Search

The initial microstructures with the unimodal and bimodal size distribution of γ′ precipitates (referred to as UD and BD, respectively) in CoNi-based superalloy are obtained utilizing different heat treatment regimes. In the present work, compression tests are carried out to investigate the anomalous temperature dependence of strength and the effect of initial microstructure on the deformation mechanism of CoNi-based superalloys. Results show that the anomalous yield occurs above 600 °C, and BD specimen exhibits higher yield stress than UD specimen. In UD specimen, the major deformation mechanisms are pairs of a/2 < 110 > dislocations coupled by APB (antiphase boundary) shearing γ′ precipitates (600–700 °C), and dislocations bypassing the γ′ precipitates accompanied with the SISFs (superlattice intrinsic stacking faults) shear above the peak temperature (900 °C). In BD specimen, the predominant deformation mechanism transitioned from the APB shearing (600 °C) to APB shearing and SISFs shearing (700 °C), and then to the dislocations bypassing the γ′ precipitates together with the SISFs shearing (900 °C). Meanwhile, the relationship between the initial microstructure and deformation mechanism is discussed. Moreover, an attempt is made to quantitatively assess the contribution of secondary γ′ precipitates to the strength at the peak temperature. [ABSTRACT FROM AUTHOR]

Internal Mg diffusion (IMD) process can produce MgB 2 superconducting wires with engineering critical current density several times higher than that of traditional powder in tube processed wires, which makes it an attractive and promising method for mass producing practical MgB 2 wires. However, the MgB 2 layer growth stops shortly after the onset of the heat treatment and unreacted B always remained in the MgB 2 layer within MgB 2 wires, negatively affecting the J e performance. Thus it is of great importance to have an in-depth understanding of the mechanism of reaction between Mg rod and B powder forming the MgB 2 layer within IMD wires during heat treatment, and identify the critical factor that controls the formation rate of MgB 2 layer. In present work, the kinetics mechanism of reaction between Mg and B forming MgB 2 layer in the internal Mg diffusion (IMD) processed MgB 2 wires were systemically studied in present work. It was found that the reaction between Mg and B forming MgB 2 layer during the heating treatment is controlled by varied mechanisms. At initial stage, the formation of MgB 2 layer is mainly determined by the rate of chemical reaction between liquid Mg and B powder. As the reaction processes and the thickness of synthesized MgB 2 layer increases, the slow Mg diffusion-limited mechanism gradually becomes dominant at final stage. On the basis, herein Mg rod with a thin Cu coating is proposed to replace normal Mg rod in IMD procedure to accelerate the formation of MgB 2 layer within wires. As a result, Cu coating can change the kinetics mechanism of MgB 2 layer formation and enable the formation of MgB 2 layer to get rid of the restriction of slow Mg diffusion. Complete dense MgB 2 layer without B-rich or unreacted B regions was successfully synthesized within Cu coated IMD wires with larger diameter (1.03 mm) at temperature as low as 600 °C (below Mg melting point). The kinetics mechanism of MgB 2 layer formation determined in our work can provide a valuable guide for optimizing processing parameters of IMD MgB 2 wires. Moreover, the Cu coating technique proposed here opens a promising way to fabricate practical high performance IMD wires at low heating temperature. [ABSTRACT FROM AUTHOR]

