Enhanced Magnetocaloric Properties of the (MnNi)0.6Si0.62(FeCo)0.4Ge0.38 High-Entropy Alloy Obtained by Co Substitution

In order to improve the magnetocaloric properties of MnNiSi-based alloys, a new type of high-entropy magnetocaloric alloy was constructed. In this work, Mn_0.6Ni_1-<i>_x</i>Si_0.62Fe_0.4Co<i>_x</i>Ge_0.38 (<i>x</i> = 0.4, 0.45, and 0.5) are found to exhibit magnetostructural first-order phase transitions from high-temperature Ni₂In-type phases to low-temperature TiNiSi-type phases so that the alloys can achieve giant magnetocaloric effects. We investigate why <i>c_hexagonal</i>/<i>a_hexagonal</i> (<i>c_hexa</i>/<i>a_hexa</i>) gradually increases upon Co substitution, while phase transition temperature (<i>T_tr</i>) and isothermal magnetic entropy change (Δ<i>S_M</i>) tend to gradually decrease. In particular, the <i>x</i> = 0.4 alloy with remarkable magnetocaloric properties is obtained by tuning Co/Ni, which shows a giant entropy change of 48.5 J∙kg^-1K^-1 at 309 K for 5 T and an adiabatic temperature change (Δ<i>T_ad</i>) of 8.6 K at 306.5 K. Moreover, the <i>x</i> = 0.55 HEA shows great hardness and compressive strength with values of 552 HV2 and 267 MPa, respectively, indicating that the mechanical properties undergo an effective enhancement. The large Δ<i>S_M</i> and Δ<i>T_ad</i> may enable the MnNiSi-based HEAs to become a potential commercialized magnetocaloric material.