As-quenched State Research Articles

Nanocrystallization and Core-loss properties of Fe-rich FeSiBPNbCu nanocrystalline alloy

The present work investigates the structure, soft-magnetic and core-loss properties of 25 mm wide Fe83Si2B9P4Nb1Cu1 nanocrystalline ribbons. The melt-spun precursor ribbons of the thickness of 22 and 28 µm were prepared using commercial raw materials under ambient atmosphere. The XRD and DSC results show predominantly amorphous and hetero-amorphous structure for 22 and 28 µm ribbons in the as-quenched state. Further, the partial crystallization annealing leads to thickness dependent nanocrystallization process, wherein the thinner 22 µm ribbon shows sluggish primary(α-Fe(Si)) and delayed secondary(Fe3(B0.8P0.2)) crystallization process compared to 28 µm ribbons. The difference is explained through as-quenched precursor matrix structure and selective solute re-distribution of the intergranular region during nanocrystallization. Moreover, the higher crystallite size (D) and volume fraction(Vcr) of α-Fe(Si) nanocrystallites are observed for the 28 µm ribbon in the optimal annealing window. Under optimal annealing conditions, the 28 µm nanocrystalline ribbon (733 K) shows better AC soft-magnetic properties including B800 of 1.62 T, Hc of 15.7 A/m, Br/Bs ratio ≥ 0.8 compared to B800 of 1.59 T, Hc of 17 A/m and Br/Bs ratio ≥ 0.7 of 22 µm (743 K) ribbon. The 28 µm ribbon shows a low core loss (P) of 0.34 W/kg under 50 Hz, 1.5 T compared to P > 0.65 W/kg for 22 µm ribbons. The lower core-loss behaviour of thicker 28 µm in the sub-kHz frequency regime has been explained based on the nanocrystalline microstructure and loss coefficients (hysteresis and eddy current). The work compares the AC core-loss properties with reported Fe-rich nanocrystalline alloys and also discusses the scope of improving B800 beyond 1.65 T for the present alloys.

Natural aging behaviors of Al-Cu-Li alloy: PLC effect, properties and microstructure evolution

To facilitate the processing and manufacturing of heat-treatable aluminum alloy components, the initial temper of the billet material is usually adjusted to the as-quenched (AQ) temper by solid solution treatment. However, for most as-quenched aluminum alloys, the obvious natural aging (NA) may occur and cause sensitive time scheduling for the subsequent forming processes. Taking the 2195 Al-Cu-Li alloy as the case material, from the aspects of Portevin-Le-Chatelier (PLC) effect, properties and microstructure evolution, this study deeply and systematically explores the NA behaviors and reveals the underlying mechanism. The combined interaction of Cu and Li solute atoms with mobile dislocations is the major mechanism for the PLC effect in this alloy. The NA induced decreasing of the interaction of solutes with the dislocations makes the number of dislocations in the PLC bands reduce, resulting in its character to change from type B to type A during NA. The Al-Cu-Li alloy exhibits a strong response at the early stage of NA, viz., the tensile strength increases rapidly in the initial 4 h and then reaches the plateau 24 h later. Major sources of strengthening in the NA state are the δ′/β' precipitates, GP zones and solute atoms. The extended age hardening model were used to predict the evolution of the YS and microstructure variables of the alloy during NA. The results indicate that the contribution of solute atoms ( σ ss ), δ′/β' precipitates ( σ δ ′ ) and GP zones ( σ GP ) to strength in NA state is ~161 MPa, ~80 MPa, and ~ 141 MPa, respectively. The electrochemical measurement results indicate that the NA state has a higher corrosion resistance than the AQ state. The pitting pores form associating with the negative potential of the spherical δ′ phase in NA state. • The reduction in the number of dislocations in the PLC bands causes its character to change from type B to A during NA. • The contribution of σ ss , the σ δ ′ and σ GP to YS in NA state is ~161 MPa, ~80 MPa, and ~ 141 MPa, respectively. • The pitting pores form associating with the negative potential of the spherical δ′ phase in NA state.

