Strong and ductile beta Ti–18Zr–13Mo alloy with multimodal twinning

Abstract

Body-centred cubic (BCC) Ti-18Zr-13Mo (wt%) alloy displays excellent yield strength (≈ 800 MPa), stable hardening (rate > 1500 MPa) and uniform ductility > 18%, resulting from multi-TWIP (multiple twinning-induced plasticity) strengthening effect. This multimodal mechanisms include microscale {332} < 113 > deformation twinning (DT), nano-scale {112} < 111 > DT and a rare {5 8 11} < 135 > DT mode. Martensitic phase transformation is completely suppressed and the sample stays a single-phase solid solution throughout the deformation. In situ electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the multi-TWIP and dynamic Hall-Patch effect, with dislocation slip and large grain distortions at twin interfaces. IMPACT STATEMENT The addition of Zr solutes stabilizes β Ti-alloy and completely suppresses phase transformations, allowing {332} < 113 > , {112} < 111 > and {5 8 11} < 135 > multimodal twinning to give a better combination of strength, strain-hardening rate and ductility.

Hierarchical {332}<113> twinning in a metastable β Ti-alloy showing tolerance to strain localization

“…As observed in the deformed samples, traces of all three plane families can be seen when measuring the interface orientation of the slip bands, probably suggesting a cross-slip activity (lines marked in Figure 3(a)). Similar phenomena have been reported in reference [4]. Likewise, note that the bright-field contrast of the intersection interface between matrix and secondary 332 T was also observed in Figure 3(a), which could be regarded as cross-slip bands based on the slip trace analysis [14].…”

“…The yield stress and ultimate tensile stress are about 730 and 1090 MPa, respectively, which represent much higher values than the Ti-12Mo strength reported elsewhere [1]. This can be attributed to the full suppression of SIMT and the solution strengthening effect provided by the zirconium addition [4]. A series of frames of the DIC video shows the 2D full-field strain maps at different strain levels (Figure 1(a)).…”

“…It is assumed that the stability of solution-treated Ti-12Mo-10Zr should be situated between Ti-12Mo and Ti-13Mo-18Zr, because the 1st deformation mechanism of Ti-12Mo is TRIP + TWIP 332T [1,20] and the Ti-13Mo-18Zr is TWIP 332T+112T mode [4]. Similarly in the selection of 2 nd /3 rd accommodation mechanisms, the Ti-12Mo presents 2 nd martensite and 2 nd /3 rd 112T in the 1st 332T, whereas the TWIP Ti-13Mo-18Zr presents low volume fraction of 2 nd 332 twinning and the formation of a special {5 8 11} twinning [4]. Comparing to the triple 332T observed in the Ti-12Mo-10Zr, these differences might suggest that the selection of 2 nd and 3 rd accommodation mechanisms is also affected by the β chemical stability.…”

“…The hierarchical structures of primary, secondary and tertiary 332T, at multiple length scales, construct a 'composite band' [1] under the uniaxial tensile load in Ti-12Mo-10Zr. The 332T bands have been proved to contribute to the strain hardening effect in TWIP Ti-alloys, which probably results from both the dynamic reduction of the mean free path for dislocation glide by the growing twinning network [2,4] and the mechanical contrast effect of the twins/matrix [4]. The shear strain concentration starting at a strain of 0.02 until 0.32 suggests that intensive dislocation slip and 332T happened in the same volume of the sample all along the tensile deformation to fracture.…”

Microstructural Evolution and Strain-Hardening in TWIP Ti Alloys

“…Tensile stress-strain curves of TWIP Ti alloys at room temperature [23,24,26,[28][29][30]32] . The yield stress can achieve over 600 MPa with controlled grain size and solid solution hardening.…”