The Preparation And Magnetocaloric Effect Of Rare Earth-base High-entropy Bulk Metallic Glasses

Recently,the high-entropy bulk metallic glasses（HE BMGs）have attracted extensive attention due to the collective advantages of high-entropy alloys（HEAs）and BMGs.The HEAs generally show excellent mechanical properties,physical and chemical properties which are different from the traditional metallic alloys due to the specific structure,thus they possess good application prospect;BMGs especially the rare earth（RE）-based BMGs have many fascinating properties,such as large glass-forming ability（GFA）,the spin-glass（SG）behavior at low temperature and considerable magnetocaloric effect（MCE）,obtaining enough concerns in combination with the advantages of tailorable magnetic transition temperature,small eddy current heating,good corrosion resistant and broad magnetic entropy change.In comparison with the conventional gas compression-expansion refrigeration technique,magnetic refrigeration technique has a variety of particular properties,i.e.,being pollution-free,low noise,high energy efficiency,small cubage and reliable operation.Therefore,it is significant to investigate the RE-based HE BMGs in this context,and the thermal stability,glass-forming ability,SG behavior and MCE were systematically investigated in this work.All of these properties advance our understanding of the nature of glassy alloys as well as extend the application area of glassy alloys.First of all,the quaternary Gd₂₅RE₂₅Co₂₅Al₂₅（RE=Tb,Dy,Ho）high-entropy glassy ribbons were prepared on the basis of Gd-Co-Al alloy system by adding another RE element as the fourth principal element,and the effects of different RE elements on thermal stability,SG behavior and MCE were investigated and discussed.In terms of the sequence of the Tb,Dy and Ho,the values of glass transition temperature（T_g）and crystallization temperature（T_x）increase gradually,indicating the thermal stability of this glassy alloy system increases gradually.Due to the strong random magnetic anisotropy（RMA）caused by Tb,Dy and Ho elements,each alloy shows obvious SG behavior which causes the intersection phenomenon in isothermal magnetization curves and Arrott plots at low temperature and low magnetic field.The positive slope of Arrott plots indicates the alloys in this work undergo a second order phase transition from ferromagnetic to paramagnetic state.These alloys show excellent MCE,and they are promising candidate materials as the magnetic refrigerant materials at the temperature range of liquid nitrogen and liquid helium.Secondly,in view of the low glass-forming ability of the quaternary alloy system,we designed the pentabasic Er₂₀Dy₂₀Co₂₀Al₂₀RE₂₀（RE=Gd,Tb,Tm）HEAs by adding the fifth principal element in order to improve the entropy,and the effects of different RE elements on the glass-forming ability,thermal stability,SG behavior and MCE were investigated and discussed.The glassy rods with the diameter of 1.5 mm were prepared successfully for RE=Gd and Tb,and the glassy rod with the diameter of 1 mm was prepared for RE=Tm.In terms of the sequence of the Gd,Tb and Tm,the values of T_g and T_x increase gradually,indicating the thermal stability of this HE BMGs system increases gradually.The influence of SG behavior on the isothermal magnetization curves and Arrott plots at low temperature and low magnetic field is similar with the quaternary alloy system above.The peak of magnetic entropy change(-Δ_M^max)of 11.9 Jkg^-1K^-1 and refrigerant capacity（RC）of 619 Jkg^-1 are obtained for alloys with RE=Tm and Gd under the applied magnetic field of 5 T,respectively.Finally,in order to improve the glass-forming ability of Er₂₀Dy₂₀Co₂₀Al₂₀Gd₂₀ HE BMG,we investigated the effects of the addition of Si element on the glass-forming ability,thermal stability,SG behavior and MCE of this alloy and found that microalloying strategy could dramaticallyimprovetheglass-formingability.Asaresult,thenew(Er₂₀Dy₂₀Co₂₀Al₂₀Gd₂₀)₉₉Si₁ BMG with diameter of 6 mm was successfully prepared.The thermal stability is significantly enhanced by the addition of Si element.The supercooled liquid region（ΔT_x=T_x-T_g）and glass-forming ability decrease gradually with the increase of Si content from 1 at.%to 4 at.%.Besides,the SG behavior is influenced by the addition of Si element,but the Si content nearly has no effect on the SG behavior.