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Al–Si–Mg alloys have received widespread application in communication, transportation and other fields on account of their attractive properties, e.g. superior castability, high ductility and excellent corrosion resistance. Comprehensive understanding of relationship between morphologies and mechanical performances under combined refiner-modifier condition is the prerequisite for the applications expansion. In this work, effect of Gd and Gd/Sr on the microstructure and mechanical properties of Al-9Si-0.4 Mg alloy was systematically studied. The results indicated that Gd had excellent refinement effect on α-Al but only partial modification on eutectic Si. Whereas simultaneous addition of Gd/Sr resulted in completely refined α-Al and modified eutectic Si. Growth of eutectic Si in untreated alloys show evidence of typical facet step growth mechanism. However, eutectic Si in Gd/Sr composite modified alloy mainly grew through impurity induce twinning mechanism and twin plane re-entrant edge mechanism, and facet step growth gradually weakened until disappearance. Based on these, both ultimate tensile strength (UTS) and elongation (El) of alloys were significantly improved and reasons were explained. [ABSTRACT FROM AUTHOR]

Transient liquid phase (TLP) bonding of IN718 Ni-based alloy and 316LN steel was carried out using BNi-2 alloy foil at 1050 °C for diverse bonding time. The formation process of intermetallics in the athermal solidification zone (ASZ) and the effects of intermetallics on the mechanical properties of joints and how they break are discussed. The effect of ASZ on the mechanical properties of joints was discussed. The analysis showed four kinds of intermetallic phases in ASZ, which are Ni23B6, CrB, Ni-Si-B, and Ni3Si. Besides, the formation of precipitates in ASZ was summarized in the following four stages: (1) γ(Ni) solid solution, (2) γ(Ni) + Ni23B6 + CrB, (3) γ(Ni) + Ni23B6 + CrB + Ni-Si-B, and (4) Ni3Si via solid-state transformation. The experimental results of mechanical properties showed that the intermetallic phases formed in ASZ cause greater stress concentration, so the mechanical properties of this kind of TLP joint are not excellent. After the shear test, the joint with a large number of intermetallic compounds at the centerline of the joint showed the characteristic of brittle fracture. In addition, SEM images of the cross section of TLP joints showed that the crack nucleates in ASZ and propagates along the centerline of the joints. When the content of intermetallics at the centerline of the joint decreases, the joint shows ductile fracture. [ABSTRACT FROM AUTHOR]

In this work, two different types of post-heat treatment processes were developed for additively manufactured CuCrZr alloy. The effect of post-heat treatment on the microstructure and properties of as-built CuCrZr alloy was studied in detail. After the direct aging of the as-built samples, the cellular substructure was preserved and a large number of fine Cu4Zr and Cr particles were precipitated in the matrix. However, after solid solution and age-hardening treatment, the grains in the sample coarsened to hundreds of microns and the annihilation of the cellular substructure occurred. Compared with the samples after solid solution and age-hardening treatment, the directly aged samples had an excellent combination of hardness (191 ± 7 HV), strength (513 ± 8 MPa) and electrical conductivity (82.2 ± 1.5 pct IACS). The high strength of the directly aged samples can be attributed to the enhancement of the strengthening mechanism of the precipitated particles and the retention of the cellular substructure. In addition, the reduction of solid solution atoms would significantly weaken the electron scattering effect, thereby improving the electrical conductivity. [ABSTRACT FROM AUTHOR]

Al-0.5Mg-0.4Si-0.1Cu alloy possessing weather resistance and oxidation resistance can obtain good surface quality (metallic luster) without chrome plating. Therefore, it is an important material to replace polluting chrome-plated aluminum profiles for automotive decorative parts. At present, studies about the extrusion process of Al-0.5Mg-0.4Si-0.1Cu alloy are very few, which affects its further application. In this work, the effect of extrusion ratios on microstructure and mechanical properties of Al-0.5Mg-0.4Si-0.1Cu alloy is investigated by optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests. The results showed that the dynamic recrystallization degree of the alloy gradually increased with the extrusion ratio increasing, which is attributed to the driving force provided by the large extrusion ratio. Meanwhile, due to the occurrence of dynamic recrystallization, the texture changed from <111> to <001> orientation. In addition, grains were obviously refined and uniform with the extrusion ratio increasing. Due to the fine grain strengthening mechanism, the tensile strength and elongation of the alloy with an extrusion ratio of 30 reached 152 MPa and 32.4%. [ABSTRACT FROM AUTHOR]

