Strain-hardenability of new strengthened TRIP/TWIP tanium alloys

A new Ti-Cr based alloy has been developed to reach a TWIP (TWinning Induced Plascity) eﬀect as the main deformaon mechanism. This new composion, involving Fe addion, was derived from a classical TRIP/TWIP alloy Ti-8.5Cr-1.5Al (wt%) (TCA). The main objecve is to achieve an opmized strength/hardenability combinaon by liming the TRIP (TRansformaon Induced Plascity) eﬀect whose crical resolved shear stress lowers the plascity threshold. This new alloy Ti-7Cr-1Al-xFe (wt%) (TCAF) displays excellent mechanical properes, with an increased yield strength (with respect to TCA alloy), a very high work-hardening rate and an extremely high fracture strength (UTS=1415MPa), while maintaining an excellent duclity (ε=0.38 at fracture). Both mechanical (tensile tests) and microstructural characterizaon at diﬀerent scales (EBSD, XRD) have been performed, evidencing a dense network of ﬁne {332}<113> mechanical twins as well as the presence of stress-induced martensite plates at twins intersecons, as a secondary mechanism. load cell and an external extensometer with a gauge length of 10mm, at a strain rate of 1.7 x 10 -3 s -1 . X-ray diﬀracon measurements were collected on fractured specimens, using a Bruker Endeavor D8 X-ray diﬀractometer mounted in the Bragg-Brentano conﬁguraon with a Co anode (0.6 mm x 10.5 mm line focus, 35 kV, 28 mA). A TCAF 5% deformed sample was electropolished using a soluon of 2-butoxyethanol (C 6 H 14 O 2 ), methanol (CH 3 OH), perchloric acid (HClO 4 ) and hydrochloric acid (HCl), held at 278 Then, electron backsca�ered diﬀracon (EBSD) was performed on the TCAF deformed specimen, using a Bruker Crystalign onto a Zeiss SUPRA40 ﬁeld emission gun scanning microscope at with a step size of 94 diﬀracted intensies for TCAF, which conﬁrms the hypothesis made when looking at the shape of the tensile curves, namely that the absence of the plateau indicates that the TRIP eﬀect is not the main deformaon mechanism for this alloy. These results also show that martensite transformaon is not taking place in large proporons at later stages during the deformaon of TCAF. If there is any strain-induced martensite in this alloy, the TRIP eﬀect remains a minor deformaon mechanism. It can also be noted that the (200) β peak, supposed to be at 66°, is not visible, probably because of the sample texture before and introduced during deformaon. only 590MPa for TCA (expressed in true stress). The absence of TRIP eﬀect as a primary deformaon mechanism is clearly seen on the TCAF work-hardening curve with the suppression of the ﬁrst stage, compared to the TCA curve. In this alloy, α’’, supposed to be the so�er phase, is believed to act as an accommodaon mechanism and helps to keep a good duclity, while the dense network of thin {332}<113> twins produces a high work-hardening rate thanks to a dynamic Hall-Petch eﬀect.

new area of applica ons for tanium alloys in aerospace, for instance for components with increased capability to endure damage. Over the last few years, several new TRIP/TWIP tanium alloys have been developed and reported in the literature [2][3][4][5][6][7][8][9][10][11][12]. However, their yield stress generally remains rather low (typically below 600MPa) [2][3][4][5][6]16]. As frequently reported before, the maximum stress required to trigger the martensi c transforma on (orthorhombic martensite) is limited [17]. As a consequence, our approach consists in limi ng the TRIP effect to improve the yield strength while preserving both strain-hardening rate and uniform duc lity. A new alloy, with the following composi on Ti-7Cr-1Al-xFe (TCAF)* (wt%), is thus derived from this design strategy, using Fe as a strong β stabilizer to delay the TRIP effect. In addi on, Fe is well-known to be an efficient solute strengthening element in Ti-alloys, which can help to further improve the yield strength. The suggested composi on is then compared in what follows with a reference alloy whose composi on is Ti-8.5Cr-1.5Al (TCA) (wt%) to evaluate only the influence of Fe addi on. TCA, reported in our previous studies, is known to be a conven onal TRIP/TWIP tanium alloy [18].