In present work, a multiphase Ni 3 Al-based intermetallic alloy with complex components of γ'+γ dendrite comprising extremely high volume fraction (81 vol %) of γ′ phase and interdendritic β phase is investigated, the microstructure evolution and phase transformation behavior tailored by different solution cooling rates are studied by applying different cooling methods of water cooling (WC, 138 °C/s), air cooling (AC, 72 °C/s), and furnace cooling (FC, 0.05 °C/s) after solution treatment at 1200 °C for 10 h. Results show that the cubic degree, size and volume fraction of cuboidal primary γ′ as well as size of spherical secondary γ′ particles in the γ'+γ dendrite are improved by decreasing cooling rates, and the average sizes of the primary and secondary γ′ particles are found to meet logarithmic relationships with cooling rates. There are three kinds of precipitates in the interdendritic β phase resulted from different cooling methods, i.e., the rod-like Cr 3 C 2 carbides, the semi-spherical α-Cr particles, and the long acicular γ′ precipitates. The lower cooling rate decreases the number of rod-like Cr 3 C 2 carbides, increases the number and size of the semi-spherical α-Cr particles, and promotes the formation of long acicular γ′ precipitates in the interdendritic β-matrix. The semi-spherical α-Cr particles play the role of pinning dislocations in the β-matrix and have the orientation relationship of [‾110] α-Cr //[‾110] β-matrix. In addition, the precipitation of semi-spherical α-Cr particles in β-matrix is observed to accomplish in two sequential stages and grow in an onion-like ordered way. Stacking faults existed in partial of α-Cr particles due to the nanotwins in β-matrix. Microtwins are observed in the interdendritic β-matrix after water cooling, which have a bimodal configuration of nanotwins at the microtwinning boundaries and interior stacking faults. Besides, γ′-envelope formed between the γ'+γ dendrite and interdendritic β phase dependent on the solution cooling processes. Image 1 • Re-precipitation sizes of primary and secondary γ′ particles have the logarithmic relationship with solution cooling rates. • Microtwins in the β-matrix have a bimodal configuration of nanotwins at their boundary and interior stacking faults. • Precipitation pattern and growth mode of semi-spherical α-Cr particles in β-matrix phase after air cooling. • Precipitation behaviors influenced by solution cooling rates of precipitates in the interdendritic β. [ABSTRACT FROM AUTHOR]

After solution treatment at 520 °C for 6 h and subsequent aging at 200 °C for 6 h, eutectic Mg 2 Si phase in Al–10%Mg 2 Si alloy transforms from long rod to short fiber-like and spherical morphologies, and a great number of nano-sized β″ particles precipitate in the Al matrix. The fine eutectic Mg 2 Si phase combined with nano-sized precipitates gives rise to enhanced hardness and high tensile strength of Al–10%Mg 2 Si alloy (increasing from 186 MPa to 234.6 MPa). [ABSTRACT FROM AUTHOR]

The evolution of η phase and its effect on the tensile properties at 700 °C in 718Plus superalloy after three solution (at 940 °C for 1 h) and single aging (at 870 °C for 25 h, 50 h and 100 h) heat treatments, were investigated. The results showed that there were several various morphologies of η phase in 718Plus samples. As aging time increased, the volume fraction of needle-like/plate-like increased continuously from 5.32%, 7.78% to 29.52%, while the volume of granular/short rod-like η phase firstly increased from 0.46% to 0.68% then decreased to 0.56%. The high temperature tensile properties were affected by η phases with various morphologies. With aging time increasing, the ultimate tensile strength was obviously improved from 797.11 MPa to 860.57 MPa, which could be attributed to a certain number of plate-like/needle-like η phases at the grain boundaries which could hinder the dislocations motion near the grain boundaries. And the elongation weakly increased firstly from 16.84% to 18.76% and then decreased to 18%, which was closely related to granular/short-rod η phases. Enhancing tensile strength and acceptable ductility could be obtained by morphology control of η phase applying SA heat treatments in the study: promoting the simultaneous precipitation of an appropriate amount of plate-like/needle-like η phases and a small amount of granular/short rod-like η phases. • The morphologies of η phases found in studied 718Plus superalloy samples are plate-like/needle-like, granular/short rod-like. • With aging time extending, the volume fraction of needle-like/plate-like increased, resulting in improving tensile strength. • By applying SA aging heat treatments, an enhancing tensile strength could be obtained by morphology control of η phase [ABSTRACT FROM AUTHOR]

