Phase Formation Rule And Deformation Mechanism Of Ti-Zr-Hf-Nb-Al High-entropy Alloy

Differing from the design concept of traditional alloys,high-entropy alloys(HEAs)which are composed of multiple major elements are developed based on a brand new alloy design strategy.The synergistic effect of multi-elements endows HEAs with high mixing entropy,which brings in many excellent properties,such as high strength/hardness,good wear resistance/corrosion resistance,excellent strength at high temperature and toughness at low-temperature.Among them,metastable high-entropy alloys have attracted extensive attention due to their excellent deformation ability and unique mechanical properties.At present,the research of metastable HEAs is still in its infancy,and a number of scientific problems need to be solved urgently.Firstly,it is difficult to accurately design metastable HEAs by traditional calculation methods because of the large number of principal components and the wide range of components.Second,the composition-structure-property relationship in metastable HEAs has not been systematically studied and established.In addition,metastable alloys have the shape memory effect due to their unique deformation behavior,while little research has been done on metastable HEAs in this field.In this thesis,Ti-rich Ti-Zr-Hf-Nb-Al HEAs are taken as the research object.The influence of components on phase composition in these HEAs is explored.The quinary Ti-rich MHEAs with stress-induced transformation characteristics have been successfully predicted by machine learning method based on the database consisting the quaternary HEAs alloys and traditional titanium alloys.The superelastic and mechanical properties of the MHEAs were optimized by deformation and heat treatment,and the strengthening and toughening mechanism were systematically studied.Firstly,the effects of composition in quaternary Ti-Zr-Nb-Al and Ti-Zr-Hf-Al alloys on the phase composition and mechanical properties were studied.In the Ti-Zr-Nb-Al system,the contents of Nb and Al affected the formation of metastable ?phase.When the contents of Nb and Al are low,the as-cast alloy consists of metastable ? phase and ?" martensite;when the content of Nb is high,the alloys consists of stable ? phase;excessive Al promotes the formation of intermetallic compounds and deteriorates the mechanical properties.In the(Ti2ZrHf)100-xAlx(x=12,14,16,18,20)system,Al acts as a weak ?-stable element and has important influence on the phase composition and mechanical properties.With the increase of Al content,the phase composition of the alloys changes from a' phase with hexagonal close-packed(HCP)structure to a" phase with orthorhombic structure,and then to a ? phase with body-centered cubic(BCC)structure.In A120,the existence of metastable ? phase results in stress-induced martensitic transformation during tensile process.The database for metastable alloy was established based on the two quaternary systems and metastable titanium alloy data.Phase prediction of the quinary Ti-Zr-Hf-Nb-Al system has been proformed by using the machine learning(ML)method based on the database.The existence of metastable alloys was predicted.Experimental result has verified that the machine learning model can realize the composition design of HEAs due to its excellent data mining ability.Based on the prediction results of ML for Ti-rich HEAs,the phase composition,phase stability,mechanical properties and deformation mechanisms of as-cast Ti(55-x)Zr20Hf15Al10Nbx(named as T-Nbx in the following text)alloys were deeply analyzed.The addition of Nb promotes the transformation from HCP to BCC structure,and the stability of ? phase increases gradually.The T-Nb1-3 HEAs containing ?'/?'+?" phase show high strength and low plasticity due to the highly solution strengthening.The T-Nb4-8 HEAs displayed double-yield phenomenon during tension,in which the de-twinning process of self-accommodating martensite occurs in T-Nb4 HEA and stress-induced ???" transformation happens in T-Nb5-8 HEAs.There are no double yielding or obvious work-hardening effect in T-Nb9 HEA due to the stable ? phase.The mechanical properties and deformation mechanism of the Ti-Zr-Hf-Nb-Al HEAs mainly containing metastable ? phase after cold-rolling and heat treatment processing have been studied.After annealing at different temperatures,the cold-rolled T-Nb7 alloy completely recrystallized,and grain size increased with the annealing temperature.The grain size has obvious effect on the free energy of martensitic transformation.In the T-Nb7-800 alloy annealed at 800? with smaller grain size,stress-induced martensitic transformation from ? to ?" occurs firstly resulting in different martensitic variants,and then martensitic variants rearrange;while in the T-Nb7-900 alloy annealed at 900? with larger grain size,the twinning deformation of ?" occurs after the stress-induced martensitic transformation which endows the alloy with excellent deformation capability.The superelasticity effect and deformation mechanism of the Ti-Zr-Hf-Nb-Al HEAs have been studied in this thesis.It has been found that T-Nb6 alloy with cold rolling+900 C/0.5h heat treatment has good strength and superelasticity.The maximum recoverable strain is up to 5.2%,and the corresponding maximum tensile stress is 900MPa.In situ synchrotron X-ray diffraction study of T-Nb5 alloy with cold rolling+800?/0.5h showed that the superelasticity during tensile process is caused by the reversible ?(?)?" transformation.Beyond a certain amount of deformation,martensite variants rearrange and the superelasticity disappears.