Formation of equiaxed α during ageing in a severely deformed metastable β Ti alloy

Zafari, Ahmad; Xia, Ke

doi:10.1016/j.scriptamat.2016.07.012

Cited by 27 publications

(20 citation statements)

References 24 publications

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“…in HPT deformed specimens these bands form concentric circles (not shown here) [15]. Other studies claim that these "dark bands" are in fact shear bands with apparently enhanced density of lattice defects as compared to surrounding areas [7,9,10,19]. However, EDS investigation proved that these DBs characterized by lower amount of Mo (molybdenum is β-stabilizing element) are areas with appropriate conditions for easy precipitation and diffusion-controlled growth of the α-phase.…”

Section: Discussionmentioning

confidence: 71%

Heterogeneous Precipitation of the α-Phase in Ti15Mo Alloy Subjected to High Pressure Torsion

et al. 2018

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Ti15Mo alloy is a binary metastable beta titanium alloy with excellent mechanical properties and biocompatibility. Therefore, it can potentially replace the most commonly used Ti6Al4V alloy in biomedical applications. Further processing of the alloy by the methods of severe plastic deformation provides an opportunity to achieve exceptional grain refinement and to enhance mechanical properties. Another way how to increase the strength of the alloy is thermomechanical processing which leads to the formation of the α-phase. α-phase is known to precipitate heterogeneously, in particular at grain boundaries. Refinement of parent β matrix therefore affects the size and the morphology of α precipitates. Ultrafine grained Ti15Mo alloy was processed by high pressure torsion. The precipitation behavior of ultrafine grained and benchmark coarse-grained material during isothermal ageing was investigated by ex situ scanning electron microscopy. In the coarse-grained material the α-phase precipitates both as grain boundary α-phase and as acicular α-phase inside the β-grains. On the other hand, in ultrafine grained alloy, only equiaxed sub-micrometer α particles precipitated at dense network of grain boundaries.

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Section: Discussionmentioning

confidence: 71%

Heterogeneous Precipitation of the α-Phase in Ti15Mo Alloy Subjected to High Pressure Torsion

et al. 2018

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“…At this temperature significant recrystallization and α precipitation occurred at the heavily stabilized nanocrystalline region, which is a temperature close to the estimated recrystallization temperature (~700 °C) of cold rolled VT-22 alloy 48 . Simultaneous occurrence of β recrystallization and α precipitation is possible, as proposed in the HPT processed Ti5553 alloy 34 . The increase of sample holder temperature to 833 °C (sample temperature ~733 °C) led to extensive β phase recrystallization and α precipitation in the whole material (Fig.…”

Section: Discussionmentioning

confidence: 90%

“…reported that during isothermal aging at 350 °C for 90 mins refined α precipitates formed in Ti5553 alloy 47 . Moreover, α precipitation also occurred in a HPT-processed Ti-5553 alloy during a short ageing at 600 °C for 30 s 34 .…”

Section: Discussionmentioning

confidence: 94%

“…The abundant dislocation and subgrain boundaries in the nanocrystalline regions resulted in a higher driving force for recrystallization. Moreover, these defects can act as preferential nucleation sites for α precipitation and enhanced atomic diffusivity can accelerate α nucleation and growth 31 , 33 , 34 . The heating experiments showed that nucleation of precipitates started from the heavily stabilized nanocrystalline region when the sample holder temperature reached 613 °C (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Recently, Xu et al . reported that an ultrafine equiaxed duplex (α + β) structure can be achieved in metastable β alloys via high pressure torsion (HPT)/ equal-channel angular pressing (ECAP) +400–600 °C annealing 31 – 34 . Xu et al .…”

Section: Introductionmentioning

confidence: 99%

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Segregation mediated heterogeneous structure in a metastable β titanium alloy with a superior combination of strength and ductility

Gao

Nutter

Liu

et al. 2018

Sci Rep

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In β titanium alloys, the β stabilizers segregate easily and considerable effort has been devoted to alleviate/eliminate the segregation. In this work, instead of addressing the segregation problems, the segregation was utilized to develop a novel microstructure consisting of a nanometre-grained duplex (α+β) structure and micrometre scale β phase with superior mechanical properties. An as-cast Ti-9Mo-6W alloy exhibited segregation of Mo and W at the tens of micrometre scale. This was subjected to cold rolling and flash annealing at 820 oC for 2 and 5 mins. The solidification segregation of Mo and W leads to a locally different microstructure after cold rolling (i.e., nanostructured β phase in the regions rich in Mo and W and plate-like martensite and β phase in regions relatively poor in Mo and W), which play a decisive role in the formation of the heterogeneous microstructure. Tensile tests showed that this alloy exhibited a superior combination of high yield strength (692 MPa), high tensile strength (1115 MPa), high work hardening rate and large uniform elongation (33.5%). More importantly, the new technique proposed in this work could be potentially applicable to other alloy systems with segregation problems.

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Progress in Severe Plastic Deformation of Metastable Beta Ti Alloys

Zafari

Xia

2019

Adv Eng Mater

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Metastable β Ti alloys can undergo martensitic transformation under stress. It is shown that such stress‐induced martensitic transformation (SIMT) can significantly facilitate grain refinement by severe plastic deformation (SPD), achieving β grain sizes of <50 nm, compared with >100 nm in β Ti alloys without SIMT. The martensitic α″ formed partitions the β grains and blocks dislocation movement, but undergoes reverse transformation back into β at large strains, leaving a pure β grain structure. The grain refinement can be further enhanced by increasing the strain rate to the order of 10–100 s−1, leading to β grains of <10 nm. SPD also has significant effect on the morphology of the α precipitates upon aging, transforming it from acicular to equiaxed. Although the α nucleus formed between recrystallized β grains of <≈10 nm follows the Burgers orientation relationship (BOR), the semicoherent interface is lost upon β grain growth during aging, causing its growth into equiaxed shape.

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Formation of equiaxed α during ageing in a severely deformed metastable β Ti alloy

Cited by 27 publications

References 24 publications

Heterogeneous Precipitation of the α-Phase in Ti15Mo Alloy Subjected to High Pressure Torsion

Heterogeneous Precipitation of the α-Phase in Ti15Mo Alloy Subjected to High Pressure Torsion

Segregation mediated heterogeneous structure in a metastable β titanium alloy with a superior combination of strength and ductility

Progress in Severe Plastic Deformation of Metastable Beta Ti Alloys

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