Large Internal Stresses Research Articles

Enhancing room and cryogenic temperatures mechanical properties of FeCoCrNiMn high entropy alloys with high content of inexpensive element P

FeCoCrNiMn high-entropy alloys (HEAs) exhibit excellent ductility, particularly at cryogenic temperatures. However, their yield strength is relatively low. In this study, an inexpensive element, phosphorus (P), was added to HEAs at a high concentration (0.4 %). The results indicate that adding P not only enhances both strength and ductility, but also avoids embrittlement. The P-added HEAs were single-phase random solid solutions, with P distributed inside the grains and partially segregated at the grain boundaries. Within the grains, regions rich in P and poor in P were observed, along with significant tensile and compressive strain fields in the P-rich regions. These strain fields may lead to large local internal stresses. The presence of P prevents brittleness due to its specific atomic distribution, whether in coarse or fine grains. The yield strength increased from 185.27 MPa in P-free homogenized HEAs to 296.1 MPa in P-added HEAs, representing an increase of approximately 60 % at 298 K. At 77 K, the strengths further increased, irrespective of grain size. Further, P added reduced the stacking fault energy. At room temperature (298 K), P-free HEAs formed dislocation cells and high-density dislocation wall structures, while P-added HEAs formed additional microbands, deformation twins, and stacking faults due to increased lattice frictional stress. P-added HEAs demonstrated excellent mechanical properties at cryogenic temperatures, with good strength-ductility synergy and strain-hardening capacity. These enhancements can be attributed to the synergistic effects of nano deformation twins, hierarchical nano-spaced stacking fault networks, and Lomer-Cottrell locks, as well as their extensive interactions.

Large internal stress induced nonlinear current-voltage behavior in nanodiamond strengthened ZnO ceramics

The modulation of the electrostatic potential barrier at grain boundaries determines the performance of many ceramic-based electronics such as varistors. However, conventional protocols relying on complex doping and annealing processes inevitably increase the inhomogeneity of microstructure, which may jeopardize the performance stability and mechanical reliability in service. Instead of doping, herein we demonstrate an effective strategy to modulate the potential barrier in ZnO-based low-voltage varistors by exploiting internal stress-induced piezoelectric polarization. The local residual stress as large as ~1 GPa can be created in the ZnO matrix by incorporating ultra-stiff nanodiamond particles using a cold sintering process. As a result, the composite with only 2 wt% of nanodiamond exhibits a prominent nonlinear current-voltage response at a low switch voltage of 15.7 V/mm, which is ascribed to the depressed barrier height induced by the distinct effects of positively and negatively charged polarization on grain boundaries. More strikingly, the large internal stress can significantly enhance the strength of the composite by more than 230% compared with the monolith, owing to the highly strengthened grain boundaries and crack-tip bridging from prestressed nanodiamonds. These findings add internal stress as a new dimension to design mechanically robust ceramic electronics with high performance.

In situ growth of large-scale and ultrathin carbon nitride films shield common metal substrates from corrosion and oxidation

A centimeter-size carbon nitride (CN) film, with nanoscale thickness, is in situ deposited on various metals through the thermal condensation of melamine, achieving broad and uniform coverage, without any pre- and post-treatments. As the temperature rises, the nitrogen content in the film increases, leading to the part of CN to transform into g-C3N4. Benefit from higher Gibbs adsorption free energies of g-C3N4 with Cl- and O2 in aqueous environment, a 30 nm CN film deposited at 475 ℃ with large g-C3N4 network, minimal segregation and low internal stress enables pure metals and alloys to achieve over 90 % protection efficiency.

Theoretical design and performance of three-dimensional, pillared FeS2 cathodes

Three-dimensional (3D) electrode design can provide improved capacities and rate capabilities over conventional two-dimensional electrodes by enhancing electrical and ionic transport. Expanding upon our previous modeling efforts for conversion chemistry lithium-ion batteries, we develop a pseudo-four-dimensional (P4D) approach that is subsequently used to investigate the design of a pillared FeS2 electrode. The model considers transport in three dimensions with an additional “fourth” dimension corresponding to the solid-state lithium transport within the active material particles. Additionally, we allow for expansion of the active material during the conversion reaction to understand how internal stresses impact the electrochemical performance of the cell. By optimizing the model with respect to areal capacity, we are able to predict areal capacities up to 16.8mAh/cm2 for an areal current density of 1.78mA/cm2 and a 103% improvement for the three-dimensional electrodes over planar electrodes of equal volume. Despite the promising results, our simulations suggest that 3D design may be difficult for conversion cathode materials due to the large internal stresses that arise during conversion. Nevertheless, the model is robust and adaptable to other materials that may be more suitable for 3D electrodes due to a lesser change in volume during discharge.

