Control Of Thermal Expansion Properties Of （Hf,Ta）Fe₂ Matrix Composites

Hexagonal Mg Zn₂ type Hf_1-xTa_xFe₂（（Hf,Ta）Fe₂）alloy is a typical type of magnetic phase transition alloy,which undergoes composition-dependent antiferro/paramagnetic phase transition during cooling.This magnetic phase transition is accompanied by significant volume expansion,indicating that this kind of alloy is a negative thermal expansion material.It can be composited with positive thermal expansion materials to construct zero thermal expansion composite materials.The latter has important application value in the field of precision instrument manufacturing.However,zero thermal expansion composites based on（Hf,Ta）Fe₂and their preparation methods are rarely reported.In this paper,a positive thermal expansion phase is generated in the（Hf,Ta）Fe₂ matrix to compensate for the negative thermal expansion effect associated with the magnetic phase transition of the matrix,and then a（Hf,Ta）Fe₂ based composite material with zero thermal expansion at room temperature is designed and prepared.The specific research results of this paper are as follows:（1）The microstructure of the as-cast（Hf,Ta）Fe₂ alloy was studied,and it was found that the as-cast（Hf,Ta）Fe₂ had Fe-rich grain boundaries;after annealing,the Fe-rich grain boundaries disappeared.This phenomenon motivates this study to create a large amount of Fe-rich phase in the（Hf,Ta）Fe₂ matrix by adding additional Fe elements without annealing.As a positive thermal expansion phase,the Fe-rich phase can compensate the negative thermal expansion effect of the matrix,thereby achieving zero thermal expansion.After reasonably adjusting the ratio of Hf/Ta and content of Fe element,this paper realized the wide temperature range（265K-350K（（35）T=85K））room temperature zero thermal expansion effect in Hf_0.80Ta_0.20Fe_2.5 alloy（the thermal expansion coefficient is 0.352ppm/K）.（2）Hf_0.80Ta_0.20Fe_2-x alloy was selected as the research object,by reducing the content of Fe element in the matrix,promote the formation of Hf-rich phase;the negative thermal expansion effect of（Hf,Ta）Fe2 matrix was compensated by the positive thermal expansion effect of Hf-rich phase,and then zero thermal expansion was achieved.When the sample composition is Hf_0.80Ta_0.20Fe_1.6,in the wide temperature range of 255K-345K（（35）T=90K）,the thermal expansion coefficient of the alloy reaches 0.97ppm/K,achieving the zero thermal expansion effect at room temperature.（3）Firstly,the Hf_0.87Ta_0.13Fe_2-xNi_x alloy was selected as the research object,and Ni element was used to replace Fe element to promote the formation of C15 phase.As the positive thermal expansion phase,the C15 phase compensates the negative thermal expansion of the C14 phase of the（Hf,Ta）Fe₂matrix,thus reducing the degree of negative expansion.However,the introduction of Ni greatly reduces the magnetic phase transition temperature,so that the Hf_0.87Ta_0.13Fe_2-xNi_xalloy does not exhibit zero thermal expansion at room temperature.In order to achieve zero thermal expansion at room temperature,this study adjusted the Hf/Ta ratio to9:1 to reduce the degree of negative thermal expansion of the matrix.In the Hf_0.9Ta_0.1Fe_2-xNi_xalloy,the sample with x=0.12 exhibits zero thermal expansion（coefficient of 0.353ppm/K）in a wide temperature range of 225K-294K（（35）T=69K,including room temperature）.