Effect Of Microstructure On Magnetocaloric Properties Of High Entropy Alloys

In comparison to the conventional gas compression-expansion refrigeration technique,the magnetic refrigeration（MR）,which is based on magnetocaloric effect（MCE）,shows some advantages,e.g.,environmental friendliness and relatively high cooling efficiency.Therefore,MR has been widely considered as the next-generation refrigeration technique.As a new kind of alloy,high-entropy alloys（HEAs）developed quickly in recent years.HEAs encompass vast compositional design space.Some HEAs show comparable MCE properties.Therefore,there is a large possibility to explore and obtain HEA with outstanding MCE properties.Compared with the laminar and particle MCE alloys,the MCE alloys with the wire shape in micro-size diameters are more suitable for refrigeration cycle.The alloys with multi-phase structure show a wider working temperature span and enhanced MCE properties compared to those of alloys with single-phase structure.This thesis mainly focuses on the effect of the microstructure on the magnetocaloric properties of HEAs.Based on the classic compositions of first-generation MCE HEAs,we design the compositions of second-generation MCE HEAs with equiatomic compositions and multi-phase structure,non-equiatomic compositions and single-phase structure,and non-equiatomic compositions and dual-phase structure.We use melt-extraction technique to prepare the corresponding microwire samples.The current annealing is conducted to tune the microstructure of microwires.The above research will be helpful for establishing the relationship between the microstructure and the MCE properties of HEAs,and exploring the HEAs with high MCE properties.Through the compositional design and melt-extraction technique,we obtain three kinds of multi-phase-structure HEA microwires,which are crystalline/nanocrystalline(Gd₂₀Dy₂₀Ho₂₀Tb₂₀Er₂₀,Gd₂₅Dy₂₅Ho₂₅Tb₂₅ and Gd₂₅Dy₂₅Ho₂₅Er₂₅),amorphous/nanocrystalline(Gd₂₅Tb₂₅Co₂₅Al₂₅)and nanocrystalline（two phase structures）/amorphous(Gd₂₅Tb₂₅Co₂₅Fe₂₅)microwires.Among these microwires,Gd₂₅Tb₂₅Co₂₅Al₂₅ microwires possess several advantages,which are relatively easily tunable structure（it is convenient to further explore the influence of microstructure on the MCE properties of HEAs）,relatively high transition temperature（78 K,which is higher than the limitation of low working temperature（<60 K）of first-generation rare-earth（RE）containing high-entropy metallic-glasses（HE-MGs））,second-order-magnetic-transition（this overcomes the limitation of magnetic and thermal hysteresis of first-generation RE crystalline HEAs）and relatively high MCE properties(under a magnetic field change of 5 T,the maximum isothermal magnetic entropy change(|ΔS_M^pk|)is 8.9 J kg^-1 K^-1,overcoming the limitation of low MCE properties of first-generation RE-free HEAs).Therefore,we select this group of elements to do the further research.In order to study the specific influence mechanism of nanocrystalline phase on HE-MG,we firstly prepare fully amorphous HEAs.The Gd₃₆Tb₂₀Co₂₀Al₂₄ is designed by using binary eutectic clusters.For verifying the extensive validity of this method for the compositional design of single-phase HE-MGs,we design Dy₃₆Tb₂₀Co₂₀Al₂₄and Ho₃₆Tb₂₀Co₂₀Al₂₄ by the same method.All microwires with above compositions show the similar glass forming ability and fully amorphous structure at room temperature,proving the extensive validity of this compositional design method.The high energy synchrotron X-ray diffraction is conducted to explore the structure evolution of all microwires from room temperature to cryogenic temperature.As the temperature decreases,all microwires keep the fully amorphous structure.Among these microwires,the amorphous structure of Gd₃₆Tb₂₀Co₂₀Al₂₄ microwires possesses the largest dispersion of local clusters,resulting in the widest working temperature span and the highest cooling efficiency of the microwires.In comparison to Gd₂₅Tb₂₅Co₂₅Al₂₅ microwires,Gd₃₆Tb₂₀Co₂₀Al₂₄ microwires possess a similar transition temperature and|ΔS_M^pk|,a wider working temperature span and an enhanced cooling efficiency.In order to obtain the HE-MGs with higher transition temperatures and higher MCE properties,we design the compositions of(Gd₃₆Tb₂₀Co₂₀Al₂₄)_100-xFe_x（x=1,2and 3 at.%）.The glass forming ability of the Fe-doped microwires is decreased due to the negative and small heat of mixing between Fe and RE elements.This,combining with the low cooling speed of the free surface of the adhesion layer during the melt-extraction process,favors the amorphous/nanocrystalline dual-phase structure of Fe-doped alloy microwires.The content of nanocrystals increases with increasing Fe fraction.Fe-doping has a slight influence on the|ΔS_M^pk|(|ΔS_M^pk|of Fe-doped Gd Tb Co Al alloy microwires range 7.6-8.9 J kg^-1K^-1（5 T）).The compositional difference between nanocrystalline phase and amorphous matrix leads to the Curie temperature difference between two phases.Therefore,the values of working temperature span and relative cooling power（RCP）of Fe-doped alloy microwires constantly increase with the increase of Fe content.Besides,Fe-doping increases the Curie temperatures from 81 K（x=0）to 108 K（x=3）,above the temperature limitation of the first-generation RE-containing HE-MGs（<60 K）and towards a temperature range of natural gas liquefaction.Due to the limited fraction of the nanocrystalline phase,the highest increase of RCP values of Fe-doped Gd Tb Co Al microwires is only 7%.In addition,only the broadening effect of nanocrystalline phase on the global MCE response is observed.In order to further explore the influence mechanism of the nanocrystalline phase on the MCE properties and critical behavior of RE-containing HE-MGs,it is essential to increase the content of the nanocrystals.Accordingly,the current annealing technique is used to treat(Gd₃₆Tb₂₀Co₂₀Al₂₄)₉₇Fe₃ microwires with the aim to tune their microstructure.The results indicate that the fraction of nanocrystalline phase and the compositional difference between amorphous matrix and nanocrystalline phase of the microwires increase with increasing current density.The compositional difference of two phases broadens the working temperature span.In addition,the|ΔS_M^pk|of the microwires annealed with low current density values are similar to that of as-cast microwires.These increase the RCP values of the annealed microwires to be larger than reported values of many conventional magnetocaloric alloys with single-phase or multi-phase structure.In addition,our annealed microwires exhibit the working temperatures beyond the typical<60 K limit of first-generation RE-containing HE-MGs,while maintaining the comparable MCE properties.