Step quenching and tempering (SQT) treatment was applied on HSLA steels to achieve a multi-phase microstructure with a superior deformation performance. Intercritical quenching and tempering (IQT) as well as direct quenching and tempering (DQT) were also processed for comparison. Moreover, effects of intercritical temperatures before quench for different treatment methods on microstructure evolution and mechanical property were also investigated. Compared with IQT and DQT, SQT treatment produces a fine microstructure composited of soft ferrite/pearlite and hard martensite, with precipitates distributed along boundaries to maintain a large quantity of dislocations on deformation. Experiments involving tension and impact tests show that SQT samples with such multi-phase microstructure exhibit a desirable combination of mechanical properties. With increasing the intercritical temperature, the phase fraction of martensite is increased, which improves the strength but weakens the impact toughness in regardless of heat treatment type. Meanwhile, the yield ratio (YR) is found to be increased by higher intercritical temperature in all samples, due to the larger phase fraction of martensite with a large dislocation density and a finer grain structure. Such scenario is elucidated by using a model proposed through Swift's equation. That indicates that the content of multi-phase microstructure can be efficiently controlled via adjusting the intercritical temperature using SQT, which enables the optimization of phase composition according to the requirements of actual productions in engineering. [ABSTRACT FROM AUTHOR]

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni 3 Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni 3 Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni 3 Al phase with ordered face-centered L1 2 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101] γ'-envelope //[01‾1] dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L1 2 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni 3 Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr 23 C 6 carbides surrounded by the widened γ′-envelope. [ABSTRACT FROM AUTHOR]

Allvac 718Plus alloy is a newly-developed Ni-based superalloy with excellent thermal stability and mechanical performance at elevated temperatures up to 704 °C. The precipitation behavior of the rapidly-solidified Allvac 718Plus superalloy during aging is systematically investigated in this work. The microstructure evolutions of the strengthening γ' phase and the γ'/γ″ co-precipitates during aging are examined. During aging, the "duplet" γ'/γ″ and "triplet" γ″/γ'/γ″ occur and then disappear due to the dissolution of γ″ phase, which is promoted by the δ precipitation. On the contrary, the "triplet" γ'/γ″/γ' forms and remains after aging, resulting from the low diffusion rate of solute atoms through the spheroidal γ' precipitates. The lamellar δ-Ni 3 Nb phase occurs at grain boundaries. Its size and content increase with the prolongation of aging time. After long-term thermal exposure, the hexagonal η phase forms at the δ-γ interface due to the variations of local chemical composition, and then the tetragonal Cr-rich σ phase nucleates adjacent to the δ and η particles because of the segregation of Cr atoms rejected from the δ and η particles. • The rapidly-solidified γ-matrix of 718Plus is obtained with a supersaturated solid solution and a high density of defects. • The dissolution of ellipsoidal γ″ results in the disappearance of the "duplet" γ'/γ″ and "triplet" γ″/γ'/γ″. • The "triplet" γ'/γ″/γ' is remained due to the low diffusion rate of solute atoms through the spheroidal γ' precipitates. • The lamellar δ forms first and the η and σ phases successively nucleate at the δ-γ interface after long-time thermal exposure. [ABSTRACT FROM AUTHOR]

A polycrystalline Ni 3 Al-based superalloy was joined through transient liquid phase (TLP) bonding technique. A post bond heat treatment (PBHT) was successively conducted. Creep tests of the TLP and TLP + PBHT specimens were then carried out under 800 °C and 160 MPa. The creep deformation and fracture behaviors were further investigated through interrupted creep tests. During the PBHT procedure, microstructure evolution of the joint occurred with creep life prolonged from 64 h to 368 h. The superdislocations in TLP and TLP + PBHT specimens were determined to study the γ′ phase shearing modes. Both individual superdislocations and APB-coupled partial dislocation pairs are observed. The variation of dominate superdislocation configurations among different joint zones and the evolution of the superdislocations during the creep process are discovered. Besides, the formation of different γ′ rafting microstructures and the initiation of intergranular microcracks in the polycrystalline joint are both studied. [ABSTRACT FROM AUTHOR]