Large Local Internal Stress in an Elastically Bent Molecular Crystal Revealed by Raman Shifts.

The structural dynamics involved in the mechanical flexibility of molecular crystals and the internal stress in such flexible materials remain obscure. Here, the study reports an elastically bending lipidated molecular crystal that shows systematic shifts in characteristic vibrational frequencies across the bent crystal region - revealing the nature of structural changes during bending and the local internal stress distribution. The blueshifts in the bond stretching modes (such as C═O and C-H modes) in the inner arc region and redshifts in the outer arc region of the bent crystals observed via micro-Raman mapping are counterintuitive to the bending models based on intermolecular hydrogen bonds. Correlating these shifts with the trends observed from high-pressure Raman studies on the crystal reveals the local stress difference between the inner arc and outer arc regions of the bent crystal to be ≈2GPa, more than an order of magnitude higher than the previously proposed value in elastically bending crystals. High local internal stress can have direct ramifications on the properties of molecular piezoelectric energy harvesters, actuators, semiconductors, and flexible optoelectronic materials.

Optimizing the high-frequency magnetic properties of soft magnetic composites via surface nanoengineering

Owing to the presence of large residual internal stress during cold compaction, it is difficult to optimize the multiple high-frequency magnetic properties of amorphous soft magnetic composites (ASMCs) simultaneously. Here, a surface nanoengineering strategy was proposed to address the above dilemma by constructing a stress buffer layer composed of amorphous nano-particles, between amorphous powder and insulation coating. The amorphous FeSiBCCr@x wt.% FeB (x = 0.5, 1, 3) composite powders with core-shell structures were successfully prepared via an in-situ chemical reduction method. Especially, when the composite ratio of nano-particles is 1 wt.%, the comprehensive properties of the ASMC reach the best balance. Compared with the FeSiBCCr ASMC, the saturation magnetization of the modified ASMC enhances from 153 to 171 emu/g. Meanwhile, the core loss decreases by 28.25 %, while the effective permeability increases by 25 % and can stabilize to ∼20 MHz. Therefore, our work provides a strategy for achieving superior comprehensive soft magnetic properties of ASMCs via surface nanoengineering, which presents enormous application potential in high-frequency electric devices.

Experimental Observation of a New Attenuation Mechanism in hcp‐Metals That May Operate in the Earth's Inner Core

AbstractSeismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco‐ or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp‐iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation‐DIA combined with X‐radiography, at seismic frequencies (∼0.003–0.1 Hz), high pressure and temperatures up to ∼80% of melting temperature. Significant dissipation (0.077 ≤ Q−1(ω) ≤ 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (⩽7 kJ mol−1). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called “internal stress superplasticity”; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as “elastic strain mismatch superplasticity.” In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp‐iron and the Earth's inner‐core it will be a contributor to inner‐core observed seismic attenuation and constrain the maximum inner‐core grain‐size to ≲10 km.

Solute clustering and its role in a titanium alloy made by laser powder bed fusion

