| Fe-36Ni binary invar alloy has been widely used in the precision instruments because of its extremely low linear expansion coefficient.However,low strength greatly limits its industrial application.Improving its strength,together with low expansion coefficient,is very important for its application in industry.In this paper,oxidation characteristics of invar alloys with different chemical compositions during heating were studied in order to optimize their composition and reduce their loss in heat-treating.Then two kinds of invar alloys were heat-treated to study their variation in microstructure,mechanical properties and thermal expansion properties.Precipitation and structure of the secondary phases during aging for those materials were analyzed by means of electrolytic polishing&corrosion and electrolytic extraction.Following results were obtained:1.Addition of Cr in the Fe-Ni binary invar alloy significantly reduces its oxidation because it mainly distributes on the surface of the oxide layer and forms a dense oxide film.Moreover,logarithm of the oxide thickness linearly decreases with an increase in Cr equivalent of the materials.On the other hand,addition of V,Ti and Nb is not favorable to improve its oxidation resistance.EPMA results show that the V mainly distributes in the oxide layer in the form of spots or particles,the Ti and Nb seldom exist in the oxide layer in the form of isolated particles.Inletting pure N2partially reduces their oxidation,but it is difficult to avoid it completely.The oxidation degree of materials increases with an increase in time.And the hot-rolled invar alloys presents the oxidation layer much lower than the cast ingots.2.The Cr-alloyed invar alloy aged at 425?for 7 h after solutionization at 1000?for 1h presents optimum mechanical properties,e.g.Rm=644.1 MPa,Rel=370.2 MPa,A=47.5%,Z=47.8%),and the lowest thermal expansion coefficient of 3.30×10-6 K-1.Extraction on the secondary phases of the aged samples after solution treatment shows that the precipitated products are composed of Cr7C3 and Fe3C.On the other hand,increasing solution temperature is not favorable to improving strength of this invar alloy.However,its strength can be greatly increased through pre-deformation before aging.The sample pre-deforming about 42.8%before aging at 360?for 1 h after solutionization at 1100?for 1 h presents the strength almost same as that of aging at 425?for 7 h after solutionization at 1000?for 1 h.3.Addition of strong carbide-forming elements like V and Ti into Mo-alloyed invar alloy obviously delays growth of the Mo2C carbides during aging,and the Ti(C,N)and VC carbides almost always keep in nano scale during aging.Those nano-carbides are beneficial to increase strength and decrease coefficient of thermal expansion of the invar alloy.The invar alloy containing Mo,V and Ti shows the highest tensile strength of 937.7 MPa and the low expansion coefficient of 2.28×10-6 K-1 in the temperature range of-50?100?when aging at 780?for 5 h after solutionization at 1240?for 3 h. |
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