Material and experiments
Lab-scale melted TCAF and TCA bu�ons (200g) were prepared using a tungsten arc-mel ng furnace under high-purity Ar atmosphere. Ingots were solu on-treated at 1173 K for 30 minutes and then water quenched. Specimens were then cold rolled to manufacture sheets, 0.65mm thick, corresponding to a thickness reduc on of 80%. Cold-rolled sheets were re-heat treated in the b domain at 1083 K under high purity Ar atmosphere and then water quenched. Thus, the specimens exhibited a fully β recrystallized microstructure with an average grain size of 100μm. Tensile specimens are then extracted from the metal-sheets with gauge dimensions of 50 x 5 x 0.65 mm 3 . Uniaxial tensile tests were carried out at room temperature using an INSTRON5966 machine with a 10kN load cell and an external extensometer with a gauge length of 10mm, at a strain rate of 1.7 x 10 -3 s -1 . X-ray diffrac on measurements were collected on fractured specimens, using a Bruker Endeavor D8 X-ray diffractometer mounted in the Bragg-Brentano configura on with a Co anode (0.6 mm x 10.5 mm line focus, 35 kV, 28 mA). A TCAF 5% deformed sample was electropolished using a solu on of 2-butoxyethanol (C 6 H 14 O 2 ), methanol (CH 3 OH), perchloric acid (HClO 4 ) and hydrochloric acid (HCl), held at 278 K. Then, electron backsca�ered diffrac on (EBSD) was performed on the TCAF deformed specimen, using a Bruker Crystalign system mounted onto a Zeiss SUPRA40 field emission gun scanning electron microscope opera ng at 20kV, with a step size of 94 nm.