In the present work, Tungsten (W)/oxide dispersion strengthened (ODS) steel joints were fabricated by the direct solid state diffusion bonding (SSDB) technology with a multistage cooling process, and the microstructure and tensile strength of the bonded interfaces and parent materials were experimentally investigated. The results show that W and ODS steel can be successfully bonded at the temperature ranging from 900 °C to 1050 °C, without severe macroscopic deformation or obvious microscopic defects. Reaction layers generated at the bonded interfaces are evolutive with the bonding temperature, result in different fracture locations of the bonded joints. In the joint bonded at 950 °C, a higher interfacial strength of ~ 234.2 MPa is achieved, due to the formation of nano-scale intermetallic compound FeW. Microstructure of W remains stable after all the SSDB processes, while the lath structure of ODS steel is completely broken and transformed into the equiaxed grains, which should be responsible for the deterioration of strength. When the bonding temperature is higher than 950 °C, the pinning effect of precipitates M23C6 and nano-oxide particles on the movement of dislocations is observed. [ABSTRACT FROM AUTHOR]

In the present study, correlation of the microstructure and stress corrosion cracking (SCC) susceptibility in the top, middle and root regions of a multi-pass austenitic stainless steel weld joint fabricated by narrow-gap tungsten inert gas welding (NG-TIG) was investigated by slow strain rate tests (SSRT) following a microstructure characterization. The results show that from the top to root regions in both the heat-affected zone (HAZ) and weld metal (WM), the dislocation density and length fraction of the Σ3 boundaries present a decreased trend, while the grain size, length fraction of LAB and residual strain all increase. By comparing the microstructure of WM with HAZ, δ-ferrite in WM is excessive and continuous while that in HAZ is island-shaped and discontinuous. In addition, larger grain size, lower dislocation density, higher residual strain, lower length fraction of Σ3 boundaries and higher length fraction of RGB are observed in WM. As a result, the SCC susceptibility in various regions of the weld joint follows the order of root regions > middle regions > top regions and WM > HAZ. [ABSTRACT FROM AUTHOR]

In this paper, the evolution of precipitate phases in 9CrMoCoB (CB2) steel after tempering at 650 °C for different times has been investigated using transmission electron microscopy. As the tempering time prolonged from 0 to 100 h, four types of secondary phases consisting of M 3 C, M 2 X, M 7 C 3 and M 23 C 6 were identified in the experimental steel. It was found that the needle-like M 3 C carbides could be precipitated during both the air cooling process after austenization and the early stage of tempering. After tempering for 15 to 120 min, fine M 2 X particles were observed. The precipitation and growth of M 2 X were derived from the partially consumption of M 3 C carbides. After tempering for 50 h, M 2 X disappeared, while the rod-like M 7 C 3 carbides were observed to be precipitated independent of the early formed M 3 C precipitates. After tempering for 100 h, only M 23 C 6 carbides persisted, of which the number density and size kept increasing throughout 30 min to 100 h owing to their high thermostability. In addition, the precipitation mechanisms of the four types of precipitates were explained, and the relationship between the measured hardness values and microstructure was discussed. • Precipitates evolution in 9CrMoCoB (CB2) steel during tempering has been investigated. • M 3 C precipitated during both the air cooling process after austenization and the early stage of tempering. • The precipitation sequence can be given as follows: M 3 C → M 2 X → M 7 C 3 → M 23 C 6. • Hardness reduction rate of specimens decreased with the tempering time increasing. [ABSTRACT FROM AUTHOR]

