MS-9-O-2636 TEM investigation of the elementary plasticity mechanisms in TRIP/TWIP Ti-12 wt.% Mo alloy
During the past few decades, interest in titanium alloys has continuously increased due to their combination of properties such as high strength, low density, biocompatibility and good corrosion resistance. However, both their low ductility and their lack of strain-hardening limit their use in advanced applications where superior combinations of strength and ductility are required. Recently, an electronic design approach for the development of a new family of titanium alloys exhibiting a combination of high ductility and improved strain-hardening rate has been proposed and exemplified in the binary Ti–12 wt.%Mo grade [1]. The chemical formulation of such alloys was designed following the Morinaga model based on the cluster DV-Xα method by mapping electronic parameters Bo (bond order) and Md (d-orbital energy) [2]. This map is of great interest since it can be used as a tool to design new titanium alloys exhibiting specific improved performances.
Based on this approach, as-quenched Ti-12 wt.% Mo alloy with athermal ω phase has been designed to exhibit simultaneous transformation induced plasticity (TRIP) and twinning in-duced plasticity (TWIP) effects in order to improve mechanical properties of the as-quenched β phase [1]. Mechanical tests exhibit a work hardening rate never reached before in Ti alloys (Figure 1a) [3]. The monotonic raising of the strain-hardening rate reaches a maximum value of ~ 2000 MPa from the elastic limit to ε = 0.1, the early stage of the plastic deformation. Electron back scattered diffraction (EBSD) has shown the occurrence of {332} <113> twinning and β→α” stress-induced martensitic transformations during tensile loading (Figures 1b and 1c). On the other hand, high resolution transmission electron microscopy (HRTEM) revealed the presence of {112} <111> nanotwins (Figure 2a) accompanied with stress-induced ω phase (Figure 2b). The ω phase was found to nucleate on the (211) mechanical twin boundaries. The nanoscale mechanisms controlling the formation and the interaction of phase boundaries and twin boundaries have been investigated using TEM techniques including aberration corrected TEM and in-situ TEM micro/nanomechanical testing. Special efforts have been also made to elucidate the elementary mechanisms controlling the interaction of deformation dislocations (especially screw dislocations) with the stress induced interfaces. Finally, the role of these fundamental mechanisms in the remarkable mechanical properties exhibited by the Ti alloy used in the present study is discussed.
[1] M. Marteleur et al., Scripta Materialia, Vol 66, Issue 10, 2012, pp. 749-752
[2] M. Abdel-Hady et al., Scripta Materialia. Vol 55, Issue 5, 2006, pp. 477-480
[3] F. Sun et al., Acta Materialia, Vol 61, Issue 17, 2013, pp. 6406-6417
Fig. 1: Figure 1: (a) Tensile true strain/true stress curve is shown by the black line. The strain-hardening rate, dσ/dε, is plotted in black circles and the smoothed curve is shown in red. (b) EBSD map showing {332} <113> twins (orange) in β matrix (blue) in sample deformed at ε = 0.05. (c) EBSD map showing two variants of α” phase (blue and green). |
Fig. 2: Figure 2: HRTEM images taken along [110] in sample deformed at ε = 0.007, showing (a) {112} <111> mechanical twin lamella. Note the high strain field at the tip of the twin due to the presence of twinning partial dislocations stopping in the β matrix and (b) stress-induced ω phase nucleating on the (211) plane of the twin lamella. |