In situ TEM observation of FCC Ti formation at elevated temperatures

Yu, Qian; Kacher, Josh; Gammer, Christoph; Traylor, Rachel; Samanta, Amit; Yang, Zhenzhong; Minor, Andrew M.

doi:10.1016/j.scriptamat.2017.06.033

Cited by 82 publications

(27 citation statements)

References 26 publications

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“…Interestingly, this γ → ε transformation mechanism does not necessarily explain the occurrence of γ nanolaminates inside the HCP ε blocks after deformation. This may rather be due to a reverse transformation (ε → γ) inside the HCP ε block. In order to discuss this hypothesis, three factors, viz., the stacking fault energy, local temperature, and local stress−strain fields, have to be considered.…”

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confidence: 99%

“…Theoretically, a reverse transformation from the HCP ε to the FCC γ can be achieved through the motion of Shockley partials on every second {0001} basal plane with Burgers vectors

\frac{a}{3}

1 \bar{1} 00

> . They have three equivalent glide directions, which can generate stacking faults (i.e., nanoscale FCC structure) in the HCP matrix with negligible volume expansion . Upon strain loading, the sliding of Shockley partials in the HCP ε block of the dual‐phase HEA is achieved by two possible ways ( Figure ).…”

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confidence: 99%

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Bidirectional Transformation Enables Hierarchical Nanolaminate Dual‐Phase High‐Entropy Alloys

et al. 2018

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conventional nanostructures, for instance with grain sizes below a critical value of ≈50 nm, can experience softening due to grain boundary sliding. [11,12] Over the past years, a novel approach to microstructure design, known as hierarchical laminate structure, has shown to greatly improve strength and ductility of conventional materials simultaneously, such as Ti alloys and steels, through massive microstructure refinement. [13][14][15][16] Typically, such materials with hierarchical laminate structures consist of a parent phase serving as soft matrix to improve ductility and a hard product phase impeding dislocation motion to strengthen the microstructure. Unfortunately, such heterogeneous materials with high local mechanical disparity among their constituting phases are often prone to internal damage initiation, thus limiting ductility to 3.8-15% for Ti alloys, [13,14] 20-33% for medium Mn steels, [17] and 23-33% for stainless steels. [16] Therefore, the mechanical properties of such hierarchical laminate materials need to be further improved to minimize the probability of internal damage and failure. Also, creating laminate structures in bulk materials often requires imposing complex processing methods, e.g., accumulative roll bonding. [18] Interestingly, a recently developed dual-phase high-entropy alloy (HEA) also exhibits a simultaneous increase of strength and ductility after grain refinement [19][20][21][22][23] as highlighted in Figure 1. The excellent strength-ductility combination of the novel dual-phase HEA has been reported to be related to the transformationinduced plasticity (TRIP) effect, i.e., the displacive phase transformation from the face-centered cubic (FCC γ) matrix into the hexagonal close-packed (HCP ε) phase upon deformation. [19,22] Depending on the magnitude of the stacking fault energy, different compositional variants of recently developed HEAs have shown deformation modes with planar dislocation slip, [24,25] twinning induced plasticity (TWIP), [26] or TRIP effect. [22] Here, we observe another state which is thermodynamically characterized by a similar stability of two co-existing phases (FCC γ and HCP ε). This means that the stacking fault energy of the FCC matrix must assume a near-zero yet positive value so that forward (γ → ε) and backward (ε → γ) transformations can be both triggered in the same material and loading state.We studied the deformation mechanisms in the dual-phase HEA (Fe 50 Mn 30 Co 10 Cr 10 , at%) in more detail via transmission electron microscopy (TEM) analysis and found that this novel TRIP-assisted HEA gradually develops a hierarchical Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, compromising the material's ductility. Here, a fundamentally new approach that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy allo...

