A. Phase and crystal structure

Figure 1 shows the XRD patterns of melt-spun Ni–Mn–Sn–Co ribbons at room temperature, in which both R-Co0 and R-Co2 are well indexed as L2₁ austenitic phase with cubic structure. At the doping level of 4 at.% Co, a mixture of L2₁ austenite and four-layered orthorhombic (4O) martensite was identified, whereas at 6 at.% Co doping, a mixture of L2₁ austenite and five-layered orthorhombic (10M) martensite was identified. Further increase of Co doping to 8 at.% Co yields a pure seven-layered monoclinic (14M) martensitic phase in the R-Co8. Miller indices (hkl) for the L2₁, 4O, 10M, and 14M structures are indexed in Fig. 1. The corresponding lattice parameters calculated using a least-squares method based on the XRD data are listed in Table I.

FIG. 1.

XRD patterns of melt-spun Ni–Mn–Sn–Co ribbons at room temperature.

TABLE I.

Crystal structures and lattice parameters of melt-spun Ni–Mn–Sn–Co ribbons at room temperature calculated using the XRD data in Fig. 1.

Figure 2 presents the backscattered electron (BSE) images taken from the free-surface of R-Co0, R-Co4, and R-Co6, respectively. All of them show equiaxed grains with size of ∼1–2 μm. In the sample of R-Co4, martensite plates are observed within some grains [Fig. 2(b)]. However, the sample R-Co6 is almost fully filled with martensite plates [Fig. 2(c)], and similar martensitic microstructure is also observed in R-Co8 (not shown here). From the SEM images alone, no clear difference can be distinguished among these three different martensites. Therefore, the crystal structures of martensite in R-Co4, R-Co6, and R-Co8 were further characterized using TEM. Figure 3(a) shows a low-magnification TEM bright-field image of R-Co4, where plate-like martensitic variants distribute along grain boundaries, and the grain center is the austenitic matrix according to the selected area electron diffraction pattern (SADP) shown in the inset. Figure 3(b) is a magnified image taken from the highlighted white rectangular field marked in Fig. 3(a). The inserted SADP in Fig. 3(b) displays three satellite spots between the fundamental maxima (marked by white arrows), which represents typical diffraction characteristics of the modulated 4O structure. Furthermore, it is reasonable to conclude that the martensitic phase initially nucleates near the grain boundaries and then grows to the interior of the grains during the subsequent martensitic transition from Fig. 3(a). The bright-field image of 10M martensite in R-Co6 is presented in Fig. 3(c) and its corresponding SADP shows four satellite spots between the fundamental maxima. The TEM image of R-Co8 shows three groups of 14M martensitic variants with different orientations [Fig. 3(d)]. In addition, it should be noted that no secondary phase is observed within the present studied composition range according to the XRD analyses and TEM observations.

FIG. 2.

BSE images of melt-spun R-Co0 (a), R-Co4 (b), and R-Co6 (c).

FIG. 3.

TEM images and the corresponding SADPs taken from the regions highlighted with white circles in melt-spun Ni–Mn–Sn–Co ribbons, indicating the presence of cubic L2₁ austenite and four-layered orthorhombic (4O) martensite in R-Co4 (a and b), five-layered orthorhombic (10M) martensite in R-Co6 (c), and seven-layered monoclinic (14M) martensite in R-Co8 (d), respectively.

B. Martensitic transformation

Figure 4 presents the DSC charts of the Ni–Mn–Sn–Co ribbons. The curves of R-Co2, R-Co4, R-Co6, and Co8 are shifted up for clarity. It is noticed that the transformational peaks shift progressively toward higher temperature regime with the Co addition. The characteristic transition temperatures, including reverse martensitic transition start and finish temperatures (A _s and A _f) and martensitic transition start and finish temperatures (M _s and M _f) are determined by a tangent method, as shown in the example of R-Co8. The weak exothermic (above A _f) and endothermic peaks (above M _s) upon heating and cooling, respectively, correspond to the magnetic transition of austenite ($T_{\rm{C}}^{\rm{A}}$).

FIG. 4.

DSC charts of melt-spun Ni–Mn–Sn–Co ribbons. The curves of R-Co2, R-Co4, R-Co6, and R-Co8 are shifted up for clarity.

