The Impact of Strain Reversal on Microstructure Evolution and Orientation Relationships in Ti-6Al-4V with an Initial Alpha Colony Microstructure

Abstract

The effect of forward and reverse torsion on flow behavior and microstructure evolution, particularly dynamic and static spheroidization, on Ti-6Al-4V with an alpha lamella colony microstructure was studied. Testing was undertaken sub beta transus [1088 K (815°C)] at strain rates of either 0.05 or 0.5 s À1 . Quantitative metallography and electron back scatter diffraction has identified that a critical monotonic strain (e c ) in the range of 0.3 to 0.6 is required to initiate rapid dynamic spheroidization of the alpha lamella. For material deformed to strains below e c and then reversed to a zero net strain the orientation relationships between alpha colonies are close to ideal Burgers, enabling prior beta grains to be fully reconstructed. Material deformed to strains greater than e c and reversed lose Burgers and no beta reconstruction is possible, suggesting e c is the strain required to generate break-up of lamella. Static spheroidization is, however, sensitive to strain path around e c . Annealing at 1088 K (815°C) for 4 hours for material subjected to 0.25 forward + 0.25 forward strain produces 48 pct spheroidized grains while material with 0.25 forward + 0.25 reverse strain has 10 pct spheroidization. This is believed to be a direct consequence of different levels of the stored energy between these two strain paths.

Digital/virtual microstructures in application to metals engineering – A review

“…Interaction between different phases, inclusions or precipitations directly influence microstructure response to processing and exploitation conditions [4][5][6][7] and should not be described by a single homogenized flow stress model. -Development of micro forming technologies, where sample is no longer an aggregate of billions of microstructural features but contains significantly limited number e.g.…”

An Overview of the Thermomechanical Processing of α/β Titanium Alloys: Current Status and Future Research Opportunities

“…Poths, Nicolaou, Muszka, and their coworkers [131,141,142] found a similar behavior during monotonic and reversed torsion testing of Ti64 with a colony-a microstructure at 1088 K (815°C). In the work of Poths et al, [141] for example, the rate of dynamic spheroidization during reversed torsion was approximately one-half of that during monotonic (''forward'') torsion ( Figure 18).…”

“…[124,130] A similar result was found in research involving reversed torsion of Ti64 with a colony-a microstructure at 1088 K (815°C). [131] Here, the BOR was preserved during forward + reversed torsion when the imposed strain increment was less than the critical value required to initiate dynamic spheroidization, i.e., 0.3 to 0.6.…”

“…A number of investigations of hot (and warm) working of a/b (and b) titanium alloys with a lamellar microstructure have verified the occurrence of the boundary-splitting mechanism and sought to quantify the evolution of sub-boundaries and the a/b interface energy. [76,[122][123][124][125][126][127][128][129][130][131] For example, EBSD measurements have shown that the formation of sub-boundaries in a lamellae increases with strain. [76,[123][124][125] In particular, sub-boundary misorientations of the order of 5°are developed after true strains of~0.30 and increase to~15°( or greater) by strains of~1.1 -1.3 in Ti64.…”

In‐Situ High Temperature EBSD Analysis of the Effect of a Deformation Step on the Alpha to Beta Transition in Additive Manufactured Ti‐6Al‐4V

“…The addition of a rolling step to Wire Arc Additive Manufacture (WAAM) has also been found to greatly reduce the prior grain size and cause a large reduction in texture strength, leading to improved mechanical isotropy [8,9] and this will be discussed in a future publication. Here, the prime interest is in explaining the high level of refinement found when only relatively small plastic strains are applied in the WAAM rolling process, which being of the order of 10% are much lower than would be expected to cause conventional recrystallization in a Ti-6Al-4V alloy [15].…”