Effects of strain rate on tensile deformation behaviour in Ti-6Al-4V at cryogenic temperature

In this study, we inves gated the effects of strain rate on tensile deforma on behaviour in Ti-6Al-4V sheet at cryogenic temperature. X-ray diffrac on (XRD) was used to iden fy the crystallographic orienta on of rolled Ti-6Al-4V. A series of tensile tests were performed by constant strain rate method (CRS) with variable strain rates (i.e., on the order of 1x10-2 to 10-4·s-1). Liquid nitrogen (LN2) was used to mimic cryogenic environment, and for the thermal equilibrium the specimens were immersed in the vessel containing liquid nitrogen for ~10 minutes before tensile tes ng, and the temperature condi on was con nuously maintained during the tes ng. Microstructure and fracture surface was analysed by polarised light microscopy and scanning electron microscope (SEM). Electron backsca�er diffrac on (EBSD) was further used to characterise local deforma on behaviour. Deforma on twinning is occurred at cryogenic tempearture, which is rather different to the deforma on at room temperature. It is thought that the twinning induced deforma on behaviour may lead to a strength enhancement and a rate dependent duc lity improvement.


Introduc on
Titanium alloys, par cularly α + β alloys like Ti-6Al-4V (hereina�er referred to as Ti64), have been used in machining industry owing to their promising mechanical proper es. However machining using Ti64 has two problema c issues which are short tool life and rapid wear, even though processing condi ons are moderate. Previous works show that cryogenic condi on using liquid nitrogen as a coolant has been proposed to improve the tool life, where the tool life could be extended up to ~five mes [1,2], though detailed mechanisms associated with this phenomenon were not well studied. Fundamental deforma on behaviour of Ti64 on extreme environment such as cryogenic temperature have been therefore issued. It has been known that disloca on plas city is dominant deforma on mechanisms at room and/or elevated temperature and twinning is typically difficult to be found in Ti64 at room temperature [3,4]. At cryogenic temperature, however, twinning could be observed and mechanical proper es are significantly different compared to those at room temperature [5]. Twinning occurred from primary (about 3%) to secondary twinning (about 20%) depending on local plas c strains. Because a number of slip system are limited in hexagonal close packed (HCP) metal, twinning has been considered as an essen al deforma on process [6,7].
In this study, we performed a series of tensile tests of Ti64 sheet with variable strain rates at cryogenic temperature and inves gated the effects of strain rate on tensile deforma on behaviour through the analysis of stress-strain curve, fractography and microstructural features.

Material and experiments
Tensile specimens with gauge length of 25mm (ASTM 8M) were sec oned from rolled Ti64 sheet with thickness of 0.9mm, and extended along the parallel (RD) and perpendicular (TD) to the rolling direc ons (see Fig. 1). Crystallographic texture of Ti64 (see Fig. 2a) was analysed by a PANaly cal X-ray diffrac on (XRD). Table 1 shows tensile proper es with respect to loading direc ons tested at room temperature. Since RD and TD results were almost similar in the tensile test, only TD was considered in this study.   The tensile test was performed by constant strain rate method un l fracture at 77 and 293K. Strain rates of 1x10 -2 , 10 -3 and 10 -4 ·s -1 were selected and liquid nitrogen (LN 2 ) was used to mimic cryogenic environment. For the thermal equilibrium the specimen was immersed in the vessel containing liquid nitrogen (see Fig. 2b) for about 10 minutes before tensile tes ng, and the temperature condi on was con nuously maintained during the tes ng. Note that all the specimens were fractured at the gauge length. The cross-sec ons located ~2mm below the fracture surface were ground with a series of SiC papers (up to 4000 grit) and subsequently polished with OP-S (Oxide Polishing Suspensions) diluted with H 2 O by a ra o 5:1 of OP-S : H 2 O. The specimens were then etched using 2% Kroll's reagent for ~20 seconds, and a Carl Zeiss op cal microscope (AXIO Scope A1) with polarised light was used to iden fy the microstructal features. Fracture surface was carefully observed by JEOL scanning electron microscope (SEM) and electron backsca�er diffrac on (EBSD) was further used to characterise local microstructure and deforma on features, with an accelera ng voltage of 15kV, a probe current of 4.32nA and a step size of 0.25μm. determined by 0.2% proof stress and elonga on (ε f ) are significantly different between 77 and 293K. Yield strength is increased approximately 915MPa to 1401MPa but elonga on decreased 17 to 10 % in the strain rate of 10 -2 ·s -1 as temperature altered from 293 to 77K. Note that similar results can be found in the previous studies such that strength and hardness of Ti64 are increased while elonga on is decreased at cryogenic temperature [8,9]. Fig. 3b shows yield strength and elonga on distribu ons plo�ed with respect to variable strain rates. σ Y is increased as strain rate is increased, though elonga on shows opposite trend at both temperatures. An interes ng point here is that a duc lity could be somewhat improved with controlling strain rates (i.e., lower strain rate leads to higher elonga on) even at cryogenic temperature. This observa on could be linked to the work by Hong et al. [1], who inves gated the rela onship between the tool life of Ti64 and the cu ng speed under machining process with liquid nitrogen cooling. They showed that the cu ng speed (i.e., 1, 1.5, 2, 2.5 m/s) and the associated tool life (i.e., approximately 900, 300, 200, 50 seconds) have an inverse correla on in one nozzle method. It can be thought that Ti64 clearly possesses rate sensi ve material proper es at cryogenic temperature, and these proper es should be considered when developing machining processes with cryogenic cooling.

