magnetocaloric effect [21] and elastocaloric effect [22,23]. Therefore, Ni-Mn-X alloys have been considered as excellent candidates for the new generation of muti-functional materials. In Ni-Mn-X alloys where X can be Ga, In, Sn, or Al, the main-group (p-group) atoms form covalent bonds by p–d orbital hybridization with transition-metal (d-group) atoms. As a result, these alloys exhibit significant inherent grain boundary brittleness. It has been proved that polycrystalline samples of these alloys can only be deformed by compression and the compressive strain is relatively limited. Tension is difficult for these alloys. In the recently reported Ni-(Co)-Mn-Ti alloys [24,25], their closed-shell d–d electron hybridization leads to better compressive properties [26,27]. Tension is still difficult for Ni-(Co)-Mn-Ti alloys.

Generally, Ni-Mn-X alloys are single phase system, as schematically shown in Fig. 1(a). Intergranular fracture is rather easy for Ni-Mn-X alloy. Previously, efforts of alloying the fourth elements, including interstitial, transition and rare earth elements, have been taken to introduce the second phase into Ni-Mn-X alloys in order to improve their brittleness. When alloying the interstitial element B [28], a maximum value of 22.3 % for the compressive strain was reached in the (Ni₅₄Mn₂₅Ga₂₁)_99.5B_0.5 alloy due to grain refinement and the formation of a few (NiMnGa)₂₃B₆ phase distributed uniformly in the matrix. However, the (NiMnGa)₂₃B₆ phase was a hard and brittle phase. In case of higher B content, network of (NiMnGa)₂₃B₆ phase appeared and the compressive strain was decreased dramatically. Similarly, adding rare earth elements, such as Y [29], Dy [30] and Gd [31] also increased the compressive strain of Ni-Mn-X alloys by grain refinement and forming rare earth-rich phase. But the rare earth-rich phases were also brittle. Tension was still difficult for Ni-Mn-X alloys.

Tension was realized by alloying transition elements, such as Fe [32,33], Cu [32], [33], [34], [35] and Co [32,33,36], into Ni-Mn-Ga alloys. Unfortunately, the shape memory effect was severely compressed. In such cases, a large amount of soft and ductile γ phase with face-centered cubic (FCC) structure was precipitated. Tensile strains of 3.8 %, 8.2 % and 10.5 % were achieved in dual-phase Ni₅₆Mn₁₇Cu₈Ga₁₉, Ni₅₆Mn₂₁Co₄Ga₁₉ and Ni₅₆Mn₁₆Fe₉Ga₁₉ alloys with the volume fraction of γ phase of 34 %, 18 % and 21 %, respectively [33]. These alloys exhibit high plasticity. However, the functionality of shape memory in Ni-Mn-X alloys was almost completely suppressed. It was found that except particles of γ phase distributing at grain boundaries, γ phase also exist within the matrix grains, as schematically shown in Fig. 1(b). Movement of phase interface caused by the martensitic transformation was pinned by the particles of γ phase present in matrix grains [33]. Consequently, the shape memory effect was weakened. There is little report on the shape memory effect of Ni-Mn-X alloys under tension conditions.

We suppose that if the γ phase is just present at the grain boundaries, as schematically shown in Fig. 1(c), both high plasticity and obvious shape memory effect are expected in Ni-Mn-X alloys. In this work, we performed two-step heat treatment for a Ni₃₀Cu₂₀Mn_42.4Ga_7.6 alloy. Dual-phase configuration with thin layer of γ phase single crystal just distributing along grain boundaries of the martensite phase was formed at room temperature. Tensile strain up to 13 % was realized. Simultaneously, shape memory effect strain of 3.2 % was achieved under the tension condition. The phase evolution, phase structure and fracture surface were systematically analyzed. This work supplies an attractive way of grain boundary engineering to toughen functional materials.