Activation volume and the role of solute atoms in Al-Mg-Si alloy processed by equal channel angular extrusion

Grain refinement to the sub-micron scale was found to enhance both strength and plasticity. In this size range, the micro-mechanisms of deformation are not yet sufficiently specified. In the present work, strain rate sensitivity was investigated in an Al-Mg-Si alloy processed by equal-channel angular extrusion. The microstructure was controlled by conventional transmission electron microscopy and orientation imaging-ASTAR. In the as-quenched state, deformation produces grains below 100 nm in size. The activation volume V was reduced to 70b3. Based on published theoretical models, we have shown that in such alloy, pure grain boundary sliding cannot be revealed in the low-stress range, due to the intervention of solute atoms. Beyond, the stress-dependence of V consists with an inverse Cottrell-Stokes behavior. This trend was confirmed by TEM which revealed dislocation activity and grain boundary emission. Ageing at 150 °C increases the contribution of GBS whereas the residual activation volume continues to be leveled.

Nonuniformity of Magnetic Characteristics of a Cobalt-Based Amorphous Alloy in the As-Quenched State

Nonuniformity of Magnetic Characteristics of a Cobalt-Based Amorphous Alloy in the As-Quenched State

Effect of 1.5 wt% Copper Addition and Various Contents of Silicon on Mechanical Properties of 1.7102 Medium Carbon Steel.

Requirements for mechanical properties of steels are constantly increasing, and the combination of quenching and tempering is the method generally chosen for achieving high strength in medium carbon steels. This study examines the influence of various silicon contents from 1.06 to 2.49 wt% and the addition of copper (1.47 wt%) on the behavior of 1.7102 steel starting with the as-quenched state and ending with the tempered condition at the temperature of 500 °C. The microstructure was characterized by SEM and TEM, the phase composition and dislocation density were studied by XRD analysis, and mechanical properties were assessed by tensile and hardness testing, whereas tempered martensite embrittlement was assessed using Charpy impact test and the activation energy of carbide precipitation was determined by dilatometry. The benefit of copper consists in the improvement of reduction of area by tempering between 150 and 300 °C. The increase in strength due to copper precipitation occurs upon tempering at 500 °C, where strength is generally low due to a drop in dislocation density and changes in microstructure. The increasing content of silicon raises strength and dislocation density in steels, but the plastic properties of steel are limited. It was found that the silicon content of 1.5 wt% is optimum for the materials under study.

A new perspective on the precipitation sequence in a high-purity Al-1.74 at.% Cu alloy by employing positron annihilation spectroscopy: experiment and theory

We study Al-1.74 at.% Cu as a typical binary model alloy to obtain new information on the precipitation sequence. The alloy has been solution heat treated, rapidly quenched, and then isochronally annealed up to 540 °C. We reveal new effects on the evolution of the precipitation sequence by combining positron annihilation lifetime and coincidence Doppler broadening spectroscopies. Positron parameters do very sensitively respond to changes in the type of precipitates present. We find that in the as-quenched state after exposing the alloy just a few minutes to room temperature GP-I zones containing vacancies inside their copper platelet dominate. They represent about 80% of all detected GP-I zones. At 200 °C GP-II (Θ″) is the dominating type of precipitate. However, the GP-II zones have obviously expelled all structural vacancies on growing from larger GP-I zones. Also the transition from GP-II zones dissolving while precipitates of the Θ′ phase do form at about 250 °C can be clearly observed. The signals support that Θ′ precipitates contain vacancies on their Cu sublattice, since they have to grow in a copper-poor environment. Finally, our signals indicate coarsening of Θ′-precipitates for T > 400 °C and their dissolution until 450 °C. All our experimental results agree well with ab initio theoretical calculations of positron parameters. However, the formation of the equilibrium Θ-phase cannot be observed since these have a too low number density due to their large size.

On the interaction of solute atoms with vacancies in diluted Al-alloys: A paradigmatic experimental and ab-initio study on indium and tin

To study the vacancy-solute-atom interaction, diluted Al-In and Al-Sn alloys with 0.005 and 0.025 at.% indium/tin cast from very high purity elements were investigated by positron annihilation spectroscopy (PAS). Therefore, the alloys have been solution heat treated at temperatures ranging from 320 to 620 °C and then rapidly quenched into ice water freezing-in most thermal vacancies. Positron annihilation lifetime (PALS) and coincidence Doppler broadening spectroscopies (CDBS) were combined to unambiguously identify vacancy-solute-atom complexes. For enabling a direct comparison to experiment, we did employ ab-initio DFT calculations of vacancy-solute-atom complexes providing relaxed atomic coordinates, which are used to calculate PAS annihilation parameters. In the as-quenched state vacancy-solute-atom pairs as well as vacancy clusters were observed in both alloys for all concentrations. During isochronal annealing vacancy clusters, formed during quenching, dissolved at about 130 °C, leaving vacancy-solute-atoms complexes as the only remaining defects. Thus, we could unambiguously identify those by a combination of PALS and CDBS with ab-initio calculations. Employing isothermal annealing the binding energy EB of vacancies to In and Sn solute atoms was determined experimentally by PALS as well as by ab-initio calculations. We find from our experiments EB = (0.20 ± 0.03) and (0.32 ± 0.10) eV for In and Sn, respectively, which is in very good agreement with our ab-initio calculations giving 0.23 and 0.26 eV, respectively. Our results clearly identified vacancy-In/Sn complexes as responsible for the retardation of vacancy migration after quenching. The vacancies bound to In and Sn solute-atoms are released around 150 °C shifting, when added to AlCu alloys as trace elements, the transport of copper atoms to higher temperatures necessary for the formation of finely distributed ″precipitates″, which efficiently strengthen this kind of alloys.