In recent years, atomic clusters and composition undulation have been observed in a number of concentrated solid-solution alloys and were found to be beneficial for strength-ductility synergy. Some reports are also available on the observation of atomic clusters in body-centred cubic metastable β titanium alloys but the role of these atomic clusters in mechanical property development is unclear. In this study, we report the formation of solute atomic clusters in an additively manufactured and solution-treated metastable titanium alloy and investigate their role in deformation and strengthening mechanisms. It was found that the atomic clusters together with ultrafine ω precipitates present in the material effectively pin dislocation motion and contribute to an ultrahigh yield strength. The interaction between dislocations and solute atomic clusters leads to an increased shear stress necessary for dislocation motion. The atomic clusters and the resulting chemical undulation generate pronounced atomic strain fluctuations and distortions with alternating tensile and compressive strain fields, which lead to large local internal stresses and thus resistance to dislocation glide. Cross slip has also been frequently observed in the additively manufactured and solution-treated metastable titanium alloys, which is beneficial for strain hardening and ductility. We also reveal that thermal cycling in the alloy can lead to a significant influence on the microstructural evolution. During solidification and cooling upon laser powder bed fusion, only β and athermal ω precipitates form. With repetitive thermal cycling, the microstructure transforms into α + β grains together with isothermal ω particles. The present findings suggest that solute atom clustering can be introduced through proper selection of solute elements, providing a new strengthening mechanism for titanium alloys.

A green recycling process for spent lithium-ion batteries with extremely low chemical consumption

Selective recovery of lithium from spent cathodes is an attractive, green and efficient recycling method for spent lithium-ion batteries (LIBs). However, current technologies face numerous challenges including high reagent consumption, limited versatility and significant secondary pollution. In this study, we found that an intermediate phase formed during leaching exhibited high physiochemical stability, which hindered leaching of Li+ and led to increased reagent and energy consumption. A simple mechanical activation strategy was used to change the intermediate phase of the leaching process by activating the spent cathode without grinding additives. The energy input of the mechanical force resulted in defects in the activated waste, leading to a novel activated intermediate phases. Through experimental characterization and DFT analyses, this novel intermediate phase containing H+ was found to have a large specific surface area, large internal stress and low reaction energy, which enabled the leaching of the Li+. As a result, the method showed a high utilization efficiency of H+ in recycling most spent cathodes. In the end, we established a closed-loop recovery route for spent LIBs that exhibited exceptional efficiency for H+ utilization (> 97 %), obviated the need for auxiliary reagents, and substantially reduced secondary pollutant generation.

Study on Welding Deformation Correction of TC4 Titanium Alloy Panel Parts

Aiming at the problem that TC4 titanium alloy panel parts after welding have large internal residual stress and obvious panel deformation, which can’t meet the requirements of the parts, the viscoelastic plastic model of titanium alloy panel parts is established by finite element analysis, and the welding deformation-thermal alignment collaborative simulation environment is established by ANSYS, and the welding deformation and thermal alignment process are analyzed by numerical simulation. The law of shape correction when the correction temperature is 600 °C, 650 °C, 700 °C and the holding time is 3600s, 5400s and 7200s, respectively, and the calculation formula of springback rate is established to evaluate the effect of thermal shape correction, so as to optimize the heat treatment correction process. The results show that the correction temperature and holding time have obvious influence on the correction effect. When the correction temperature is 700 °C and holding time is 7200s, the correction effect is the best, and the springback rate is controlled below 25%, which meets the requirements of machining accuracy.

A molecular dynamics analysis of the influence of iron corrosion products on the healing process of bitumen

Corrosion of iron materials in the asphalt concrete pavement occurs commonly when the bitumen film peels off, and the generation of corrosion products would affect the healing performance of bitumen. To identify the affection, this research focuses on the influence of iron corrosion products on the healing process of bitumen by molecular dynamics simulation. Firstly, bitumen model and iron corrosion products model were built. Then the healing systems of sandwich structure were constructed, and the simulated temperature were applied to reach equilibrium in the healing process with NVT ensemble (constant number of atoms, volume, and temperature). Dynamic movements of bitumen were characterized by appearance qualitatively. Healing rate of crack and healing rate of bitumen aggregation were held to evaluate the healing effect. Diffusion behaviors, internal force of motivation and interaction effect were also analyzed. The results indicate the duplicity of iron corrosion products in the healing process including the ease for bitumen climbing and the obstruction of bitumen movement. The comprehensive healing index demonstrated that iron corrosion products would reduce the healing degree, which was mainly caused by the obstruction effect and large internal stress generated by severe aggregation of bitumen in the limited space. From the perspective of crack closure and bitumen aggregation degree in the corrosion area, FeO healing systems were healed best, followed by Fe3O4, Fe2O3 and FeOOH. Furthermore, diffusion period of bitumen molecules on the surface of iron corrosion products during the healing process should be regarded as the important period affecting healing.