Fig. 3. (a) Engineering stress-strain curves at room and cryogenic temperature and (b) distributions of yield strength & elongation plotted with respect to
variable strain rates. Fig. 4 shows fracture surface of Ti64 tested specimens. There are two types of fracture on fracture surface of Ti64, one is a normal failure (denoted by N) and the other is a shear failure (denoted by S). Normal and shear failure is oriented at the angle of 90° and 45°, respec vely, with respect to the tension axis. Failure surfaces oriented at the angle of 45 ° to tension axis have the dimple pa�ern that is elongated in the direc on of the shear crack propaga on (Fig. 4b), while surfaces of a normal failure have no elonga on of a dimple pa�ern (Fig. 4a) [10]. Fig. 5 shows fracture surface at 293 and 77K with variable strain rates. Normal and shear failures are present in all the fracture surface except the case for the strain rate of 10 -4 ·s -1 at 293K. At room temperature normal fracture width is increased as the strain rate gradually decreased and occurred as a whole in the case of 10 -However in cryogenic temperature, normal fracture width and ra o are decreased while elonga on is increased. This is rather different in a way that normal failure is more dominant at 293K and shear failure is dominant at 77K in the case of 10 -4 ·s -1 strain rate. Therefore, there is another apect to explain the phenomenon that lower strain rate leads to higher elonga on.   5. Fracture surface at (a), (b), (c) 293K and (d), (e), (f) 77K with strain rates of 1x10 -2 , 10 -3 and 10 -4 ·s -1 , respec vely. Fig. 6 shows the cross-sec onal area of tensile specimens below 2mm from fracture point observed using a combina on of differenial interference contrast (DIC) microscope and EBSD measurement. Plas c deforma on is fully accomodated by slip ac vi es at 293K, but deforma on twinning is observed at 77K. It is known that the cri cal resolved shear stress (CRSS) for slip ac va on is rapidly increased as the deforma on temperature is decreased, but the CRSS for twinning ac va on is rela vely independent on the deforma on temperature [12,13]. Twinning is observed regardless of the applied strain rate, though there are different trends according to loca ons on cross-sec on in Fig. 6. In higher strain rate twinning is mostly observed at near edge, while its distribu on becomes much wider as the strain rate is lowered. Fig. 7 indicates twinning area frac on calculated with respect to variable strain rates. The higher strain rate, the bigger difference the degree of twinning occurred between edge and centre zone. The occurrence of twinning and shear failure has similar tendency where both increases in the case of 10 -4 ·s -1 strain rate. Shear failure is composed of many dimples and tearing grains. At low temperatures, duc lity is improved as the amount of dimples and tearing grain increases [14]. From this point of view, as twinning occurs over a wide range, shear failure also occurs as a whole, resul ng in increasing elonga on in case of 10 -4 ·s -1 strain rate.

Conclusion
In the present study a series of tensile tests was performed at cryogenic temperature and the associated deforma on mechanisms were analysed.
The main results can be summarised as follows.
1. Tensile proper es such as yield strength and elonga on are different at 77K compared to 293K. Yield strength and elas c range have significantly increased but elonga on decreased at 77K. However, the tendency that lower strain rate leads to higher elonga on is rather similar at both 293K and 77K.