Precipitation and growth behavior of mushroom-like Ni 3 Al phase along the dual phase (γ + γ′) and eutectic γ − γ′ area interfaces of as-cast JG4246A Ni 3 Al-based alloy were investigated by using short time cyclic heat treatment. Results showed that vacancies, small angle grain boundaries and dislocations formed along the dual phase (γ + γ′) and eutectic γ − γ′ area interfaces during the cyclic heat treatment, which promoted the precipitation of Ni 3 Al phase along this location. In addition, concentration gradient of Al elements between the eutectic γ − γ′ area and dual phase (γ + γ′) areas and the more rapid pipe diffusion provided by dislocations contributed to growth of directional mushroom-like Ni 3 Al phase. [ABSTRACT FROM AUTHOR]

Avoiding the formation of cracks to ensure a reliable printability and a good stability is crucial in the laser additive manufacturing of alloys. Contrary to previous studies that have generally tried to decrease the liquid film and solidification range, in this work, we innovatively utilized segregation engineering and abundant cell boundaries to introduce liquid backfilling as well as a network of segregation phases to alleviate thermal stress, consequently eliminating hot cracking. More specifically, zirconium was introduced into a nickel-based superalloy to form a continuous interdendritic liquid film during the laser additive manufacturing process. It was found that the continuous intermetallic Ni 11 Zr 9 segregation phase decorated the cell boundaries, and cracks were completely eliminated in the as-printed Haynes 230 alloys when their Zr content reached 1 wt.%. Moreover, this continuous Ni 11 Zr 9 network layer was able to act as a "skeleton" to significantly improve the yield strength of the as-printed samples. Following appropriate heat treatment, these Zr-modified Haynes 230 alloys exhibited an extraordinary combination of strength and plasticity, which were superior to those of the previously reported Haynes 230 alloy. These findings provide a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The microstructure and corrosion behavior of Al–10%Mg 2 Si alloy after heat treatment are investigated. For Al–10%Mg 2 Si in 3.5 wt.% NaCl solution, corrosion of α -Al phase initiates in (Al + Mg 2 Si) binary eutectic zones with electrochemical polarity change of Mg 2 Si phase, and expands along the eutectic Mg 2 Si precipitates. After heat treatment, eutectic Mg 2 Si transforms from long flakes and rods to short fibers, disks and spheroidized particles. The transformation of eutectic Mg 2 Si phase suppresses the rapid expansion of corrosion pits, which leads to enhancing the corrosion resistance of Al–10%Mg 2 Si alloy. [ABSTRACT FROM AUTHOR]

The microstructure control of multiphase Ni 3 Al-based intermetallic alloy by traditional heat treatment is still limited, especially size and volume fraction of interdendritic β phase cannot be effectively reduced, due to the limitation of interdendritic β phase dissolution temperature close to the melting point of the alloy. In present work, the temperature gradient and solidification rate of the alloy under different laser remelting parameters were calculated by the Rosenthal equation. Afterward, the as-cast multiphase Ni 3 Al-based intermetallic alloy (19.4 vol% of interdendritic β phase) was remelted under different laser parameters of 3000–4500 W laser power and 5–20 mm/s laser scanning rate. And the effects of different laser remelting parameters on the microstructure, room temperature, and 980 °C high-temperature tensile properties of multiphase Ni 3 Al-based intermetallic alloy were studied. The results show that laser remelting treatment with the appropriate parameters (4000 W laser power and 10 mm/s laser scanning rate) can effectively reduce the size and volume fraction of interdendritic β phase and the morphology and size of γ′ phase in the γ′+γ dendrite, and significantly refine the microstructure of alloy, thus effectively improving the room temperature and 980 °C tensile properties of multiphase Ni 3 Al-based intermetallic alloy. Under the optimum laser remelting parameters of 4000 W-10 mm/s, the tensile strength at room temperature and 980 °C of the corresponding laser remelting alloy can reach 1215 MPa and 280 MPa, respectively, and the elongations after fracture of the corresponding alloy are 14.09% and 6.41%, respectively. [ABSTRACT FROM AUTHOR]

