The Influence of Twinning on the Strain–Hardenability in TRIP/TWIP Titanium Alloys: Role of Solute–Solution Strengthening

Abstract

Transformation Induced Plasticity and Twinning Induced Plasticity (TWIP) titanium alloys are well-known to display a good combination of strain-hardenability and ductility. However, a large range of strain-hardening rates, which cannot be predicted by the actual design method based on electronic parameters, is obtained. In order to explain this wide range of properties, two different alloys displaying a large difference of strain-hardening rates, but similar chemical stability, have been studied and compared: Ti-12Mo and Ti-8.5Cr-1.5Sn (in wt%). Evolution of both twin size and density during in situ tensile tests has been followed by SEM/electron backscatter diffraction mapping, and two distinct behaviors can be highlighted: the growth of existing twins (Ti-12Mo) and the nucleation of new twins (Ti-8.5Cr-1.5Sn) upon loading. The last one may lead to an improvement of the dynamic Hall-Petch effect by multiplication of twin/matrix interfaces, with subsequent improvement of the macroscopic strain-hardening. It is thought that this competition may be related to the crystal lattice distortion induced by the alloying elements and the subsequent reduction of the migration velocity of the twin/matrix interfaces. Impact Statement: This work reports on distinct behaviors of mechanical twins in TRIP/TWIP titanium alloys, highlighting for the first time a competition between growth of existing twins and nucleation of additional twins upon loading. This effect is assumed to be due to the solute-strengthening effect in the studied alloys and modify, as a consequence, the strain-hardenability of the TRIP/TWIP alloys.

Accommodation mechanisms in strain-transformable titanium alloys

“…Overall, the evolution of the deformed microstructure in the case of the TCAF alloy shows the progressive development of a very dense network of {332}<113> twins as the primary deformation mechanism (Figure 2, Figure 3 and Figure 5). This may explain, as already observed in previous reports [25,[34][35][36], the high strain-hardening rate observed in this alloy.…”

“…As the deformation proceeds, twins multiply in the grains, yet the deformation remains heterogeneous. The twin multiplication during the deformation is believed to explain the high and stable strain-hardening rate of the alloy, as proposed by the dynamic Hall-Petch mechanism [32][33][34] and further investigated in another study [35].…”

Powder Metallurgy Processing and Mechanical Properties of Controlled Ti-24Nb-4Zr-8Sn Heterogeneous Microstructures

“…The chemical composition of the alloy, and most particularly the oxygen content, is indeed of primary importance on the stress-induced β to α" phase transformation [53]. Grain sizes also influence the appearance of phase transformation and mechanical twinning in the deformation sequence of β-metastable alloys [54,55]. Finally, precipitation of α and/or ω phases also significantly influences the mechanical behavior of Ti-2448 [43,56].…”

A Review of In Situ Observations of Deformation‐Induced β ↔ α″ Martensite Transformations in Metastable β Ti Alloys

“…The behavior of α″ martensite on load removal was studied in Ti–27Nb, [ 57 ] Ti–24Nb–4Zr–8Sn, [ 59 ] Ti–10V–4Cr–1Al, [ 72 ] Ti5Al–5Mo–5V–3Cr, [ 63 ] Ti–12Mo–3Zr, [ 37 ] Ti–10V–2Fe–3Al, [ 34 ] and Ti–Mo [ 41,80 ] alloys.…”

“…[ 37 ] Qian et al [ 33 ] also suggested that de‐twinning processes on unloading could take place by {130}<10> α″ twins → {110}<10 > α″ twins → α″ martensite. In contrast, when Ti alloys were strained to a higher level (for example, 15% in Ti–12Mo), no reversion of α″ martensite on unloading was reported in Ti‐1023 [ 34 ] and Ti‐12Mo [ 80 ] alloys. It could be speculated that in alloys with superelastic behavior and a less stable β phase, the reversion process occurs with ease even when the fully α″ martensite microstructure is formed, but the limit of strain accumulation is not reached.…”