Influence of Austenitization Parameters on the Precipitation Sequence and the Chemical Homogenization of Austenite in a High-Performance Fe–Ni–Cr–Al–Ti–Mo Stainless Maraging Steel

MLX19 stainless maraging steel grade exhibits a mechanical strength/fracture toughness balance within the required range for landing gear applications. However, the microstructure after the heat treatment still needs to be precisely controlled to obtain a better repeatability of the mechanical properties. This work shows that austenitizing is a critical stage. The influence of austenitization treatment parameters on the microstructure obtained after quenching was thus precisely quantified. It was first revealed that, after a standard austenitization at 850 °C and for specific heating rates and holding times, undissolved β-NiAl precipitates, reaching sizes up to 500 nm, still remain in the as-quenched state, in addition to a high retained austenite fraction. It was also found that large amounts of retained austenite are the result of local heterogeneities in the chemical composition of the austenitic phase prior to quenching, while the undissolved precipitates change the overall chemical composition of the austenitic matrix. New austenitization conditions were thus proposed, leading to a better homogeneity of the chemical composition of the martensitic matrix after quenching.

Reversion of natural ageing and restoration of quick bake-hardening response in Al-Zn-Mg-Cu alloy

The high-strength Al-Zn-Mg-Cu alloy provides much better dent resistance and weight-reduction potential compared to the conventional Al alloys used for the automobile body panels. However, natural ageing (NA) significantly reduces the formability of Al-Zn-Mg-Cu alloy. The reversion of natural ageing has been systematically investigated by hardness test, tensile test, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Substantial reversion of natural ageing and thus hardness decrease occurs immediately upon thermal treatment at 120–210 °C in an Al-Zn-Mg-Cu alloy. Although the hardness of the most reverted state decreases with increasing temperature, the lowest hardness is still higher than the as-quenched state by 30HV. As revealed by the complementary DSC and TEM observations, this is ascribed to the synchronization of the dissolution and the re-precipitation of the solutes in the NA clusters during reversion ageing. Reversion at 180–210 °C for less than 30 s leads to a hardness decline of 40HV. The hardening kinetics during NA after reversion is slower than that during first-time NA due to the reduced vacancy concentration. Artificial ageing at 180 °C for 30 min after secondary NA of less than 24 h induces intensive precipitation of plate-like pre-η phases and a giant strength increase of 188–204 MPa. Potential use of high strength Al-Zn-Mg-Cu alloy in automobile body panels could be realized by appropriate reversion treatment improving the formability and the quick bake hardening response.

Corrosion resistance of UNS S41426 stainless steel in acid media and its relation to chemical partition and austenite fraction

The present work evaluates the impact of tempering treatment on austenite reversion and corrosion resistance of UNS S41426 in 0.5 M H2SO4 solution. Higher austenite fraction was related to higher tempering temperature, engendering lower hardness degrees. Despite a reduction in corrosion rates in immersion tests, Ni-rich austenite decreased corrosion resistance in more oxidizing environments. Cr-depleted regions created a secondary anodic peak in polarization tests. Chemical partitioning is presented to be the main feature that influences corrosion in supermartensitic stainless steel. Finally, tempering at 650 °C presented the best results as it had ten times less corrosion rate than as-quenched state.

Effect of Cu Addition on the Precipitation Hardening and Mechanical Properties of Al-Mg Based Cast Alloys.

