Solidification Segregation And Improvement In La(Fe,Al)₁₃ Alloy With Zero Thermal Expansion

Zero thermal expansion or negative thermal expansion materials have important applications in integrated circuits,precision instruments and other fields.Na Zn₁₃-type La(Fe,Al)₁₃ magnetic phase transition alloy is one of the most promising materials with abnormal thermal expansion.At present,the reported La(Fe,Al)₁₃ alloy is mainly prepared by arc melting method,but this method has been proved to cause solidification segregation in the research on La(Fe,Si)₁₃ alloy,i.e.different positions have different microstructure.Considering the similarity between La(Fe,Si)₁₃ alloy and La(Fe,Al)₁₃ alloy,this article suspects that the La(Fe,Al)₁₃ alloy prepared by arc melting method also has solidification segregation.If this phenomenon exists,the prepared La(Fe,Al)₁₃ alloy will have the disadvantage of different thermal expansion at different positions.This is certainly not conducive to practical application.Based on the above problems,this study first confirmed the existence of solidification segregation in arc smelting ingots,and then solved the problem by copper mold spray casting.The specific research content and results are as follows:1.In this paper,La Fe_10.3Al_2.7 prepared by arc melting method is used as the copper mold spray casting,and the microstructure of the ingot before and after annealing is observed.The as-cast ingot is composed of 1:13 phase,?-Fe(Al)phase and amorphous La-rich phase,and the 1:13 phase is mainly distributed at the bottom of the ingot.There is a"nucleus"in the central area of the as-cast sample,which is rich in?-Fe(Al)phase and rare in La-rich phase.The inhomogeneity of the structure and composition caused by the solidification segregation is mainly due to the inconsistent cooling rate of the ingot during the cooling and solidification process.During the annealing process,the?-Fe(Al)phase and the La-rich phase undergo a solid-state reaction to form the 1:13 phase,but the reaction is not sufficient,and there will still be a residual?-Fe(Al)phase.Annealing cannot eliminate segregation.Different regions of the annealed samples have different contents of?-Fe(Al)phase.This results in different thermal expansions at different locations in the alloy.2.Since the solidification segregation phenomenon originates from the uneven cooling rate,this paper uses the copper mold spray casting process to increase the cooling rate of the melt to eliminate the solidification segregation phenomenon.In this paper,a round rod with a diameter of 4mm and a thin sheet with a thickness of 1.5mm of La(Fe,Al)₁₃ alloy were prepared by the copper mold spray casting process,and their microstructure and thermal expansion properties were studied.Due to the increase of the cooling rate,there is no"nucleus"in the arc smelting ingot in the two samples,indicating that the copper mold spray casting method can effectively eliminate the solidification segregation phenomenon.Compared with the round rod sample,the annealed sheet has a finer and more evenly distributed?-Fe(Al)phase.Both the round rod and thin sheet samples exhibit zero thermal expansion effect when the Al content is 2.7 and 1.5.