The correlation of microstructure-properties of the Ni-based superalloy with coprecipitation of γ'/γ'' phases has been studied in details. To obtain the different γ'/γ'' microstructures, various heat treatments were attempted. The experimental results show that high amounts of the compact γ'/γ'' phases contribute to the tensile strength of Ni-based superalloy, which decreases with the coarsening of γ'/γ'' structures. The γ'/γ'' phases can restrain the motion of the dislocations. Further increasing the size of the γ' and γ'' phases, the deformation mechanism transforms from dislocation pairs shearing to Orowan bypassing, and then to dislocation shearing. Stacking faults (SFs) also appears in γ'/γ'' phases. Additionally, the moderate η/δ phases effectively pin the grain boundary, which enhances the alloy's ductility. [ABSTRACT FROM AUTHOR]

In this paper, post bonding heat treatments to transient liquid phase bond reduced activation ferrite/martensitic steel were conducted, with a Fe–Si–B amorphous foil as the interlayer. The results show that, the post bonding heat treatments decreased the strength difference between different regions of the transient liquid phase bonded joint and altered the shape and the amount of Cr 2 B along the interface between the diffusion affected zone and base metal. In particular, it is found that the tensile fracture position of the as-bonded joint has been altered by post bonding heat treatment, from the interface between the diffusion affected zone and the base metal to where inside the base metal. In order to unveil such deformation differences between transient liquid phase bonded joints with and without post bonding heat treatment, digital image correlation method was adopted to study the influence of post bonding heat treatment on the local strain evolution of transient liquid phase bonded joint from both macro- and micro-scale. The non-uniform strain distribution upon the nominal surface of the tensile samples during deformation showed that, the strain of the as-bonded joint was always localized at the interface of the diffusion affected zone, resulting brittle fracture eventually. While for the joint after post bonding heat treatment, the strain localization gradually transferred from the interface to the base metal during the tensile test, which is attributed to the changing deformation resistance of these regions during straining. [ABSTRACT FROM AUTHOR]

In this study, the microstructure and high-temperature mechanical properties of a 9Cr-1.5W reduced activation ferritic/martensitic (RAFM) steel processed by friction stir processing (FSP) combined with a tempering treatment were investigated. During the FSP, lath martensite microstructures containing needle-like M 3 C and nano-sized MX precipitates were formed in the steel owing to the severe plastic deformation and high cooling rate. The steel subjected to FSP and the tempering treatment at 700 °C for 60 min (FSP-T60) had a fine-grained microstructure consisting of ferrite and martensite, and a large number of M 23 C 6 precipitates formed at the grain boundaries and within the grains. The FSP-T60 steel showed an increase of ∼100 MPa in the tensile strength at 600 °C and a high elongation of 26% as compared to the steel prepared via traditional hot rolling, normalizing, and tempering. The FSP-T60 steel also exhibited improved creep resistance with a creep rupture time of 4523 h at 600 °C/210 MPa. The increase in the strength and ductility of the steel can be attributed to its refined grain structure and the high density and dispersion of the M 23 C 6 precipitates. Meanwhile, the homogeneous distribution and refinement of M 23 C 6 carbides could reduce the growth rate of the Laves phase during creep, thus extending the formation time of the creep cavities and cracks and further increasing the creep rupture time of the steel. [ABSTRACT FROM AUTHOR]

A CoNi-based alloy (various cooling rates after solution heat treatment) was utilized to investigate the microstructure-dependent coarsening behavior of γ′ precipitates. The fast cooling rate leads to the formation of initial fine γ′ precipitates with high number density. The different initial microstructures of CoNi-based alloy lead to various coarsening behaviors of γ′ precipitates during subsequent aging treatment at 900 °C. The coarsening behavior of large γ′ precipitates with low density (formed in slow cooling rate, 0.1 °C/s) is in accordance with the Lifshitz-Slyozov-encounter modified (LSEM) model. Whereas for the fine γ′ precipitates with high density (formed in fast cooling rate, 60 °C/s or 240 °C/s), its coarsening behavior is consistent with the TIDC model at early stage and LSEM model at later stage, respectively. • The different initial microstructures of Cobalt-based alloy lead to various coarsening behaviors of γ′ precipitates. • Fast cooling rate leads to the formation of fine γ′ precipitates with high density. • Coarsening of fine γ′ precipitates is consistent with the TIDC model in the early stage and LSEM model in the later stage. • Coarsening of large γ′ precipitates is in accordance with the LSEM model. [ABSTRACT FROM AUTHOR]

