Study Of Magnetocaloric Effect Of Gd-based Bulk Metallic Glass With Large Magnetic Entropy Change Near Room Temperature

Refrigeration technique plays an important role in many fields like refrigerator,air-conditioner,aerospace,precision instruments and defense components.Magnetic refrigeration technique based on magnetocaloric effect（MCE）shows advantages of eco-friendly,high efficiency,small vibration and low noise,which is a promising method to replace the traditional gas compression-expansion technique.As the core of the magnetic refrigeration technique,the development of refrigeration materials with excellent magnetocaloric effect is of great significance.Due to the excellent magnetocaloric properties and special electronic structures,rare earth（RE）-based amorphous magnetocaloric materials have attract tremendous attentions.However,the limited glass-forming ability（GFA）restricts the further investigation and application of RE-based metallic glasses.Since metallic glasses are formed by fast cooling the melts,the structural evolution and the dynamic behavior of the liquid during the cooling process can greatly influence the microstructure,GFA and physical properties.Study on the dynamic behavior of glass-forming liquid can be helpful for further understanding the nature of glass and adjusting the GFA.In this work,the effects of Fe,rare earth elements and microalloying Si on the thermal stability,magnetocaloric properties,GFA and microstructure in Gd-based metallic glasses are investigated systematically.Through the investigation of the abnormal liquid behaviors in the metallic glass-forming liquids,the underlying mechanism of the enhancement of GFA induced by microalloying Si addition is elucidated.By partly substituting Fe for Co,the effects of Fe on the thermal stability,GFA and MCE of the Gd₅₅Co₂₀Al₂₄Si₁ alloy are investigated.It is found that proper Fe substitution can improve the GFA of the alloy,and Gd₅₅Co₁₉Al₂₄Si₁Fe₁ and Gd₅₅Co₁₈Al₂₄Si₁Fe₂ bulk metallic glasses with a critical diameter of 6 mm are prepared.In addition,Fe addition can obviously increase the Curie temperature,widen the magnetic transition temperature range,and improve the refrigeration capacity.With 5 at.%Fe addition,Gd₅₅Co₁₅Al₂₄Si₁Fe₅ metallic glass exhibits Curie temperature of 126 K,which is 25%higher than that of the Fe-free alloy,a full width at half maximum of magnetic entropy change of 118 K,and refrigeration capacity of 857 J kg^-1under a field change of 5 T.Through different RE substitution,the effect of RE element on the MCE and spin glass-like behavior for Gd₂₅RE₂₅Co₂₅Al₂₅（RE=Tb,Dy,Ho）high-entropy amorphous alloys are investigated.We have found that this alloy system shows excellent MCE and tunable Curie temperature with different RE substitution.Under a field change of 5 T,Gd₂₅Ho₂₅Co₂₅Al₂₅exhibits a maximum magnetic entropy change of 9.78 J kg^-1K^-1 and refrigeration capacity of626 J kg^-1.Due to the strong random magnetic anisotropy and exchange frustration below the Curie temperature,Gd₂₅RE₂₅Co₂₅Al₂₅ alloys exhibit distinct spin glass-like behavior.The ac susceptibility measurements at different frequencies show that the relaxation time of this alloy system exhibits a divergence at the transition temperature,and the Gd₂₅Ho₂₅Co₂₅Al₂₅ alloy exhibits the smallest random magnetic anisotropy.The effects of minor Si addition on the MCE,GFA,liquidus dynamics and microstructure of Gd₅₅Co₂₀Al₂₅ metallic glass are investigated.With only 0.5 at.%Si addition,the critical diameter is enhanced from 2 to 7 mm without destruction of the magnetocaloric properties of the alloy.The viscosity change in the superheated liquids and the liquid behavior in the supercooled liquids are studied for Gd₅₅Co₂₀Al₂₅ and Gd₅₅Co₂₀Al_24.5Si_0.5 alloys,and a liquid-liquid transition in the superheated melts and fragile-to-strong transition in the supercooled liquids are observed in both alloys.Microalloying Si could dramatically decreases the strength of liquid-liquid transition,increases the strength of fragile-to-strong transition,and facilitates the formation of the short-range-ordering and medium-range-ordering,therefore,suppresses the crystallization process and improves the GFA of the alloy.To reduce the hysteresis loss and improve the MCE of the Gd₂₀Tb₁₈Dy₁₈Co₂₀Al₂₄ high-entropy metallic glass,hydrogen treatment is applied.The result shows that the hydrogen absorption can reduce the random magnetic anisotropy of the alloy thus decreases the coercivity and remnant magnetism effectively.Meanwhile,it improves the magnetic entropy change and lower the Curie temperature greatly.Amorphous Gd₂₀Tb₁₈Dy₁₈Co₂₀Al₂₄H_130.65 alloy exhibits a large magnetic entropy change of 13.6 J kg^-1K^-1 at 8 K under a field change of 5 T,which makes it a promising candidate as magnetic refrigeration at low temperature.This work is of great significance for further understanding the magnetic structure,the correlation between the abnormal dynamic transition and GFA,and adjusting the magnetocaloric properties and GFA of RE-based metallic glasses,which is helpful for developing RE-based metallic glasses with both large GFA and excellent magnetocaloric properties.