Study of plastic deformation mechanisms in TA15 titanium alloy by combination of geometrically necessary and statistically-stored dislocations

Abstract

Study of plastic deformation mechanisms in TA15 titanium alloy by combination of geometrically necessary and statistically-stored dislocationsThis study combines geometrically necessary dislocation analysis and statistically-stored dislocation identification to investigate plastic deformation mechanisms in TA15 titanium alloy. Tensile tests were conducted up to different strains at room temperature. The slip lines were observed and identified using high resolution scanning electron microscopy. Geometrically necessary dislocations were calculated based on strain gradient theories. The statisticallystored dislocations were studied using transmission electron microscopy under two beam conditions. The results indicate that the ratio of <a>-type geometrically necessary dislocation density to <c+a>-type decreases from 5 -8 to 1 -3 with increasing strain from 0.8 % to 2.0 %. The <a>-type slip is the dominant deformation mechanism at the early stage of plastic deformation, whereas <c+a>-slips become dominant with further increase in deformation strain.Keywords: Deformation mechanisms; Geometrically necessary dislocation; Slip line; Transmission electron microscopy; Electron backscatter diffraction 1. Introduction TA15, also known as Ti-6Al-2Zr-1Mo-1V, is a typical near a titanium alloy. The microstructure of TA15 is composed of dominant a lamellae (hexagonal-close-packed, hcp) and a few residual beta phases (body-centered-cubic, bcc) at room temperature [1]. Because of its high specific strength and excellent corrosion resistance at the evaluated temperature, TA15 titanium alloy is preferentially used in aviation and aerospace industries [2 -9].Generally, because of low symmetry of the hcp crystal structure and the special lamella grain morphology, the near a titanium alloy exhibits low tensile ductility, high strength and anisotropic mechanical properties compared to beta and a+b titanium alloy [10]. The deformation mechanisms in hcp metal are complex because of the competition between dislocation slip (including basal, prismatic pyramidal slip model) and twinning under different deformation conditions [11,12]. Especially, due to a relatively high Al content, twinning in alloyed TA15 tends to be suppressed and dislocation slip is the dominant plastic deformation mechanism. According to von-Mises criterion, at least five independent slip systems are required for homogeneous plastic deformation. Therefore, in addition to basal <11 20> {0002}, prismatic <11 20>{10 10} and pyramidal <11 20> {10 11} slip modes, the non-basal <c+a> <11 23>{10 11} and/or {11 22} <11 23> slips are invoked [13]. However, the effect of addition slip modes on full plastic deformation of titanium and titanium alloys is still debatable [14]. Therefore, it is very important to study deformation mecha-