On the hardenability of Nb-modified metastable beta Ti-5553 alloy

Among the commercially available titanium alloys, the metastable beta Ti-5553 alloy (Ti-5Al-5V-5Mo-3Cr-0.5Fe wt.%) is an object of great interest because it is employed in aerospace structural applications, primarily in the replacement of steel components. One of the primary advantages of this alloy is its high hardenability, which allows it to retain the beta phase at room temperature, even at low cooling rates, thereby allowing the thermoprocessing of thick parts. The aim of this investigation was to evaluate the effect of the replacement of V with Nb on the hardenability of Ti-5553. Based on the molybdenum equivalent criterion, the Nb-modified Ti-5553 alloy was designed to present 12 wt.% of Nb instead of 5 wt.% of V. Samples of both alloys were prepared by melting them in an arc furnace under an inert atmosphere, heat-treated at high temperatures for 12 h and plastic deformed using swage forging. Finally, these samples were solution heat-treated at temperatures above the beta-transus followed by cooling at different rates using water quenching, furnace cooling and a modified Jominy end quench test. Characterization was performed by measuring Vickers hardness, X-ray diffraction, and light optical, scanning electron and transmission electron microscopy. The results obtained indicate that metastable beta alpha phase can be retained when the cooling rate is higher than 21 degrees C/s for both alloys. At lower cooling rates, a phase precipitation was observed, but it appeared to be less evident in the Nb-modified Ti-5553, suggesting that the replacement of V with Nb increased the hardenability of the alloy. (c) 2016 Elsevier B.V. All rights reserved.