Aimed at evaluating the effectiveness of Zr and Hf additions on oxide particle refinement and tensile strength improvement of transformable oxide dispersion strengthened (ODS) steels, 9Cr–Zr and 9Cr–Hf ODS martensitic steels were fabricated by mechanical milling, spark plasma sintering and tempering heat treatment at 800 °C. The as-sintered 9Cr–Zr steel consisted of martensite and certain amount of transformed ferrite with M 23 C 6 (M = Fe, Cr) precipites within ferrite interior. With respect to the as-sintered 9Cr–Hf steel, the microstructure consisted of martensite and small amount of residual ferrite. The residual ferrite is characteristics of higher number density of oxide nanoparticles than those in martensite. The added Zr or Hf cannot transform all Y 2 O 3 into Y–Zr–O or Y-Hf-O complex oxides. Except for Y 2 O 3 , ZrO 2 and Y 4 Zr 3 O 12 were identified in 9Cr–Zr steel, and Y x Hf y O z and Y 2 H 2 O 7 were found in 9Cr–Hf steel. The orientation relationship and interface structure between Y–Zr–O/Y-Hf-O and the matrix were studied in detail, as well as the size distributions of oxide nanoparticles in both as-sintered and tempered conditions. Tempering is mandatory to restore ductility of martensitic steels. The tensile strength of tempered 9Cr–Hf steel is superior to most reported results, while the tempered 9Cr–Zr steel exhibits a good combination of tensile strength and ductility. The ferrite phase in both tempered 9Cr–Zr and 9Cr–Hf steels has negligible effects on ductility degradation. The inferior strength of high-angle boundaries to tempered martensite itself is suggested to be responsible for limited ductility of steels. Compared to Al and Ti constituents, Zr and Hf can be considered as potential microalloying elements to produce ODS steels with favorable mechanical performance. [ABSTRACT FROM AUTHOR]

• The major highlight of this paper is how to improve the thermal stability of γ I ′ phase by investigating its directional coarsening behavior. • The second highlight is clarifying the kinetics of γ I ′ directional coarsening and its connection mode. • The third highlight is clarifying the influence of initial radius, number, volume fraction and γ channel distance on the kinetics of γ I ′ directional coarsening. • The forth highlight is clarifying the influence of lattice misfit, elemental diffusion and interfacial composition width on γ I ′ directional coarsening. Polycrystalline Ni 3 Al-based superalloy with up to 70% γ ′ phase owns excellent strength at around 1100 °C. It has been successfully used as a substitute for turbine blade crown, which is one of the aircraft components working in the severest environment (i.e., sometimes nearly overheating). However, few studies concentrate on the coarsening resistance and phase thermal stability in this newly designed polycrystalline superalloy at the working temperature, hence, directional coarsening behavior of primary γ ′ (γ I ′) has remained restricted. This paper investigated directional coarsening behavior of γ I ′ during aging treatment (1100 °C) after overheating at 1270 °C with furnace cooling (SFA), air cooling (SAA) and water cooling (SWA) respectively. It clearly showed that γ I ′ grew into lamellar and gradually became irregular during the directional coarsening process. Moreover, its morphological combination mode was illustrated. Since parallel short lamellar γ I ′ connected each other, long lamellar formed. When perpendicular long lamellar γ I ′ connected each other, it became L -type or T-type, i.e., precursor of irregular γ I ′. The kinetics of directional coarsening regime of γ I ′ was in line with the matrix-diffusion - controlled (MDC) theory according to the evaluation of γ I ′ size. This was caused by higher migration of γ I ′ interfaces than the diffusion of solute elements. It was also confirmed that driving force of directional coarsening mainly depended on the competition of elemental diffusion, lattice misfit distribution and γ channel distance. [ABSTRACT FROM AUTHOR]

