Formation Mechanism Of ? Phase In Fe-V Alloy And Its Industrial Application

It is generally that topologically close-packed structured(TCP)? phase would always deteriorate mechanical properties of structural materials,such as Ni-or Co-based single-crystal superalloys and high-chromium ferritic steels containing transition metal(TM)elements.Therefore,predicting and controlling the formation of ? phase have been long-term research topics,not only because of the theoretical interest but also important background in practical applications.From the theoretical point of view,the formation mechanism of ? phase is beneficial to better understand the alloying behavior of transition metals,and from the viewpoint of technical perspective,it is valuable to avoid the precipitation of ? phase,and then minimizing the effect on the performance of materials.So far,although several models on the formation of ? phase have been proposed,its formation mechanism is still not clear yet.Detailed studies on the influence of atomic size are lacking,and available research mainly focuses on the electronic factors,such as electron concentration and electronegativity,etc.However,these electronic factors cannot describe the formation of TM-TM ? phase consistently and since they are not derived from the geometrical or physical models.From our point of view,the ? formation should be controlled by both the electronic factor and size factor in binary transition metal alloys with co-existence of metallic bond and unsaturated covalent bond,and the covalent bond acts as the leading role.It is the existence of covalent bond and proper atomic size that results in the formation of ?phase.In order to outline the chemical bonding in ? phase,a new concept of VET(valence electron total count)is proposed in the present work.Also,atomic size parameters R=rs/rl and 1/R=rl/rs(the subscripts s and l refer to smaller and larger atoms respectively)are introduced to reveal the requirement of Frank-Kasper coordination polyhedra formation and the influence of geometrical factor on theformation of the ? phase,based on the similarity of topological short-range order(TSRO)in liquid metal before it transforms into glasses,icosahedral quasicrystals,and TCP phases,respectively.It shows that,within a given TM-TM alloy systems defined in this work,a range of 12?VET?15 can cover all ? phases.A geometrical rule shows that ranges of 0.896<R<0.955,1.047<1/R<1.116 and 0.976<R<1,1<1/R<1.025 are favored by the formation of ? phase,and this rule can effectively differentiate the ?,Laves and ? phases but fails to distinguish the ? and A15 phases.The method is extended to the Laves phase in TM-TM systems,and a new parameter DET(d electron total count)is also developed to find out the formation rule of TM-TM Laves phases.Atomic size criterion of R >1.116 is valid for the TM-TM Laves phases forming through liquid phases,and the DET in the range of5<DET<10 favors the formation of Laves phases.Based on the fact that the formation mechanism of ? phase,we propose to suppress the precipitation of the ? phase by introducing a third larger or smaller atom into a TM-TM alloy.An Al-doped Fe-V alloy was chosen in the present work to test this possibility.Results showed that Al with larger atomic radius destabilizes the ?phase in FeV alloy and the critical Al concentration is highly consistent with the theoretical calculation value.This method provides a possible new strategy to control the precipitation of ? phase in binary,ternary or even higher order multi-component alloy systems.With regards to the high strength of ferrovanadium,we adopt the method of reverse thinking,taking the transition metal ? phase alloy formation mechanism as the main line,and finding out the key condition for the stable ? phase formation in FeV alloy to improve the crushing performance of FeV50.Based on the finding,a gradient smelting approach using tilting furnace was proposed and obtained satisfactory results in industrial production practice.