Phase Formation Of Fe-Ga Alloys During Solidification And Heat Treatment

Fe-Ga alloys are a type of magnetostrictive materials without RE-additive,have shown a bright application prospection in many fields such as aviation,aerospace,navigation and other fields,cause merits of large low-field magnetostriction,high tensile strength,good toughness and low cost.The magnetostrictive properties of Fe-Ga alloys have been widely investigated,while the phase constitution showing the large magnetostrictive properties of Fe-Ga alloys is still an open question.In this thesis,effects of composition,solidification and heat treatment processes on the formation were investigated.Then,the magnetostriction of some of the alloys were measured.Twelve alloy compositions were selected as(Fe1-xCox)100-yGay(x=0,0.1,0.2,0.3;y=19,23,25).These alloys were prepared by three solidification processes:induction melting,arc melting and levitation quenching.Then these alloys were annealed at different temperatures.The phase constitution and microstructure of these alloys were studied by X-ray diffraction,scanning electron microscopy,electron back-scattered diffraction and optical microscopy.The effects of different Ga concentration,Co concentration,solidification processes and annealing temperatures on the phase constitution and microstructure of Fe-Ga and Fe-Co-Ga alloys were studied.Fe100-yGay(y=19,23,25)and(Fe1-xCox)77Ga23(x=0.1,0.2,0.3)alloys were prepared by arc melting,then annealed at 630℃.The magnetostriction of as-cast and annealed alloys were measured.The main conclusions are drawn as follow:(1)All the Fe-Ga alloy samples solidified under different cooling rates contain A2 and D03 phases,while L12 and a small amount of D019 phases are also formed in levitation-quenched samples due to extremely high cooling rates.The addition of a certain amount of Co element promotes the formation of D03 phase and D019 phase in the arc-melted samples and the levitation-quenched samples,respectively.As a general trend,the addition of Co shifts phase equilibria towards the Fe-rich side in the process of solidification,thus promoting the solid phase transition A2→A2’+D03.(2)Under conditions of three cooling rates,the A2 phase formed in the process of solidification undergoes a solid phase transformation during annealing process.With a higher Ga concentration and the addition of Co,the decomposition products in the A2 phase are produced by spinodal decomposition.The D019 phase is easily precipitated when annealed at intermediate temperature in Fe-Ga alloys.The L12 phase is precipitated when annealed at low temperature in induction-melted alloys.(3)For Fe-Ga alloys,magnetostriction and magnetic response rates of as-cast alloys are better than those of annealed alloys.In the as-cast samples,Co alloying can reduce the magnetostriction and the magnetic response rate.It was determined that the low and medium cooling rates have significant effects on the phase constitution during annealing processes.A2 and D03 phases are favorable for magnetostriction,whereas D019 and L12 phases are detrimental to the magnetostriction.Because the annealed(Fe0.7Co0.3)77Ga23 alloy contains a distorted D03 phase,it shows the most significant magnetostriction among annealed alloys.