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confidence: 99%

“…Theoretically, a reverse transformation from the HCP ε to the FCC γ can be achieved through the motion of Shockley partials on every second {0001} basal plane with Burgers vectors

\frac{a}{3}

1 \bar{1} 00

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confidence: 99%

Bidirectional Transformation Enables Hierarchical Nanolaminate Dual‐Phase High‐Entropy Alloys

et al. 2018

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“…The mechanical properties of α + β alloys depend greatly on the content of the β-stabilizer element and on the type of heat treatment. Moreover, a metastable phase with a face-centred cubic (fcc) structure (called γ phase) has been reported in pure Ti and Ti-based alloys [21][22][23][24][25][26][27].…”

Section: Introductionmentioning

confidence: 99%

Thermodynamic Analysis of the Formation of FCC and BCC Solid Solutions of Ti-Based Ternary Alloys by Mechanical Alloying

Aguilar

Martínez

Tello

et al. 2020

Metals

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A thermodynamic analysis of the synthesis of face-centred cubic (fcc) and body-centred cubic (bcc) solid solutions of Ti-based alloys produced by mechanical alloying was performed. Four Ti-based alloys were analysed: (i) Ti-13Ta-3Sn (at.%), (ii) Ti-30Nb-13Ta (at.%), (iii) Ti-20Nb-30Ta (wt. %) and (iv) Ti-33Nb-4Mn (at.%). The milled powders were characterized by X-ray diffraction, and the crystallite size and microstrain were determined using the Rietveld and Williamson-Hall methods. The Gibbs free energy of mixing for the formation of a solid solution of the three ternary systems (Ti-Ta-Sn, Ti-Nb-Ta and Ti-Nb-Mn) was calculated using an extended Miedema's model, applying the Materials Analysis Applying Thermodynamics (MAAT) software. The values of the activity of each component were determined by MAAT. It was found that increasing the density of crystalline defects, such as dislocations and crystallite boundaries, changed the solubility limit in these ternary systems. Therefore, at longer milling times, the Gibbs free energy increases, so there is a driving force to form solid solutions from elemental powders. Finally, there is agreement between experimental and thermodynamic data confirming the formation of solid solutions.Author Contributions: Conceptualization, C.A. and C.M.; methodology, C.A. and F.S.M.; software, C.A.; validation, I.A.; formal Analysis, C.A., C.M. and S.P.; investigation, C.A., F.S.M. and K.T.; writing-original draft preparation, C.A., S.P. and A.D.; writing-review and editing, S.P., A.D., K.T. and I.A.; visualisation, C.A. and I.A.; project administration, C.A.; funding acquisition, C.A. and K.T. All authors have read and agreed to the published version of the manuscript. Funding:The authors would like to acknowledge the financial support received from the FONDECYT project n 1190797 and FONDEQUIP/EQM project n 140095. Conflicts of Interest:The authors declare no conflict of interest.

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“…Experimental observations showed that the presence of the FCC phase was associated with thin films, interfaces, powders, high levels of plastic deformation, and heat-treated Ti alloys. Recent work shows that the small FCC precipitates formed in freestanding thin foils during in situ transmission electron microscope (TEM) heating 8 . Experimental results indicate that the stability of FCC Ti in thin films is related to the film thickness, surface orientation, and temperature [9][10][11][12][13][14] .…”

Section: Introductionmentioning

confidence: 99%

Deformation Behavior of Pure Titanium With a Rare HCP/FCC Boundary: An Atomistic Study

et al. 2020

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The compressive and tensile behaviors in a Ti nanopillar with a biphasic hexagonal close-packed (HCP) /face-centered cubic (FCC) phase boundary are theoretically researched using classic molecular dynamic simulation. The results indicate that the HCP/FCC interface and free surface of the nanopillar are the sources of dislocation nucleation. The plastic deformation is mainly concentrated in the metastable FCC phase of the biphasic nanopillar. Under compressive loading, a reverse phase transformation of FCC to the HCP phase is induced by the dislocation glide of multiple Shockley partial dislocations 1 6 <121> under compressive loading. However, for tensile loading a large number of Lomer-Cottrell sessile dislocations and stacking fault nets are formed when the partial dislocations react, which leads to an increase in stress. The formation mechanism of a Lomer-Cottrell sessile dislocation is also studied in detail. Shockley partial dislocations are the dominant mode of plastic deformation behaviors in the metastable FCC phase of the biphasic nanopillar.

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In situ TEM observation of FCC Ti formation at elevated temperatures

Cited by 82 publications

References 26 publications

Bidirectional Transformation Enables Hierarchical Nanolaminate Dual‐Phase High‐Entropy Alloys

Bidirectional Transformation Enables Hierarchical Nanolaminate Dual‐Phase High‐Entropy Alloys

Thermodynamic Analysis of the Formation of FCC and BCC Solid Solutions of Ti-Based Ternary Alloys by Mechanical Alloying

Deformation Behavior of Pure Titanium With a Rare HCP/FCC Boundary: An Atomistic Study

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