Composition Design Approach Based On Cluster Structure Model Of Multi-component Ti Alloys And Their Properties

Titanium alloys have been used extensively in aerospace,biomedical,and chemical fields due to their high specific strength,low modulus,excellent corrosion resistances,as well as good fatigue and creep resistances.These prominent comprehensive properties of Ti alloys are derived from their multi-component alloying.However,the multi-component additions will cause a chemical composition complexity and further affect the structural stability of alloy.Therefore,the species and amounts of alloying elements could not be decided in quantitative yet,resulting in a poor efficiency to design multi-component alloys with high performance.Actually,the chemical short-range orders(CSROs)are the most typical structural characteristics of solid solution alloys due to the obvious local structural heterogeneities with respect to the average crystal structure,which plays an important role to the precipitation of ordered phases and the various properties of alloys.Therefore,the present thesis introduced the cluster-plus-glue-atom model which characterizes the CSROs to explore the evolution among the local atomic distributions of alloying elements in different phase structures of Ti alloys.A cluster-formula composition design approach was then proposed and applied in composition interpretation of multi-component Ti alloys based on the cluster-plus-glue-atom model.Then near ?-Ti alloys with high-temperature oxidation resistance and metastable ?-Ti alloys with low Young's modulus were developed by this method.Meanwhile,the structural stabilities and mechanical properties of these new alloys were characterized by theory and experiments.The relationships among cluster-formula composition,structural stability,and properties was established finally.The details are as follows.1.The cluster-plus-glue-atom model was introduced to explore the relationships among the local atomic distributions of alloying elements in different phase structures of Ti alloys,including ?-Ti,?-Ti,co-Ti,a"-Ti,a2-Ti3Al,y-TiAl,O-Ti2AlNb,B2-Ti(Al,Nb).With the guide of the Friedel oscillation theory for electron-structure stabilization,specific cluster structural units,i.e.,cluster formulas,for these phases were determined,being[Al-Ti6Ti6]Ti3 for a-Ti,[A]-Ti2Ti12]Ti1 for ?-Ti,[Al-Ti4Ti4Ti2Ti2](Ti2Ti1)for a"-Ti,[Al-Ti8Ti6]Ti1]for p-Ti,[Al-Ti6Ti6]Al3 for a2-Ti3Al,[Al-Al4Ti8]Al3 for ?-TiAl,[Al-Ti4Ti2Ti2Nb2Nb2](Al2Al,)for O-Ti2AlNb,and[Al-Ti8(A1,Nb)6](Al,Nb)1 for B2-Ti(Al,Nb),respectively.The total atom number(Z)values in these cluster structural units are all Z= 16.Furthermore,the composition rules of industrial multi-component Ti alloys based on these phases were generalized in light of these cluster structural units.Thus a general cluster formula was formed,which will open up a new route towards designing high-performance Ti alloys with complex compositions.2.The near ?-Ti alloys with multiple elements were designed to improve the high-temperature oxidation resistance based on the cluster formula approach.A series of new alloyswith Nb/Ta substitution for Mo in Ti-1100(Ti-6A1-2.75Sn-4Zr-0.4Mo-0.45Si,wt.%)alloy were designed to improve the oxidation resistance above 600?,which is based on the uniform cluster formula[Al-(Ti,Zr)12](Ti2(Al,M1)(M=Sn,Mo,Nb,Ta,W,Si)for the near a-Ti alloys.The experimental results showed that the new[Al-(Ti11.7Zr0.3)](Ti2Al0.69Sn0.18Mo0.01Ta0.01Nb0.01,Si0.1)(Ti-6.0Al-2.8Sn-3.6Zr-0.1 3Mo-0.12Nb-0.24Ta-0.37Si,wt.%)alloy with the co-addition of Mo.Ta,and Nb exhibits a high tensile strength and good ductility at both room and high temperatures.An obvious improvement of oxidation resistance achieves in another alloy[Al-(Ti11.7Zr0.3)](Ti2Alo.69Sn0.18Tao.03Si0.1)(Ti-6.0Al-2.8Sn-3.6Zr-0.72Ta-0.37Si,wt.%)with Ta alloying alone,in which the weight gain oxidized at 800 °C is only one-seventh of the value of Ti-1 1 00 alloy.3.Metastable ?-Ti alloys with lower Young's modulus were designed via the cluster formula approach,because the Young's modulus of currently developed ?-Ti alloys is still 50-80 GPa.The cluster-formula approach can achieve the optimum combinations of different elements,[?-Ti stabilizers(Mo,Nb)and low-E elements(Sn,Zr),to lower the E further on the premise of 3 stability.The specific design steps for multi-component low-E"(3-Ti alloys were then determined,in which the lowest Young's modulus with ?= 43 GPa reaches at the[(Sno.5Moo.5)-(Ti13Z?)]Nb-(Ti-10.0Zr-5.2Mo-6.5Sn-10.2Nb,wt.%)composition.It is attributed to the multi-component alloying,which stabilizes the(3-Ti phase by inhibiting the precipitation of ?"and? phases with relatively high E.Based on the cluster formula,a new Mo equivalent(Moeq)Q was proposed through the slopes of the boundary lines between the ?and(?±?)phase zones in binary Ti-M phase diagrams.In specific,the equivalent coefficient of each element M is the slope ratio of the[?/(a+?)]boundary line of the binary Ti-M phase diagram to that of the Ti-Mo.It is expresses with the equation of(Moeq)Q-1.OMo+1.25V+0.59W+0.33Nb+0.25Ta+l.93Fe±l.84Cr+1.50Cu+2.46Ni+2.67Co+2.26Mn+0.30Sn+0.31Zr+0.19Hf+3.01Si-1.47Al(wt.%),where the critical lower limit ?c for ?stabilization is ?c= 11.8 wt.%.This new(Moeq)Q can characterize the P stability of multi-component ?3-Ti alloys more reliably,especially for those alloys with the Mo equivalent around the ?c.The relationship between(Moeg)Q and Young' s modulus of multi-component Ti-Mo-Sn-Zr-Nb alloys was also established to guide the design of low-E ?-Ti alloys.4.The effects of cold rolling and heat treatments on the structural stability and mechanical behavior of[(Sn0.5Mo0.5)-(Tii3Zr1)]Nbl and[Sn-Ti14]Nb3 alloys were investigated,in an attempt to develop metastable ?-Ti alloys with both low modulus and high strength.It was found that the cold-rolled[(Sn0.5Mo0.5)-(Ti,3Zr1)]Nb1 alloy with fine grain and rolling textures exhibits a much lower Young's modulus with E?39 GPa.After aging at 400? for 20 min,a much higher tensile strength of OUTS=1369 MPa could be obtained,which is attributed to the formation of a",and high-density dislocations,not the brittle ? phase induced by the cold rolling.After the cold rolling and aging at 400 ?/20 min,the tensile strength of another ?-[Sn-Ti14]Nb3 alloy with a good ductility could be enhanced to ?UTS=1049 MPa,compared with the value of ?UTS?447 MPa at suction-cast state.In addition,this alloy has a high structural stability since no ? phase precipitates during the serial treatments,including cold rolling,solution treatment followed by water quenching,and aging.