Magnetic and structural characteristics of self-assembly Co-Ni-Zn ternary nanocrystals

With respect to the rapid growth of high-performance applications for magneto-electric alloys, extensive attempts have been made to develop the electroplating of nanocrystalline magnets coatings within high coercivity and excellent corrosion resistance. Herein we proposed a low-cost and practical electroplating of the substitution of Zn2 + ions in Ni-Co bath for the ternary Ni-Co-Zn soft-magnetic alloys. Anomalous co-deposition of Ni0.12Co0.88-xZnx (x = 0–0.36) alloys were aligned from sulfamate electrolytes within L-ascorbic acid (H2Asc) as complexant, in which the ratio (x) of atomic molar was regulated by dropping ZnCl2 concentrated solution based on Nernst theory and Cyclic-Voltammetry (CV) tests. Results validated that it yielded crystal growth within a narrow size of 150–250 nm, allowing the structural recombination of the spider-web-like ZnO tangled with Ni-Co nanocrystals. In addition to the enhanced corrosion behavior of the ternary Ni-Co-Zn alloys compared to binary Ni-Co, it was attributed to the coexistence of γ-Ni5Zn21 phase and [Zn(OH)4]2- colloidal-like corrosion products. More impressively, it depicted high coercivity (Hc) of ∼15.3 Oe and superior saturation magnetization (Ms) of ∼153.2 emu/g from VSM tests for Ni0.12Co0.64Zn0.24 (x = 0.24), which was due to the self-assembly of ZnO-rich dispersoids twined around GB regions to be magnetic isolation and restrict the movement of domain walls for higher coercivity. Especially for the sample (x = 0.24) after aged at 450 °C for 2 h, it achieved a superior Hc value of 45.8 Oe that was 3 times higher than those without ageing, which was correlated well with large internal stresses by the disordered stacking among hetero-atoms. So the integrated Ni-Co-Zn ternary soft magnets offered a promising approach to achieving high coercivity with excellent anti-corrosive properties, satisfying the stringent requirements of potential applications used in magnetic memory devices, etc.

Quantifying Internal Stress and Demagnetization Effects for Natural Multidomain Magnetite and Magnetite‐Ilmenite Intergrowths

AbstractDemagnetizing effects and internal stress are difficult to distinguish in natural magnetite samples, but quantitative stress estimates can provide valuable information about microstructure formation, surface oxidation, impacts, tectonic stresses, or interface properties in exsolution structures. Quantifying demagnetizing effects informs about magnetite particle shape, magnetostatic interaction, or anisotropic texture. Here, we establish an improved measurement workflow to separate demagnetizing effects from internal stress for natural magnetite. The method is based on temperature‐dependent hysteresis measurements, and for natural samples require accurate estimates of Curie temperature and temperature‐dependent saturation magnetization to ensure that near‐end‐member magnetite is the dominant magnetic mineral, and to calibrate the temperature‐dependent scaled reversible work (SRW). SRW is the fundamental quantity to determine stress and demagnetizing factor. The improved SRW method is applied to three natural samples with different stress histories where it proves that large magnetite crystals in the metamorphosed Modum complex (Norway) have low internal stress (<100 MPa), while in highly exsolved magnetite‐ilmenite intergrowths from Taberg (Sweden) and Bushveld (South Africa) the magnetite component is highly stressed (>230 MPa). This confirms experimentally that interface strain in complex microstructures due to spinodal decomposition and partial oxidation creates large average internal stress in the magnetite minerals. Because sister specimens have similar internal stress but noticeably (>20%) different demagnetizing factors, textural, and shape anisotropy contribute substantially to SRW in these samples.

