On the Influence of Thermal History on the Martensitic Transformation in Ti-24Nb-4Zr-8Sn (wt%)

Abstract

Metastable β titanium alloys, such as Ti-24Nb-4Zr-8Sn, have potential for application in vibration damping systems due to their ability to display superelastic behaviour. However, their use is currently limited due to large variations in the temperature range over which this behaviour is stable, which can additionally be shown to depend on the thermal history of the sample. This study demonstrates the sensitivity of the microstructure to thermal history and highlights a possible cause for this variability. Currently held theories of ω formation supressing the martensitic transformation have been called into question and an alternative mechanism based on a total stress approach has been suggested. Understanding this variability enables better design of alloys and processing routes in order to achieve materials with the desired properties required for industrial application.

Understanding the Room Temperature Recovery Behavior in a Ti–24Nb–4Zr–8Sn Superelastic Alloy

“…The ω iso has been shown to form precipitates of a similar size to the initial ω ath in Ti–2448, and hence any formation of ω should be detectable as a change in an ω reflection peak profile. [ 10 ] However, there was no evidence of a change in the {002} ω peak profile, indicating that the volume fraction of ω phase was unchanged during the RT hold, meaning this cannot be responsible for the changes in mechanical properties in this case. Furthermore, no extra reflections were seen after aging, indicating that there has not been formation of any other precipitates or phases during the age.…”

“…This phase is often observed at RT following the solution heat treatment of metastable β alloys, with TEM studies in the literature showing that following air cooling of Ti-2448, it is the compositionally indistinct athermal variant (ω ath ) that forms. [10,30] The microstructure for the initial condition is shown in Figure 1b. Equiaxed grains of a single phase were observed, with the SXRD pattern indicating this phase as β.…”

“…In the literature studies, the samples were quenched post heat treatment, which may introduce these internal stresses into the material. [ 10 ] This would have a similar effect as mechanical cycling, with a lower value of σ SIM and hysteresis area relative to an air‐cooled sample. [ 10 ] The subsequent relaxation of the internal stresses, by the movement of interstitials, could contribute to an increase in σ SIM and hysteresis area, with any rise in σ SIM exacerbated by the formation of ω iso .…”

“…[2][3][4][5][6][7][8] The α 00 martensite is formed on loading above a critical applied stress, σ SIM , which is acutely sensitive to the alloy composition, and processing history. [9][10][11] During repeated loading, the magnitude of σ SIM decreases, with the most dramatic changes observed within the first few cycles. Most studies agree that the transformation from β to α 00 results in the generation of dislocations that form to accommodate the irrational β/α 00 phase boundary.…”

Effect of Cooling and Annealing Conditions on the Microstructure, Mechanical and Superelastic Behavior of a Rotary Forged Ti–18Zr–15Nb (at. %) Bar Stock for Spinal Implants

“…It must be noted that, after all the TMT sequences, the alloy must be cooled down and the cooling rates may differ significantly, such as for the Ti–Nb alloy in [ 50 ], wherein the experimentally measured cooling rates in the 700 to 200 °C temperature range varied from 4–1 °C/s for air cooling (AC) to 350–100 °C/s for water quenching (WQ). In [ 51 , 52 ], the effect of the cooling rate on the structure, texture and functional properties of Ti–24Nb–4Zr–8Sn (wt. %) alloy was investigated.…”