Figure 5 shows the compositional dependence of Co content on the martensitic/magnetic transition temperatures (M _s, M _f, A _s, A _f, and $T_{\rm{C}}^{\rm{A}}$) and transformation hysteresis (ΔT). The transformation hysteresis (ΔT) is defined as the difference between the heat flow peak temperatures, which falls in the range around 18–20 K and varies slightly for all ribbons. The martensitic transition temperatures show a good linear relationship with the Co content, increasing about 8 K per at.% Co addition. Generally, the change in martensitic transition temperatures can be interpreted from the following two aspects. First, the phase transformation is closely associated with the valence electron concentration (e/a). It has been supposed that the L2₁-structured austenite is stabilized because the Fermi surface just touches the (110) Brillouin zone. Increasing e/a would overlap the Fermi surface and the (110) Brillouin zone, and thus lead to electrons above Fermi level moving to the corner states of the Brillouin zone. As a consequence, the excessive increase in system energy leads to the lattice distortion to minimize the free energy, i.e., the formation of martensite.^{Reference Smit20} In the present study, e/a increases from 8.11 to 8.18 with increasing Co content from 0 to 8 at.%. Here, it is assumed that the valence electrons per atom are 10 (3d ⁸4s ²) for Ni, 7 (3d ⁵4s ²) for Mn, 4 (5s ²5p ²) for Sn, and 9 (3d ⁷4s ²) for Co, respectively. It is clear that our results are in good agreement with the e/a-dependence rule.^{Reference Zayak, Adeagbo, Entel and Rabe21,Reference Krenke, Moya, Aksoy, Acet, Entel, Mañosa, Planes, Elerman, Yücel and Wassermann22} Second, the atom size effect originating from the unit-cell contraction of the high-temperature austenite would promote the martensitic transformation due to changes of the relative positions between the Fermi surface and the Brillouin zone.^{Reference Kokorin, Osipenko and Shirina23} The atom radii are 0.125 nm for Ni, 0.135 nm for Mn, 0.163 nm for Sn, and 0.126 nm for Co, respectively.^{Reference Chen, Yang, Lu and Qin24} As discussed above, since no secondary phase is formed in the studied ribbons, it is reasonably assumed that the unit-cell of the high-temperature L2₁ austenite would shrink with addition of smaller Co atoms. The calculated lattice parameters listed in Table I also support this point. In fact, the lattice contraction is evidenced from the fact that the diffraction peaks from L2₁ austenite shift toward high angles with Co addition in Fig. 1.Therefore, the atom size influences on the change of the transition temperatures to some extent in the present work.

FIG. 5.

Dependence of martensitic/magnetic transition temperatures and transformation hysteresis on the Co content.

Recently, the magnetic valence number (Z _m) was proposed by Karaman et al. ^{Reference Dogan, Karaman, Singh, Chivukula, Thawabi and Arroyave25} to describe the transition temperatures based on the fact that the spin-up d-band lies either entirely above or below the Fermi level in strong magnetic systems.^{Reference Williams, Moruzzi, Malozemoff and Terakura26} In the present study, the Curie temperature of austenite for all ribbons is higher than the martensitic transition start temperature, that is, the austenite is ferromagnetic prior to the martensitic transformation for all ribbons. The magnetic valence numbers are 0, 3, –4, and 1 for Ni, Mn, Sn, and Co, respectively,^{Reference Williams, Moruzzi, Malozemoff and Terakura26} and the Z _m value increases monotonously from 0.69 to 0.715 with Co addition. However, recall that for the change of phase transition temperatures of the present ribbons, it is noticed that this is opposite to the Z _m rule obtained in Co–Ni–Ga alloy system.^{Reference Dogan, Karaman, Singh, Chivukula, Thawabi and Arroyave25} There may be two reasons for this abnormal phenomenon: (1) the Z _m rule is valid only in case of relatively lower e/a difference; and (2) the crystal structures of martensite in Ni–Mn–Sn–Co ribbons are complex in contrast to Co–Ni–Ga where only tetragonal martensite forms.^{Reference Dogan, Karaman, Singh, Chivukula, Thawabi and Arroyave25} The evolution of martensitic crystal structures in Ni–Mn–Sn–Co ribbons nicely fits our previous work in Ni–Mn–Sn bulk alloys.^{Reference Zheng, Wang, Xue, Zhai, Frenzel and Luo27,Reference Wang, Yu, Zhai, Luo and Zheng28}

C. Magnetic transition of austenite

From Fig. 5, it is found that the magnetic transition temperature of austenite ($T_{\rm{C}}^{\rm{A}}$) shows an increasing tendency with Co content, and the Z _m factor applies well to the change in this case. It is well accepted that the magnetic transition strongly depends on the magnetic moment degree of matrix in Heusler Ni–Mn-based materials.^{Reference Dogan, Karaman, Singh, Chivukula, Thawabi and Arroyave25} Fecher et al. ^{Reference Fecher, Kandpal, Wurmehl, Felser and Schönhense29} reported that the Curie temperature of Co₂Fe-based Heusler alloys is linearly related to the total magnetic moments. The Curie temperature increases with increasing Z _m in the Ni–Mn–Sn–Co ribbons, which is in agreement with the literature.^{Reference Dogan, Karaman, Singh, Chivukula, Thawabi and Arroyave25,Reference Sato, Okazaki, Furuya and Wuttig30,Reference Liu, Xia, Huang, Zheng and Li31} This fact can be explained as follows. First, Co atoms would work as ferromagnetic activator with Co addition to Ni–Mn–Sn and thus lead to ferromagnetic alignment of most Mn moments in the austenite.^{Reference Şaşıoğlu, Sandratskii and Bruno32–Reference Stager and Campbell34} Second, it has been proved that the Co–Mn ferromagnetic exchange interaction in Co₂MnSn is several times larger than the Ni–Mn in Ni₂MnSn based on density-functional studies.^{Reference Kurtulus, Dronskowski, Samolyuk and Antropov35} Similar results were presented by Han et al. in Mn₂Ni_1.64–xCo_xSn_0.36 alloys.^{Reference Han, Chen, Qian, Zhang, Jiang, Wang and Du36} It can be expected to strengthen the magnetic moments through partially replacing Ni–Mn exchange interaction by Co–Mn in Ni–Mn–Sn–Co ribbons. Lastly, the unit-cell shrinkage of austenite also favors the exchange interaction with doping smaller Co atoms. That is to say, the Co addition to Ni–Mn–Sn could effectively enhance ferromagnetic interaction of austenite and it is inevitable to cause an increase of the magnetic driving force against the thermal motion of lattice, and thus to promote the occurrence of the magnetic transition.