Abstract The decomposition of metastable β phase and microstructural evolution of a forged Ti-22Al-25Nb orthorhombic alloys were investigated by the electron microscopes and phase identification. For the near-Ti 2 AlNb alloy, the processing temperatures were super-transus (over the transition temperature of the β phase). β phase was ordered before its decomposition and cooling rate could strongly influence the precipitation of the explored Ti-22Al-25Nb alloy. Water quenching could prevent the transformation related to diffusion and air cooling hindered the occurrence of fine lenticular O phase in the matrix. Comparison with previous researches and selected area diffraction patterns revealed that there did exist a displacive precursor that provided nucleation sites for the α 2 phase during B2 → α 2 transition. Due to the transient structure and diffusive gradient of Nb component during the transformation, a "cellular structure" was assembled and quantities of equiaxed α 2 precipitates were acquired. Simultaneously allotriomorph α 2 precipitates inclined to grow into coarse plates on the boundaries, which would have a detrimental effect on the mechanical properties of the alloy. Widmanstätten structure was formed during the (B2+α 2 +O) field and massive transformation of lenticular O phase occurred in the (B2+O) field. Highlights • Effect of cooling rates on the decomposition of B2 phase in the Ti 2 AlNb-based alloy is presented. • Precipitation and microstructural evolution during slow-cooling process are discussed in detail. • The mechanism of the formation of cellular structure was put forward and verified. [ABSTRACT FROM AUTHOR]

Ti2AlNb‐based alloys are synthesized by spark plasma sintering from pre‐alloyed and ball‐milled Ti–22Al–25Nb powder, and the structure of the alloys is regulated by aging at 800 °C for 0.5, 1, 2, and 3 h. Comparison of phase composition, microstructure, and mechanical properties are made in this study. Aging in the B2 + O phase region refers to the reversible transformation of B2 ↔ O. Complete B2 + O Widmanstätten structure is obtained in the aged alloys synthesized from pre‐alloyed powder, and the ones from milled powder contain acicular O and B2 + O Widmanstätten structure. Ball milling eliminates the B2 + O colonies, facilitates the continuous transformation of O → B2, and induces acicular O in the alloys; however, the hardness of these alloys is decreased in contrast with that of the ones synthesized from pre‐alloyed powder. Owing to the precipitation strengthening based on a large amount of fine O‐phase precipitates, the 3 h aged alloy from pre‐alloyed powder exhibits comprehensive mechanical properties with Vickers hardness of 448 ± 9 HV, ultimate tensile strength of 730 MPa, and elongation of 0.43%. [ABSTRACT FROM AUTHOR]

The hot deformation behavior of Nb–V–Ti microalloyed ultra-high strength steel was investigated by isothermal compression at 900–1200 °C with strain rates from 0.01 to 10 s−1. The microstructure evolution and precipitation behavior were studied using an optical microscope and a transmission electron microscope Results indicate that the peak stress of experimental steel increases with increasing the strain rate and decreasing the deformation temperature. The constitutive equation of hot deformation was developed with the activation energy Q being about 407.29 kJ/mol. The processing maps were also obtained to identify the instable regions of the flow behavior and to evaluate the efficiency of hot deformation. The size of dynamically recrystallized grains increases gradually with a decrease in the strain rate. Three types of carbides were identified, namely M3C, rich-Ti MC, and rich-Nb MC. With the increase of the deformation rate, the amounts of carbides increase, and the average sizes of the carbides decrease gradually. [ABSTRACT FROM PUBLISHER]