In the present study, a two-phase Ti 2 AlNb-based alloy with a nominal composition of Ti–22Al–25Nb (at%) comprising the dominant O + B2/β phase was explored. The microstructural evolution and tensile properties are studied by applying different heat treatment methods of water-cooling (WC, 140 °C/s) and furnace-cooling (FC, 0.08 °C/s) after heat treatment at 1015 °C for 40 min, followed by aging treatment at 800 °C for 1, 5, and 100 h. The results indicated that the morphologies of solution-treated alloys after different cooling methods have a significant distinction. The WC alloy consists of a continuous B2/β phase and a few remnant O phase, whereas the FC alloy comprises a mass of lath O phase and discontinuous B2/β phase. The Widmanstätten O + B2 structures were generated in the following aging treatment. The α 2 phase was not detected during the heat treatments in the present work. The B2 phase and O phase in the aged WC and FC alloys follow the classical orientation relationship: (110) B 2 / / (001) O , [ 1 ‾ 11 ] B 2 / / [ 1 1 ‾ 0 ] O. The room-temperature ultimate tensile strength (UTS) and elongation to failure (EL) of the FC and aged FC alloys are higher than the WC and aged WC alloys, except for the 100 h-aging condition. On the contrary, the high-temperature UTS and EL of the aged WC alloys are higher than the aged FC alloys. The existence of the O/O phase boundaries, the coarsening of the B2 grain boundary and discontinuous precipitation harmed the performances of the studied alloys. Beyond that, the fracture mode of the explored alloys with different heat treatment methods was also analyzed. • The morphologies of solution-treated Ti 2 AlNb-based alloys after different cooling methods have a significant distinction. • The B2 phase and O phase in the studied alloys follow the classical orientation relationship: 110B2//001O, [111]B2//[110]O. • Tensile properties and fracture mode are significantly influenced by different cooling rates and aging treatments. • The O/O boundaries, coarsening of the grain boundary and discontinuous precipitation harmed the properties of the alloys. [ABSTRACT FROM AUTHOR]

The precipitation of η phase during the solution treatments of Allvac 718Plus was investigated in this study. The results showed that there was free-γ′ zone appearing around η phase which was consistent with the hexagonal η-Ni 3 (Al/Ti) 0.5 Nb 0.5 after heat treatments. The granular η phase was found to be the transition of the plate-like η phase and have no pinning effect against the grain boundary, while the plate-like η phase has a pinning effect on the grain boundary and leads to the formation of serrated grain boundaries. With the solution temperature increasing, the size and fraction of η phase were observed to increase and then decrease with the morphology changing from granular to plate-like and then to granular. With the solution time prolonging, the fraction and size of η phase were increased, and the η phase formed in a cellular mode and then grow in a Widmanstätten morphology into the grain with its morphology changing from the dominant granular/plate-like to Widmanstättan structure. It was found that the increase in room temperature microhardness with the solution time changing from 6 h to 24 h was caused by the formation of Widmanstätten η phase. • The granular η phase is the transition of the plate-like η phase,having no pinning effect against the grain boundary. • The content and morphology of η phase changes with the solution temperature increasing. • The η phase forms in a cellular mode and then grow in a Widmanstätten morphology with the solution time prolonging. • The increase in microhardness with the solution time from 6 h to 24 h is closely related to the Widmanstätten η phase. [ABSTRACT FROM AUTHOR]

