Martensite Formation and Decomposition during Traditional and AM Processing of Two-Phase Titanium Alloys—An Overview

Abstract

Titanium alloys have been considered as unique materials for many years. Even their microstructure and operational properties have been well known and described in details, the new technologies introduced—e.g., 3D printing—have restored the need for further research in this area. It is understood that martensitic transformation is usually applied in heat treatment of hardenable alloys (e.g., Fe alloys), but in the case of titanium alloys, it also occurs during the thermomechanical processing or advanced additive manufacturing. The paper summarizes previous knowledge on martensite formation and decomposition processes in two-phase titanium alloys. It emphasizes their important role in microstructure development during conventional and modern industrial processing.

Mechanical and microstructural characterization of titanium gr.5 parts produced by different manufacturing routes

“…Therefore, as a general consideration, the forged and machined parts are characterized by a Widmanstatten microstructure for which α and β lamellae are subsequent to the growth from the prior β grains [33]. It is worth to note that the thickness of the lamellae is visibly higher for the forged and machined parts compared to the solely machined ones: this result, as expected, is subsequent to the material heating above the β transus temperature for the forging process [34] and the slow cooling rate of the produced part. On the other hand, it can be seen from the microstructure of the forged and machined part that the original microstructure induced by the forging process was retained during the machining step, except for an appreciable grain growth that occurred.…”

Mechanical Behavior Assessment of Ti-6Al-4V ELI Alloy Produced by Laser Powder Bed Fusion

“…Due to the non-equilibrium solidifications inherent to LPBF, the metastable acicular martensitic α (hcp) phase would form after LPBF, and it was experimentally observed to remain in the ASR samples that underwent stress-relief heat treatment at 670 • C for 5 h followed by furnace cooling, as presented in Figure 7a. These metastable acicular martensitic α phases are considered hard and brittle [45,46], in comparison to the equilibrium mixture of α and β phases, and would contribute to the high strength, low ductility, and low MOT observed.…”

Heat Treatments of Metastable β Titanium Alloy Ti-24Nb-4Zr-8Sn Processed by Laser Powder Bed Fusion

“…In the as-built condition, only minor reflections of the α″ phase are observable and, in addition, some of the peaks of other phases are broadened, probably due to the overlay with the α″ phase peaks, see black arrows in Figure 2 . It is assumed, that due to insufficient undercooling during the process and/or decomposition through in situ heat treatments, the formation of α″ martensite is inhibited, suppressed, or reversed [ 34 , 70 , 71 ]. Heat treatments, including slow cooling or aging, enable the decomposition of α″ and the precipitation of the α phase [ 72 ].…”

“…In the condition of energy disturbance, such as heat treatment and deformation, the metastable bcc β phase decomposes into the hcp α phase, the hcp α′ martensite phase, the orthorhombic α″ martensite, or/and the ω phase [ 27 , 28 , 29 , 30 , 31 , 32 ]. The α′ martensite formation in titanium alloys, in general, results in increasing tensile strength and hardness accompanied by decreasing plasticity [ 33 , 34 ]. The formation of α″ martensite under rapid quenching tends to reduce hardness, tensile, and fatigue strength [ 18 , 35 , 36 , 37 , 38 ].…”