| βTi-Nb alloy is used as human implants with good biocompatibility, low elastic modulus, high strength, good wear resistance and corrosion resistance, that it is one of the best medical titanium alloy.Research of plastic deformation mechanisms of Ti-Nb binary alloy help us better understand plastic deformation of multicomponent βTi-Nb alloy, and help design of βTi-Nb alloy with excellent mechanical properties. In this article, we firstly analyze the orientation relationships among various phases, and simulate compound diffraction patterns of β twins, ω, α"and β in order to effectively analyze the actual diffraction patterns; transmission electron microscopy(TEM) is used to study phase stability, plastic deformation mechanisms and their relationships in Ti-(40, 50, 65, 85)Nb(wt.%) al oys. We come to the following conclusions:1. There are only four kinds of {112} <111> twins with different orientation in bcc alloy, and each has 3 possible {112} twinning planes parallel to the same <111> axis. In the compound diffraction patterns along any of [011]β, [012]βand [1-13]β of both {112} <111> twins and the matrix, only 2 kinds of {112} <111> twins can be distinguished from the matrix and the others have no more diffraction spots than the matrix. Each variant of ω phase also has 3 possible {112} habit planes parallel to the same <111> axis, and the diffraction laws of ω phase is identical to that of {112} <111> twins along any of [011]β, [012]β and [1-13]β, i.e. when the electron beam is parallel to a possible habit plane, the diffraction pattern can be distinguished. And only 2 variants of ω phase among 4 variants can be distinguished from the compound diffraction pattern along one of above axises. There are respectively 2, 1, and 3 variants among 6 α " variants, which can be distinguished from the compound diffraction pattern along [100]β, [011]β, and [1-13]β. Based on the fact that the diffraction spots of experiments are more than that of simulated, it can be known that double diffraction occurs in each diffraction of ω or α".2. TEM characterizations of the microstructure of Ti-Nb alloy before and after deformation indicate: stress- induced α " transformation and dislocation slip occur in Ti-40 Nb during plastic deformation; in Ti-50 Nb during plastic deformation, {112} <111> twinning and stress- induced ω phase transformation occur, and V-shaped ω lamellas zigzag configuration of twin and ω are peculiarly produced; in Ti-65 Nb, it is mainly dislocation slip, there is little stress- induced ω transformation; in Ti-85 Nb, only dislocation slip occurs. The relative diffraction intensity of althermal ω phase indicates β phase stability in Ti-Nb alloys increases with increasing Nb content. So we come to the conclusions: in Ti-Nb alloy with lower β phase stability, α" martensitic transformation is prone to occur during plastic deformation; when β phase stability is moderate, stress- induced phase transformation ω and {112} <111> twinning can occur; when β phase stability is high, the alloy can only be deformed through dislocation glide. Dislocation motion is a indispensable way of plastic deformation in β Ti-Nb al oy, and the larger density of dislocations the higher yield strength of the al oy.3. Zigzag configuration of {112} <111> twin and stress- induced ω phase is observed in Ti-50 Nb. The habit planes are respectively(1-21) and(11-2) with common <111> axis. The zigzag configuration can be formed by the movement of three- layer homogeneous 1/6<111> and inhomogeneous 1/12<111>, 1/3<111>, and 1/12<111> shears on consecutive {112} planes, respectively resulting fro m dissociation of a perfect 1/2<111> screw dislocation and the cross-slip of the screw dislocation when it is blocked at the obstacle. To form zigzag configuration following requirements need to be satisfied in the Ti-Nb alloy: 1) high β phase stability; 2) stacking fault energy of {112} plane is moderate; 3)(0-11) plane can block the partial dislocations; 4) stress is asymmetrically distributed on two habit planes. |
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