Microalloying of an Al-15%Mg 2 Si alloy (wt%) with 1%Cu produced an enhanced age-hardening response, increasing peak hardness and ultimate tensile strength due to a modified precipitation process. The solid solubility of Cu in Al-15%Mg 2 Si alloy effectively promotes the formation of the high number density of Guinier-Preston-Bagaryasky (GPB) zones combined with S precipitates. It suppresses β′ and β″ formation during ageing at 200 °C. Consequently, Al-15%Mg 2 Si-1%Cu alloy with the high density of GPB zones and S phase exhibited high peak hardness (121 HVN) and ultimate tensile strength (284 MPa), 42.4% and 49.5% higher than that of the Al-15%Mg 2 Si alloy, respectively. • The addition of 1% Cu in Al-15%Mg 2 Si alloy has a significant influence on precipitation during ageing at 200 °C. • Cu addition promotes the formation of GPB zones and S phase, instead of β′ and β″ precipitates. • The high number density of GPB zones and S phase produces an enhanced age-hardening response. [ABSTRACT FROM AUTHOR]

The Nb 3 (Al 1-x Sn x) superconductors (x = 0.05, 0.15, 0.25, 0.35) were prepared by mechanical alloying and subsequent low-temperature sintering. It was found that the critical current density (J c) of the obtained Nb 3 (Al 1-x Sn x) sample is enhanced significantly compared to undoped Nb 3 Al prepared using the same method. The maximum value of the J c reaches up to 5.10 × 104 A·cm−2 (4.2 K, 5 T) for Nb 3 (Al 0.85 Sn 0.15) sample. Combined with the phase composition analysis and microstructure observation, it is indicated that Sn enters the A15 crystal structure of Nb 3 Al forming the Nb 3 (Al 1-x Sn x) phase after low-temperature sintering. What's more, Sn doping improves the grain connectivity as well as refines the grain size of Nb 3 Al, which are the main reasons for the enhancement of J c in the Sn doped samples. Based on the exploration of the phase evolution of Nb–Al–Sn system during the mechanical alloying and subsequent low-temperature sintering process, it is also confirmed that Nb 3 Sn forms firstly during the sintering process and then can serve as the perfect nucleus of Nb 3 Al to increase its nucleation rate, leading to the significant decrease of the grain size of Nb 3 Al. • The J c of Nb 3 Al reaches up to 5.10×104 A·cm−2 (4.2 K, 5 T) after Sn doping. • The Sn doping sharply reduced the grain size of Nb 3 Al (from 80-200 nm to 25–40 nm). • During the low-temperature sintering process, the Nb 3 Sn formed firstly and then served as the perfect nucleus of Nb 3 Al to increase its nucleation rate. [ABSTRACT FROM AUTHOR]

Microstructure evolution, static coarsening mechanism and coarsening kinetics of a pre-deformed Ti 2 AlNb-based alloy during aging at 945 °C and 970 °C were investigated in this study. The corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results show that the pre-deformed alloy exhibited different morphologies aged at 945 °C and 970 °C. When the alloy was aged at 970 °C, the coarsening mechanism of the O precipitates was mainly controlled by the Ostwald ripening mechanism at first aging stage; after that, it was controlled by the boundary splitting mechanism and termination migration mechanism. Due to the difference in interface energy between the terminals and the long-axis direction of the O lath, diffusion process of solute atoms leads to the coarsening and fragmenting of O lath. The dimension of the O precipitates in the studied alloy continued to grow with the prolonged time during aging at 945 °C or 970 °C while the coarsening rate decreased. The static coarsening kinetic of the studied alloy was established in term of the modified Lifshitz-Slyozov-Wagner (LSW) theory. Static coarsening behavior of the pre-deformed alloy was mainly controlled by bulk diffusion at 945 °C and interfacial diffusion at 970 °C, respectively. • The thickness of O precipitates increased with aging temperature and time. • The microstructure consisted of lamellar O at 945 °C and changed from spherical to lath with aging time at 970 °C-aging. • Static coarsening behavior was controlled by Ostwald ripening, boundary splitting and termination migration mechanism. • Coarsening kinetics of the studied alloy can be established according to modified LSW theory. [ABSTRACT FROM AUTHOR]