Phase Relation And Thermodynamic Investigation Of The Nd-Pr-Fe-B Rare-earth Permanent Magnets Alloys System

Nd-Fe-B-based rare-earth permanent magnets show the highest energy product of all magnetic materials.In recent years,magnets were used more and more with the development of the renewable energy industry.The investigations on the Nd-Fe-B-based rare-earth permanent magnets have caused more and more attention.Nowadays,there are two main targets of the investigations on the magnets The first one is to improve the magnetic performance and the second one is to reduce the cost.To add the light rare-earth elements into the Nd-Fe-B-based magnets might be an promising method to get progress in both of the aspects.High performance commercial Nd-Fe-B-based magnets usually contains excessive rare-earth elements including some heavy rare-earth elements such as Tb and Dy.By forming the high coercivity RE2Fe14B phase with heavy rare-earth elements,magnetic properties of the magnets can be improved.Usually,the price of heavy rare-earth elements is about 5 to 10 times of that of Nd with the same purity.A small amount of addition with heavy rare-earth elements can lead to a doubled price of the magnets.Addition with light rare-earth element Pr can improve the coercivity of Nd-Fe-B-based magnets and achieve a higher energy product.On the other hand,the additional with Pr but not heavy rare-earth elements can significantly reduce the cost of the magnets.The phase diagrams and phase relations are the most vital and fundamental information for alloy designing.Thus,investigations on the Nd-Pr-Fe-B alloy system are meaningful.So far,the Nd-Fe-B ternary system has been widely tested and calculated.Besides,only the Pr-Fe-B system was partially determined.Thus in the present work,investigation were focused on the following aspects.1.The literature data of the Nd-Fe-B system was thermodynamically assessed.The isothermal sections of 298,873,973,1073,1173 and 1273 K and a series of vertical sections were reported.Based on the reported experimental phase diagram data and the thermodynamic properties of the intermetallics,thermodynamic models of the phases in the Nd-Fe-B system were selected.Thermodynamic parameters of the Nd-B and Nd-Fe-B system were re-optimized.The calculation can well reproduce the available experimental data.2.The liquidus surface projection and isothermal sections at 873 and 673 K and a part of the isothermal phase relation at 933 K of the Pr-Nd-Fe system were determined.Based on the experimental results and thermodynamic parameters of the binary systems,optimization of the Pr-Nd-Fe system were carried out.Based on the optimized thermodynamic parameters,the calculation can well reproduce the experimental data.It is hard for Fe17Nds to crystallize directly from liquid while the precipitated Fe17Nds can be stable until its peritectic reaction temperature.By thermal analysis,melting temperature of Fe17Nds and the isopleth Fe57.5Nd42.5-Fe72Pr28 were determined.The calculated result can well reproduce the melting temperature of Fe17Nds in this isopleth.3.The liquidus surface projection of the Pr-Fe-B system with B-content below 20 at.%was determined.Based on the solidification processes and phase transformation temperatures of the alloys,some ternary invariant reactions were determined.Based on the experimental results and literature data,thermodynamic parameters of the phases in the Pr-Fe-B system were optimized.Based on the obtained thermodynamic parameters,the calculation can well reproduce the experimental data.4.Based on the thermodynamic parameters of the Nd-Fe-B,Pr-Nd-Fe and Pr-Fe-B systems,and the extrapolation of the thermodynamic parameters of the Pr-Nd-B system,the thermodynamic database of the Nd-Pr-Fe-B quaternary system were constructed.The available experimental isopleth can be well reproduced.Solidification simulation of the Nd-Fe-B and Pr-Fe-B alloys with Fe-content within 52-96 at.%was carried out with the Scheil model and equilibrium phase relation.Relations between phase contents and composition and temperature of the alloys were provided.Addition with the experimental magnetic properties of the alloys,relations between the phase compositions and thermodynamic properties and magnetic properties of the alloys were investigated.Besides,the liquidus surface projection and isothermal section at 1073 K of the Al-Co-Ti system were experimentally determined.Based on the result and literature data,thermodynamic models of the individual phases in this system were selected and thermodynamic optimization of this system was carried out.Based on the obtained thermodynamic parameters,the experimental data can be generally reproduced.