Cross-interface growth mechanism of nanotwins in extremely high stacking-fault energy ceramic layer

Twins have been proven to greatly enhance the hardness, toughness, thermal stability, and wear resistance of materials. However, the formation of twins in transition nitride ceramics is exceptionally difficult due to their prohibitively high stacking-fault energy. Consequently, reports on nanotwinned ceramic films have been limited in scope. In this study, we utilize the template effect of multilayer structures to induce the growth of nanotwins within the high stacking-fault energy TiN and AlN layers. Theoretical analysis reveals that the growth of nanotwins across the interface is facilitated by the template effect and large internal stress. The former fosters the epitaxial growth of nanotwinned two-sided structures into the TiN and AlN layers. Simultaneously, the pronounced internal stress engenders substantial lateral stress in the TiN and AlN layers immediately above the underlying nanotwin region, ultimately leading to the formation of nanotwins. In addition, due to the differences in lattice constants and crystal structures of TiN and AlN, the maximum thickness of TiN layer that can sustaining stable growth of nanotwins is about 10 nm, whereas for AlN layer is just about 1.5 nm. This study proposes a promising approach to introduce twins in ceramic films with exceedingly high stacking-fault energies and enable advancements in the toughening of ceramic films.

Microstructure Refinement and Work-Hardening Behaviors of NiAl Alloy Prepared by Combustion Synthesis and Hot Pressing Technique

Most methods used to synthesize and prepare NiAl intermetallics and their alloys have the disadvantages of complexity and high cost. In this paper, the NiAl alloy was prepared by a Combustion Synthesis and Hot Pressing (CSHP) technique under rapid solidification. The grain size of the NiAl alloy is significantly refined to 60–80 μm, which reduces the stress concentration during deformation and improves the fracture strength and fracture hardness. Moreover, the large internal stress and greater amount of dislocations in the as-cast microstructure are produced by their formation under pressure due to the fast cooling rate in the solidification process. The high dislocation density strengthens the NiAl alloy, giving it higher strength, hardness, and work-hardening ability. The high compression properties are also present in the NiAl intermetallics at room temperature, in which the fracture strength is around 1005 MPa and the fracture strain reaches 21.6%. The compressive fracture strength at room temperature is higher than that of the pure NiAl alloy prepared by the Hot-Pressing-Aided Exothermic Synthesis (HPES, about 632 MPa), while it is slightly lower than that of pure NiAl alloy treated by HPES and Hot Isostatic Pressing (HIP, 1050 MPa). The fracture strain is significantly higher than that of the NiAl alloy prepared by other methods. This study can provide guidance for the successful preparation of the NiAl alloy with high strength and toughness.

The cracking behavior of the new Ni-based superalloy GH4151 in the triple melting process

In this paper, the mechanism of crack formation in VIM, ESR, and VAR of GH4151 alloy ingots is discussed. The cracks in VIM ingots are hot tearing, mainly caused by severe segregation of elements such as Nb, Ti, and Al in VIM electrodes, which promotes the precipitation of (γ+γ′) eutectic and Laves phases and thus leads to the formation of hot tearing. On the other hand, cracks in ESR and VAR ingots are cold cracks caused by the uneven distribution of γ′ phases. The temperature range for the precipitation of (γ+γ′) eutectic phases is 1160°C–1148 °C, while that for γ′ phases is 1137°C–1072 °C. At temperatures below 884 °C, the uneven precipitation of γ′ phases results in a maximum internal stress of 1096 MPa at room temperature. Based on computational fluid dynamics (CFD) simulations, to avoid large internal stresses caused by the precipitation of γ′ phase, the ingot should be removed from the water-cooled crystallizer within 57 min. Annealing is an effective measure to improve the quality of electrodes. After annealing treatment, the γ′ phase in the alloy transforms from short rod-shaped to square-shaped, and the decomposition of Laves phase promotes the precipitation of μ phase and σ phase from the matrix, improving the mechanical properties of the alloy.

High-density twin boundaries in transition metal nitride coating with boron doping

The formation mechanism of twin boundaries in transition metal nitrides has remained largely unexplored owing to their high stacking fault energy that hinders twin boundary formation. Herein, we fabricated TiB0.11N1.16 coatings with high-density twin boundaries, including coherent twin boundaries and incoherent twin boundaries. The experimental results revealed that boron doping facilitates the coating orientation transition from [200] to [111], which provides an orientation advantage and indirectly promoting the formation of twin boundaries. Theoretical analyses indicated that boron segregation leads to the formation and stabilization of incoherent twin boundaries via two aspects: i) it reduces incoherent twin boundary energy and ii) forms large internal stress, which stabilizes the reduced incoherent twin boundary energy and prevents detwinning. The coupling of the two effects not only forms and stabilizes the incoherent twin boundaries, but also provides a possible precursor structure for the formation of coherent twin boundaries. Owing to the stress field change caused by the nonuniform distribution of boron, some twinned columns transform into the matrix orientation driven by system energy minimization, thus forming coherent twin boundaries. Moreover, the hardness and toughness of the TiB0.11N1.16 coatings were superior to those of twin-free ceramic coatings. The proposed mechanisms will help to introduce twins in materials with high stacking fault energy.

