The dynamic response of a β titanium alloy to high strain rates and elevated temperatures

Abstract

The stress-strain behaviour and microstructural evolution of the Ti-6Cr-5Mo-5V-4Al (Ti6554) alloy was systematically investigated using Split Hopkinson Pressure Bar (SHPB) tests over a wide range of strain rates from 1000 s -1 to 10000 s -1 and initial temperatures from 293K to 1173K. Dislocation slip is the main deformation mechanism for plastic flow of the Ti6554 alloy at high strain rates. The flow stress increases with increasing strain rate and decreasing temperature. Also the flow stress is more sensitive to temperature than to strain rate. For high strain rate deformations, the strain hardening rate is found to be negative at 293K and increases with increasing temperatures. Flow softening observed at 293K is potentially caused by adiabatic heating. The increment in the strain hardening rate with increasing temperatures may be the result of interactions between thermally activated solute Cr atoms and mobile dislocations. When the temperature is raised to 873K, a novel Į precipitate 2 morphology consisting of globular Į aligned in strings was observed in specimens deformed at strain rates of 4000 and 10000 s -1 . It has hardening effects on the ȕ matrix and is purported to nucleate on dislocations introduced by the high strain rate deformation. Adiabatic shear bands were observed in specimens deformed at higher temperatures (873K). The microstructure inside the shear bands is harder than that outside of the shear bands in the Ti6554 alloy.

The dynamic response of β titanium alloys to high strain rates at room and elevated temperatures

“…Furthermore, flow stress of the Ti6554 alloy is more sensitive to temperature than strain rate. Flow curves at 293K all exhibit a negative strain hardening rate while the strain hardening rate will increase to become positive with increasing temperature, which has been attributed to dynamic strain aging (DSA) caused by the activation of solute Cr atoms in our previous study [30]. The flow curve from the test at 1173K and 4000 s -1 in Fig.3 (b) reveals a small peak at a strain of 0.125 followed by a gradual drop of stress towards a plateau, which is the main characteristic of dynamic recrystallization (DRX).…”

“…When the incident pulse reaches the interface between the input bar and the specimen, part of the pulse (reflected pulse) will be reflected back to the Input bar while the rest (transmitted pulse) will be transmitted to the output bar through the specimen. These pulses will be recorded by the strain gages mounted on the bars [30]. The flow stress σ, strain ε and strain rate ̇are calculated using equations as follow: …”

“…Other similar β titanium alloys such as Ti-7Mo-3Nb-3Cr-3Al (Ti7333) [27] and Ti-6Cr-5Mo-5V-4Al (Ti6554) [28,29], with ultrahigh strength and excellent ductility, have also been developed recently for aerospace applications. As the volume fraction, distribution and morphology of α phase precipitates have great effects on the mechanical properties of β titanium alloys [30], a large number of experiments have been conducted on the effects of heat treatment on the microstructures and tensile…”

“…Instead of using conventional screw-driven or servo-hydraulic methods, SHPB has been used extensively to test the dynamic stress-strain behaviour of materials at high strain rates ranging from 10 2 to 10 4 s -1 [30]. The data obtained by SHPB can be used to fit constitutive models.…”

“…The most crucial limitation is that the maximum strain rate reached by these machines is quite low. The highest strain rate for testing in servo-hydraulic machines reported in the past literature was only 100s Instead of using conventional screw-driven or servo-hydraulic methods, SHPB has been used extensively to test the dynamic stress-strain behaviour of materials at high strain rates ranging from 10 2 to 10 4 s -1 [30]. The data obtained by SHPB can be used to fit constitutive models.…”