Results and discussion
Tensile true stress-true strain curves and the associated work-hardening rates as a func on of strain of both TCA and TCAF are presented in  Table 1. TCA alloy exhibits typical mechanical proper es of TRIP/TWIP tanium alloys commonly found in the literature [2][3][4] with a combina on of high strength (UTS=1100MPa) and high duc lity (uniform deforma on of 0.42) resul ng from a large strain-hardening. TCA stress-strain and work-hardening rate curves show a nonmonotonous evolu on, involving a typical plateau, due to the ac va on of different deforma on mechanisms, as classically observed in materials with mul ple plas city phenomena [19][20][21]. The work-hardening rate curve can be split into two domains. In the first stage, from the yield strength to the hump vertex (ε≈0.15), the work-hardening rate increases significantly and in the second stage, from the vertex to 2 MATEC Web of Conferences 321, 11056 (2020) https://doi.org/10.1051/matecconf/202032111056 The 14 th World Conference on Titanium the fracture, it slowly decreases. The shape of the curve is characteris c of TRIP/TWIP tanium alloys, where the first stage corresponds to a combina on of strain-induced α'' precipita on, nuclea on/growth of twins and disloca on glide. The end of the plateau, corresponding to the vertex on the work-hardening curve, is considered to be the signature of the beginning of plas c deforma on of the martensite created during the first stage [22][23][24]. Lilensten et al. followed the deforma on of such an alloy with SXRD (Synchrotron X-Ray diffrac on) and, showed that the volume frac on of martensite strongly increases during the first stage and then exhibits a slight increase during the second stage. Thus it is assumed that the martensite is mainly formed during the first stage and its volume frac on does not change significantly in the following stage [6]. The newly designed alloy, namely TCAF, exhibits excellent mechanical proper es with an increase of the yield strength (YS=650MPa) of 80 MPa compared to TCA with same grain size for both alloys, a very high ul mate tensile strength (UTS=1415MPa) and a high work-hardening rate. The improvement of these mechanical proper es does not lead to a large loss of duc lity (0.38 in uniform deforma on). In addi on to 3 MATEC Web of Conferences 321, 11056 (2020) https://doi.org/10.1051/matecconf/202032111056 The 14 th World Conference on Titanium the substan al yield strength increase, the main feature of this alloy appears to be its remarkable high strain-hardening. The difference between the ul mate tensile strength and the yield strength (expressed in true stress) for the TCAF is about 765 MPa compared to 590 MPa for the TCA. This value is one of the highest found in the literature in comparison with other β-metastable tanium alloys [2,3,6]. It is worth no ng that the TCAF tensile curve shape significantly differs from that of the TCA alloy. Work-hardening rate as a func on of strain shows a monotonous decrease, similar to the second stage of usual TRIP/TWIP tanium alloys curves as presented previously. Nevertheless, the work-hardening rate and the stress of TCAF remains larger than, or equal to, the TCA for a given strain value. The difference of shape between TCA and TCAF tensile curves, in par cular the absence of plateau for TCAF, where the martensite is supposed to form in large amount, may indicate a difference of deforma on mechanisms for TCAF compared to usual TRIP/TWIP tanium alloys such as TCA. These tensile curves suggest that the TRIP effect is no longer the main deforma on mechanism in the TCAF, as purposely designed, leading to a yield strength improvement in comparison to TCA without too much impact on the elonga on to fracture. XRD diffractograms, shown in Fig.2, are recorded on the surface of fractured tensile specimens for both alloys. Martensite (α'') is clearly detected for TCA but seems to provide much weaker diffracted intensi es for TCAF, which confirms the hypothesis made when looking at the shape of the tensile curves, namely that the absence of the plateau indicates that the TRIP effect is not the main deforma on mechanism for this alloy. These results also show that martensite transforma on is not taking place in large propor ons at later stages during the deforma on of TCAF. If there is any strain-induced martensite in this alloy, the TRIP effect remains a minor deforma on mechanism. It can also be noted that the (200) β peak, supposed to be at 66°, is not visible, probably because of the sample texture before and introduced during deforma on.  EBSD maps of TCAF specimen deformed to a tensile strain of 0.05 are presented in Fig.3. EBSD measurements show that the strain is accommodated by a dense network of twins indexed as {332}<113> in some of the β phase grains. It is admi�ed in the literature that this profuse twinning reduces the disloca ons mean free path, leading to a dynamic Hall-Petch effect, which can account for the high workhardening of TRIP/TWIP tanium alloys [25].
EBSD phase map seems to exhibit no bands of α'' in the β-matrix similar to the martensite needles of TRIP alloys but shows the presence of α'' within the twins, and especially at the intersec on of twin variants. Therefore, in this alloy, α'' martensite, hardly visible on the XRD diffractogram because of its small size and low volume frac on, is not observed as a primary deforma on mechanism but is revealed by EBSD as a secondary one. These observa ons confirm the hypothesis made from the tensile curves, namely, in TCAF, α'' appears only as a secondary deforma on mechanism with a very limited volume frac on, typically like in the second stage of usual TRIP/TWIP tanium alloys.  The addi on of Fe in the TCAF seems to have stabilized the β phase, leading to an increase of the cri cal shear stress required to form α'' and thus explains the delay of the TRIP effect. α'' appears as a secondary deforma on mechanism and does not limit the yield strength improvement. Lai et al. showed that α'' may be the so� phase in these materials, where α'' plates can be cut through by disloca on moving and twin propaga on [26]. Regarding the previous conclusion, martensite transforma on appears to be an accommoda on mechanism taking place at high stress concentra on loca ons, which can contribute to explain the high duc lity of TCAF combined with a very high stress level at fracture. These first results open new promising outcomes in the design strategy of TRIP/TWIP tanium alloys, involving fine tuning of mechanical twins as the main strengthening effect and martensi c transforma on as an accommoda on mechanism. A new solid-solu on strengthened TRIP/TWIP tanium alloy, Ti-7Cr-1Al-xFe, is proposed and the rela onship between mechanical proper es and deforma on mechanisms is highlighted with reference to a conven onal TRIP/TWIP alloy Ti-8.5Cr-1.5Al (TCA).

Conclusion
By stabilizing the β phase with Fe addi on, it appears that the TRIP effect is triggered only as a secondary deforma on mechanism.
Combined with its strengthening behavior, the addi on of Fe leads to an improvement of the yield strength by 80MPa and a higher workhardening with a 765MPa difference between the yield strength and the ul mate tensile strength, compared to only 590MPa for TCA (expressed in true stress). The absence of TRIP effect as a primary deforma on mechanism is clearly seen on the TCAF work-hardening curve with the suppression of the first stage, compared to the TCA curve. In this alloy, α'', supposed to be the so�er phase, is believed to act as an accommoda on mechanism and helps to keep a good duc lity, while the dense network of thin {332}<113> twins produces a high workhardening rate thanks to a dynamic Hall-Petch effect.