Study Of Improving Invar Alloy Performance By Paramagnetism Element

Lower strength is the characteristic of Fe-Ni low expansion alloy. In recent year,the high strength low thermal expansion (HSLTE) alloy with low coefficient of thermalexpansion (CTE) and high strength has been concerned because of the alloy extendedapplication fields. More and more researchers pay attention to study how to balance thelow coefficient of thermal expansion and high strength and to apply the functionalmaterial as special structural materials.The author chooses the alloying elements with paramagnetic property and grainrefinement ability, based on the metal thermal expansion theory and the comprehensivestrengthening theory of primarily to fine-grain strengthening and the result of Fe-Ni-Xphase diagram by the Factsage software. The key point of influencing the CTE, strengthand stability of Fe-Ni invar alloy was investigated. Then, through the experiment ofmelting under gas shielded, forging process, heat treatment, the key factors ofinfluencing on the invar alloy performance are found out. This provides fundamentalsupport to formulate reasonable process route.(1) The rusults of alloying CTE are obtained. The influence order of single-elementalloying on CTE is W, Nb and Mo, which value has increased about50%,50%and110%, compared with the unalloyed sample. The matrix CTE on binary elemen W+Moand W+Nb is similar and improved62.5%, however that of Mo+Nb improved125%,compared with the unalloyed sample. The CET of alloyed with W, Nb, and W+Nb iscorrespondence with standard requirements, except of alloyed with Mo and Mo+Nb.This influence factors on CTE are analysis according to the theory of magnetostric andthe quality magnetization coefficient.(2) The effects of alloying grain size are analyzed. For single-element W, Mo andNb alloying, the grain size of the matrix has greatly decreased that compared with theunalloyed sample. The results show that the high content W and Nb have similarinfluence on the grain size which is5.0μm and5.3μm respectively. The grain size is 6.3μm and6.7μm after adding high content Mo and low content Nb. The grain size is10.0μm when adding low content W. For binary-element W+Mo, W+Nb and Mo+Nb,the grain size is4.8μm,4.5μm and7.69μm respectively.(3) The effects of alloying microhardness HV0.2are analyzed. The single-elementNb and Mo have great influence on microhardness. The HV0.2of high content Nb is172.6, that has improved36.0%compared with the unalloyed sample. The HV0.2oflow content Nb and Mo is nearly163. The HV0.2of the content W is150, it hasimproved12.7%compared with the unalloyed sample. The influence order ofbinary-element alloying on HV0.2is W+Nb, Mo+Nb and W+Mo, which value is170.3,162.0and151.(4) The impact of single element alloying on improving matrix performance is highcontent Nb, low content Nb, W. That of binary-element is W+Nb, W+Mo and Mo+Nb.The microstructure characteristic is annealing twins.The experimental results show that alloying with ferromagnetic W, Mo and Nb cangreatly fine the grain size of the Fe-Ni invar alloy. The increased strengthening andlower CTE mainly owe to fine grain strengthening, annealing twins sub-structurestrengthening. The effect of Nb alloying on the grain size is the most significant, whichmakes the strength of matrix increase and maintain a low CTE.