This study examines the formation of different phases of Al-6 mass% Mg-xCu (x = 1 and 3 mass%) alloys in as-cast condition. Further, it investigates the dissolution of these phases upon solution heat treatment (SHT) and studies the precipitation behavior of these ternary alloys. Scanning electron microscopy with energy-dispersive spectrometry and high resolution X-ray diffraction analyses show the presence of the second phases of Al₃Mg₂ (β), Al6CuMg₄ (T), and Al₂CuMg (S) in Alloy I (Al-6Mg-1Cu), whereas Alloy II (Al-6Mg-3Cu) had only T and S second phases (with a much higher number of S phases). Upon SHT, a significant number of eutectic phases were dissolved in Alloy I, whereas in Alloy II, the number of undissolved S phases was relatively high. A differential scanning calorimetry (DSC) analysis of experimental alloys in as-quenched states reveals two exothermic peaks related to the formation of nanoclusters and S″ or S' metastable phases. Both alloys undergo a rapid hardening stage during the aging process, in which approximately 50%-60% of total hardness was achieved. This is attributed to the formation of nanoclusters. The maximum yield strength achieved at the peak hardness condition was approximately 200 MPa for Alloy I, whereas it was approximately 160 MPa for alloy II. Alloy I took a long time to reach peak hardness, which is correlated with the stability of nanoclusters for a longer time. Earlier peak hardness in Alloy II, despite having nanoclusters, is correlated with undissolved eutectic phases acting as heterogeneous nucleation sites for the formation of S″ or S' metastable phases.

УЛУЧШЕНИЕ ПРОКАЛИВАЕМОСТИ МАССИВНЫХ ПОКОВОК ИЗ СПЛАВОВ СИСТЕМЫ Al–Zn–Mg–Cu ПУТЕМ СБАЛАНСИРОВАННОГО ЛЕГИРОВАНИЯ ЦИНКОМ И МАГНИЕМ

The paper presents the results of studies of the influence of the Zn:Mg ratio on the hardenability, structure and complex of mechanical and corrosive characteristics of forgings with 150 mm thickness made of alloys based on Al–Zn–Mg–Cu system, manufactured in the production conditions of metallurgical plants of JSC AMR and Public Corporation “KUMZ”. It has been determined that an alloy of the Al–Zn–Mg–Cu system with Zn:Mg ratio of not less than 4,1:1 and copper mass content of not more than 1.2% has a higher hardenability compared to the 1933 alloy of standard composition, which ensures homogeneous structure and indicators of mechanical and corrosion properties over the thickness of semi-finished pro-ducts; in metallurgical industry at the stage of manufacturing this allows reducing the residual quenching stresses in massive forgings by means of straightening by reduction in an as-quenched state.

Austenite reversion through subzero transformation and tempering of a boron-doped strong and ductile medium-Mn lightweight steel

Effects of subzero treatment and B doping on austenite reversion are investigated in quenched and tempered Fe–9Mn–5Al-0.3C and Fe–9Mn–5Al-0.3C-0.005B (wt.%) lightweight steels. In the as-quenched condition, the steel microstructure consist of a triplex structure of austenite, ferrite, and martensite. B doping leads to a reduction in the prior austenite grain size by grain boundary segregation and precipitation of boro-carbides, which increases the stability of austenite against the athermal martensitic transformation. After tempering at 200 °C for 2 h, nano-lath reverted austenite is formed by the C-enrichment instead of the carbide precipitation owing to the high Al content. This reversion effect is promoted further by the subzero treatment at −196 °C for 0.5 h prior to tempering, which enables the remaining austenite in the as-quenched state to transform and, thus, provides additional sites for austenite reversion. In addition, the subzero treatment and B doping result in the synergistic effect of the delay of crack initiation through the transformation of retained austenite in contact with ferrite and the improvement of bonding strength. Thus, the B-doped steel subjected to quenching, subzero treatment, and tempering exhibits a very high yield strength of approximately 1 GPa, the tensile strength of over 1.3 GPa, and an excellent elongation of 29.4%, which outperform the tensile properties of conventional austenitic or (austenite + ferrite) duplex lightweight steels.

