Effects Of Different Preparation Conditions On Microstructure And Thermal Expansion Of La（Fe,Co）_10.3Al_2.7 Alloy

Abnormal thermal expansion materials are widely used in aerospace,precision instruments and so on.Most thermal expansion materials are anisotropic,while the Na Zn₁₃-type La（Fe,Al）₁₃alloy has isotropic thermal expansion properties and has a wider range of applications.In existing reports,the preparation of La（Fe,Al）₁₃alloy requires annealing of the alloy for 7-14 days,and the zero thermal expansion temperature region of La（Fe,Al）₁₃alloy is lower than room temperature,which is not conducive to practical research and application.The existing research results show that the preparation of La（Fe,Al）₁₃alloy requires annealing of the alloy for 7-14 days,and the zero thermal expansion temperature region of the obtained La（Fe,Al）₁₃alloy is lower than room temperature,which is not conducive to its application.In this work,we achieved effective control of the microstructure by controlling the preparation conditions,shortened the subsequent heat treatment time,and obtained a La(Fe_0.92Co_0.08)_10.3Al_2.7bulk sample whose zero thermal expansion temperature region covering room temperature.The detailed research content and results are as follows:（1）LaFe_10.3Al_2.7alloy rods with a diameter of 4 mm were prepared by directional solidification.It was found thatα-Fe phase coarsening in directionally solidified rods and the LaFe_10.3Al_2.7rod samples exhibits positive thermal expansion under the conditions of different drawing rates and addition of different extra La.（2）A copper mold spray casting method is used to prepare LaFe_10.3Al_2.7alloy rod samples with a diameter of 4 mm and LaFe_10.3Al_2.7alloy sheets with a thickness of 2 mm.It is found that theα-Fe in the sheet samples before annealing is finer and more uniform than that in the bar samples;After 2 days of annealing,the sheet sample is only composed ofα-Fe phase and Na Zn₁₃type compound phase as the La-rich phase has basically disappeared.The alloy sheet can effectively shorten the annealing time and ensure zero thermal expansion effect at the same time.（3）The effect of Co doping on the zero thermal expansion temperature zone of the LaFe_10.3Al_2.7alloy sheet was studied.It was found that the La(Fe_0.92Co_0.08)_10.3Al_2.7alloy sheet with a thickness of 2 mm which prepared by copper mold injection casting has the thermal expansion coefficient was as low as 0.764 ppm/K,the upper limit of the zero thermal expansion temperature zone reached 296.0 K,realizing the wide temperature zone（150 K-296K（（35）T=146 K））zero thermal expansion effect at room temperature.