Effects Of Heavy Rare-earth Addition On Properties Of FeRENbB Bulk Metallic Glasses

As an alloy family with diversified elements,Fe-based bulk metallic glasses（BMG）have the potential to be used as magnetic refrigeration materials near room temperature because of their wide adjustable range of components,large span of magnetic phase transition temperature and low manufacturing cost.On the other hand,the Fe-based BMGs with the long-range disordered structure can lift the restrictions on grain size effect in microforming,which are expected to become a more ideal microforming magnetic material.However,it is difficult to realize their application as near room temperature magnetic cooling medium or the thermoplastic processing in supercooled liquid region（SCLR）due to the limitation of glass-forming ability（GFA）and small SCLR.In order to develop Fe-based BMGs with high GFA and large SCLR,the technology of microalloying has been applied in this work by doping minor rare-earth（RE）elements in Fe Nb B ternary alloy.The GFA,thermal stability,magnetic properties,mechanical properties and electronic structure of FeRENbB glassy alloys were characterized by differential scanning calorimeter,X-ray photoelectron spectrometer,vibrating sample magnetometer,testing machine and so on.In this paper,we investigated and analyzed in detail on the influence of the contents and species of RE elements on the properties and structure of this alloy system,eventually developed Fe-based BMGs with good magnetic refrigerant performance in temperature range of 340-550 K and good thermoplastic forming ability.Firstly,the influences of Ho addition on the thermal stability of supercooled liquids,thermoplastic deformation as well as mechanical and soft-magnetic behaviors of Fe_71-xHo_xNb₆B₂₃ BMGs were investigated.With increasing Ho content from 1 to 5 at.%,the SCLR increased from 48 to 90 K,and the thermal stability is largely enhanced through remained chemical short-or medium-ordering.Due to the competitive formation of the complex Fe₂₃B₆and Ho₂Fe₁₄B phases,the GFA of the alloy system was improved,allowing for the fabrication of glassy rods up to 3 mm in diameter.Due to the large SCLR and GFA,the Fe₆₆Ho₅Nb₆B₂₃BMG with high fracture strength of 3.45 GPa shows great thermoplastic forming ability accompanied with good soft magnetic performance.Secondly,the comprehensive characteristics of Fe_71-xEr_xNb₆B₂₃（x=0,1,3,5,7 at.%）BMGs,including the thermal properties,GFA,magnetocaloric effect and mechanical properties were investigated.As a result,with increasing Er content up to 5 at.%,the supercooled liquid region increases to 90 K,and BMG with a critical diameter of 4 mm is fabricated by copper mold casting.The regulation of Curie temperature is achieved by controlling the amount of Er.The resulting glass alloy system exhibits Curie temperature of 340-550 K,magnetic entropy change of 0.74-1.41 J/kg K,and refrigerant capacity of 56-112 J/kg in external magnetic field of 1.5 T.Moreover,the Fe_71-xEr_xNb₆B₂₃ BMGs exhibit high saturation magnetic flux density of0.43-1.09 T,high fracture strength of 3.09-4.65 GPa and high Vickers hardness of 1030-1090kg/mm²,respectively.Considering these good overall characteristics,this glass alloy system can be employed as magnetic-refrigerant materials in temperature range of 340-550 K.Finally,with the aim of investigating how the species of doped RE elements affect the GFA,5 at.%different RE elements were added in Fe₇₁Nb₆B₂₃alloy to substitute Fe.Compared with the Fe₆₆RE₅Nb₆B₂₃（RE=Ho or Dy）,Fe₆₆RE₅Nb₆B₂₃（RE=Er or Tm）not only possesses the same Fe₂₃B₆and Ho₂Fe₁₄B₂ phases,but also has additional Fe₃B phase precipitation,which leads to a better GFA by stabilizing the liquid phase structure.Moreover,Fe₆₆Tm₅Nb₆B₂₃ shows the lowest density of states near the Fermi level among four alloys,which contributes to its best GFA.The influences of RE addition on the thermal,magnetic,and mechanical behaviors were also investigated.With increasing atomic number of RE elements from Dy to Tm,the saturation magnetic flux density increases obviously accompanied by the improved flowability in the supercooled liquid region,which make Fe₆₆Tm₅Nb₆B₂₃ as a potential functional soft-magnetic material for thermoplastic machining.