Loss analysis of magnetization annealed amorphous alloy replacement stator teeth

With the development of electric motors, high speed and high efficiency have become the new development trend. Therefore, reducing the iron loss of motors has become the focus of academia and industry. Amorphous alloys are gradually being used in high speed motors by virtue of their low loss characteristics at high frequencies. Amorphous alloys generate large internal stresses during the manufacturing process, and annealing is usually required to eliminate the effect of internal stresses on magnetism. Amorphous transformer cores are usually annealed by applying a magnetic field in the direction of the magnetic circuit to improve the magnetic domain state, increase the saturation density in that direction, and reduce iron loss. Since the magnetic field of the motor stator teeth is pulsating, and amorphous annealed under longitudinal magnetic field (AALM) has the most performance along the direction of the annealed magnetic field. Therefore, in this paper, the magnetic properties of AALM, annealed amorphous (AA), and unannealed amorphous (UA) are tested to prove the advantages of the material, and then AALM is spliced as the tooth of the motor. The advantages of this material in the motor species are further demonstrated by designing a comparative simulation scheme in the longitudinal direction (same material with different structure) and in the transverse direction (same structure with different material).

Causes´ Analysis of Cracks During Welding of High-Pressure Vessel Body Made of Titanium Alloy

Purpose of reseach is to identify the causes of cracks during welding of high-pressure vessel body made of titanium alloy.Methods. Features of welded joint microstructure of high-pressure vessel body made of titanium alloy are described and studied by color flaw detection methods, fluoroscopy, and examination of the cuts. The causes of cracks in the bodies of pressure vessels are identified. The causes are a combination of internal stresses and structures features.Results. Based on the analysis of cut samples microstructure, it follows that the stamping of the power element was supplied without preliminary heat treatment. The formation of crack foci on the inner surface of the power element, at the place of alloy transition to the area of the attachment site, is primarily associated with large volumetric internal stresses, which are additionally summed up with surface stresses from machining and thermal stresses during welding. Based on the obtained data, it follows that when using a specialized tooling with a copper backing that provides pressurization of inert gas, welding stresses in a weld area are reduced by several orders of magnitude compared to the area where the tooling was without copper backing. As a result of the study, it was revealed that the causes of cracks in the bodies of high-pressure vessels are a combination of internal stresses due to the lack of preliminary thermal treatment of power elements made of titanium alloy, which are part of a high-pressure vessel, as well as stresses arising during welding of the power element with shells, considering design features.Conclusion. To eliminate the cause of cracks, additional experimental work was carried out. Surfacing the manual ADF of reinforcing roller into the zone of maximum stresses of the power element was done. The proposed solution met expectations - all the bodies of high-pressure vessels passed hydraulic tests, which made it possible to preserve material part.

Phase transformation induced twinning in commercially pure titanium: An in-situ study

In this work, in-situ high temperature electron back-scatter diffraction (HT-EBSD) was designed to investigate the phase transformation in Grade 1 commercially pure titanium (CP Ti). The transformation from α-Ti to β-Ti during the heating process mostly followed the Burgers orientation relationship (BOR). However, {112¯1}<1¯1¯26>tensile twinning was found to occur during the β-Ti to α-Ti transformation in the continuous cooling process, which led to the formation of Type 2 α-variants that defied the BOR. This was not reported before for CP Ti, while similar previous observations were all associated with Ti alloys containing β-stabilizer(s). We attribute this observation to the relatively fast cooling process (16 °C/s) where the lattice mismatch between the β-Ti and α-Ti phases has introduced a sufficiently large internal stress within a short time. Diffusional-displacive mixed transformation was proposed for the β-Ti to α-Ti transformation, which can reasonably explain the observations in this work.