Carbon redistribution in quenched and tempered lath martensite

Tempering-induced carbon redistribution in martensitic steels from clustering to carbide precipitation is relevant to various steel processing operations and resulting property combinations. In low carbon steels with martensite start temperatures well above room temperature, carbon redistribution is already initiated during quenching. The extent of autotempering varies locally across the as-quenched microstructure, even within a single prior austenite grain, depending on the sequential transformation of martensite laths. To overcome this practical limitation in the study of carbon redistribution, we systematically track the tempering-induced microstructure evolution within a single martensite lath. This is achieved through interrupted tempering treatments and microstructure analyses by electron channeling contrast imaging and atom probe tomography, both correlated with crystallographic orientation information from electron backscatter diffraction. The results show plate-shaped carbon clusters parallel to {100} martensite lattice planes in the as-quenched state, where the maximum carbon compositions remain below ε- or η-carbide compositions. The driving force for initial cluster formation is discussed in relation to spinodal decomposition - analogous to room temperature aged Fe-Ni-C alloys after martensitic transformation during cryogenic quenching. Upon tempering, direct evidence for cluster dissolution and simultaneous cementite nucleation at carbon-rich dislocation cores is obtained. A crucial role is ascribed to the dislocation network which serves as a carbon diffusion path from clusters to cementite nucleation sites.

Comparative Study of the Tempering Behavior of Different Martensitic Steels by Means of In-Situ Diffractometry and Dilatometry.

Martensitic steels are tempered to increase the toughness of the metastable martensite, which is brittle in the as-quenched state, and to achieve a more stable microstructure. During the tempering of steels, several particular overlapping effects can arise. Classical dilatometric investigations can only detect effects by monitoring the integral length change of the sample. Additional in-situ diffractometry allowed a differentiation of the individual effects such as transformation of retained austenite and formation of cementite during tempering. Additionally, the lattice parameters of martensite and therefrom the tetragonality was analyzed. Two low-alloy steels with carbon contents of 0.4 and 1.0 wt.% and a high-alloy 5Cr-1Mo-steel with 0.4 wt.% carbon were investigated by dilatometry and in-situ diffractometry. In this paper, microstructural effects during tempering of the investigated steels are discussed by a comparative study of dilatometric and diffractometric experiments. The influence of the chemical composition on the tempering behavior is illustrated by comparing the determined effects of the three steels. The kinetics of tempering is similar for the low-alloy steels and shifted to much higher temperatures for the high-alloy steel. During tempering, the tetragonality of martensite in the steel with 1.0 wt% carbon shifts towards a low carbon behavior, as in the steels with 0.4 wt.% carbon.

Structure and phase transformations of amorphous-nanocrystalline Al-based alloy

Structure evolutions of rapidly quenched Al65Ni10Si25 alloy were investigated using DTA, X-ray diffraction, SEM, and EDX methods. The processes of structural transformation from the as-quenched state into a nanocrystalline state were studied. The common and distinctive features of structural transformation processes induced by heating, isothermal annealing, and laser irradiation were observed. The formation of the nanoscale structure was observed in all ways of treatment. Two metastable phases that were formed at different temperatures were observed: hexagonal H-phase and Al9Ni2. The phase composition in the equilibrium state was observed as Al + Si + Al3Ni. A high-temperature phase of Al6Ni3Si was found in the alloy after laser heating that shows the possibility of “freezing” the high-temperature phases in the ribbon structure using laser treatment. The fractal structure was observed on the ribbon surface after irradiation.

Impact of uniaxial stress on soft-magnetic and magneto-impedance properties of vitrified magnetostrictive microwires

Abstract The investigation addresses the impact of stress on soft magnetic and giant magneto-impedance (GMI) of rapidly quenched Fe77.5Si7.5B15, (Co94Fe6)72.5Si12.5B15 and (Co94Fe6)72.5Si12.5B12.5Nb0.5Cr2 microwires. Differential scanning calorimetry revealed interesting phase stability in the later alloys. The vitrified microwires contained dominant Co and a lean Fe content showed much superior soft magnetic properties and high magnetoimpedance. The alloy microwires incorporated with Nb and Cr manifested lowest coercivity value of 0.034 Oe with a high GMImax value of 425% in the as-quenched state. The uniaxial stress applied on the microwires modified the shape of the hysteresis loops and GMI plots. The hysteresis loops showed stress dependence of coercivity based on alloy chemistry, phase stability and consequent magnetostriction. The GMImax revealed sensitive change with respect to the applied stress in all the alloy microwires. In addition to GMImax interesting features were observed in the GMI profile pertaining influence of stress on the anisotropy field. The anisotropy field shifted systematically in one of the alloy microwires which displayed symmetric dual GMI peaks. The stress was also found to modify the asymmetric characteristics in a microwire.

Structural transformations and magnetic properties of plastically deformed FeNi-based alloys synthesized from meteoritic matter

In this work we investigated FeNi-based alloys synthesized from the cosmic matter of Morasko Meteorite (MM). The fall of Morasko iron meteorite is one of the biggest known in Central Europe and it has been dated to take place about five thousand years ago. It is known that iron meteorites contain hard magnetic tetrataenite phase (L10 FeNi), which has not been synthesized on the Earth in significant amount in the bulk form yet, because of very sluggish diffusion of Fe and Ni atoms below the disorder-order temperature of 320 °C. Firstly we doped the metallic part of Morasko Meteorite with Ni to obtain nearly equiatomic composition Fe51Ni49, which is the composition of pure tetrataenite.Further synthesis steps (melt-spinning, high pressure torsion, isothermal annealing at Ta = 320 °C for τa = 720 h) were undertaken to induce structural disorder and to observe changes in the structure/microstructure of the meteorite melt with nickel and its magnetic behavior. The presence of fcc FeNi with slightly varying lattice constant was confirmed at each step of the treatment. It evolved from the initial value of a = 3.589 ± 0.001 Å in the as-quenched state, through 3.591 ± 0.001 Å after 10 revolutions in the high pressure torsion process, to the maximum value of 3.593 ± 0.001 Å after 30 revolutions of anvils. No significant impact of further application of stress (50 revolutions) was noted, as the lattice parameter changed within the uncertainty limit. After annealing, the lattice parameter decreased to the initial value of the as-quenched sample (a = 3.589 ± 0.001 Å). The observed broadening of the peaks in the X-ray diffraction patterns may be attributed to the presence of microstrains and refinement of crystallites. Both contributions were determined using the Williamson-Hall method. In the initial state, the domain sizes were of about 100 ± 17 nm and diminished to 36 ± 11 nm after application of stress. Further isothermal annealing did not change the crystallites size, which was evaluated to be 35 ± 9 nm. Microstrains in the initial state were determined as (0.039 ± 0.017)10−3 and increased significantly to (0.562 ± 0.060)10−3 after 30 revolutions of anvils. Isothermal annealing decreased microstrains to (0.215 ± 0.071)10−3. After application of stress, the coercivity and remanence were enhanced, while subsequent isothermal annealing caused their decrease. Moreover, severe plastic deformation facilitated saturation of magnetization. As-quenched sample reached saturation at a magnetic field of about 2 T, while for high pressure torsion processed alloy this value was significantly reduced.

Influence of Polymer Coating on the Magnetic Characteristics of Ribbon of an Amorphous Cobalt-Based Alloy in the As-Quenched State

Samples of cobalt-based amorphous soft magnetic alloy AMAG-172 (Co–Ni–Fe–Cr–Mn–Si–B) with near-zero negative values of saturation magnetostriction were used to study the effect of a polymer coating (applied upon the production of magnetic screens) and of the conditions of its formation on the distribution of magnetization and magnetic properties. The coating was applied onto the ribbon in the as-quenched state at temperatures of 90–130°C. It has been shown that the polymer coating leads to an increase in magnetic permeability and to a good time stability of magnetic characteristics. With an increase in the temperature of the formation of the coating, its effect on the magnetic characteristics is weakened because of an increase of the Young modulus as a result of the heat treatment upon the production of the coating.

Exploration of Nano-scale Structural Instabilities in Metastable β Titanium Alloys Using Advanced Electron Microscopy

A variety of nano-scale structural instabilities formed in different metastable β titanium alloys have been systematically investigated using advanced characterization techniques. The characteristics of three different types of nano-scale structural instabilities, the transformation mechanisms and pathways involved and the critical experimental conditions to generate such nano-scale phases will be reviewed and summarized, including athermal ω phase with hexagonal structure, O’ phase with orthorhombic structure, and incommensurate modulated nanodomains. The athermal ω phase has been observed in the as-quenched state in Ti-xMo (x=12, 15 and 181), Ti-18Mo-5Al, Ti-20V, Ti-5Fe, Ti-5Al-5Mo-5V-3Cr (Ti-5553) and Ti-24Nb-4Zr-8Sn (Ti-2448). O’ phase has been characterized to co-exist with athermal ω phase in the as-quenched state isomorphous titanium alloys, including Ti-26Zr-2Al (at.%), Ti-18Mo, Ti-18Mo-5Al, Ti-5553 and Ti-2448. Incommensurate modulated nanodomains were found in compositionally graded Ti-xFe alloy when the athermal ω phase is suppressed. These various nano-scale structural instabilities need to be taken into consideration when designing novel metastable β titanium alloys to optimize the mechanical performance by microstructure engineering.