TiAlNb-based alloys synthesized at 1223 K (950 °C) by spark plasma sintering were aged at 973 K, 1023 K, 1073 K, and 1123 K (700 °C, 750 °C, 800 °C, and 850 °C), respectively. Phase composition, microstructure, and microhardness of the aged alloys were investigated in this study. Equiaxed O grains and Widmanstätten B2/ β + O laths were formed in the aged alloys, and the microhardness was improved in contrast with the spark plasma-sintered alloy without aging. The microhardness relies largely on the O-phase content, as well as the length and width of the O laths. In particular, complete Widmanstätten B2/ β + O laths, with locally finely dispersed β precipitates, were obtained in the alloy aged at 1073 K (800 °C), and the alloy exhibited the best microhardness performance. Such fine structure is due to the temperature-dependent transformations O→O + B2/ β , O→O + B2/ β , and B2/ β →O. [ABSTRACT FROM AUTHOR]

The constitutive equation was established based on the consideration of strain compensation to describe the hot deformation behavior of low carbon reduced activation ferritic/martensitic (RAFM) steels at the temperatures of 850–1050 °C and the strain rates of 0.01–10 s−1. The result indicates that the flow stress is increased with the increase of strain rate but decreased with increase of deformation temperature. During the hot deformation process, the increase of temperature is beneficial to attain the complete dynamic recrystallization (DRX). However, excessively high temperature leads to grow up of dynamic recrystallized grain. Higher strain rate leads to finer recrystallized grains. The material constants (α, n, A) and deformation activation energy (Q) are calculated by the regression analysis. The increase of strain caused the decrease of Q, indicating the DRX occurred more easily. In addition, the developed constitutive equation could accurately predict the hot deformation behavior of the low carbon RAFM steel. [ABSTRACT FROM PUBLISHER]

Cold-rolled (CR) type 347H austenitic heat-resistant steel is investigated in this study, and the main strengthening phase, NbC, experiences the procedures of dissolution and precipitation twice, with the increase of deformation degree from 30% to 90%. Most of the primary NbC particles, remained during the solution treatment, dissolve in the austenitic matrix when the deformation degree is 30%, and rod-like NbC generates along the prior austenite grain boundary. Further deformation leads to the fragmentation and decomposition of the rod-like NbC. Until the cold rolling reduction reaches 90%, the dissolved NbC secondarily precipitates into nanoparticles and disperses throughout the martensite induced by deformation. The tensile strength of the 90% cold-deformed specimen, 1418 MPa, is enhanced by three times in contrast with the undeformed one. [ABSTRACT FROM AUTHOR]

Microstructural observation and high-resolution dilatometry were employed to investigate kinetics of martensitic transformation in high Cr ferritic creep-resistant steel upon different quenching/cooling rates. By incorporating the classical athermal nucleation and impingement correction, a non-instantaneous growth model for martensitic transformation has been developed. The developed model describes austenite/martensite interface mobility during martensite growth. The growth rate of martensite is found to be varied from 1 × 10 to 3 × 10 m/s. The low interface mobility suggests that it is not appropriate to presume the instantaneous growth behavior of martensite. Moreover, based on the proposed model, nucleation rate of martensite under different cooling rates is found to be nearly the same, while the growth rate of martensite is promoted by increasing the cooling rate. [ABSTRACT FROM AUTHOR]

Sn addition improves bulk superconducting properties of FeSe 0.5 Te 0.5 with the increase of onset transition temperature and sharpens the transition width. The critical current density ( J c ) of Sn-added sample is about 7×10 3 A cm −2 in self field and maintains a stable value of 3×10 2 A cm −2 even beyond 60,000 Oe at 4.3 K, much higher than that of the Sn-free one. Phase compositions and morphology characterizations indicate that minor Sn addition facilitates the formation of the superconducting phase whose composition is closer to the ideal one, and suppresses the formation of Fe 7 Se 8 impurity and amorphous phase, which contribute to the improvement of J c . [ABSTRACT FROM AUTHOR]

To investigate the effects of cold rolling on the microstructure, the precipitation behavior and the morphology of δ-phase, Inconel 718 alloy samples with different cold rolling reductions were aged for different periods at temperatures range from 850 °C to 1000 °C. Detailed microstructural observations and quantitative measurements were conducted to characterize the evolution of the δ-phase during aging. The results show that the microstructure consists of large deformed grains as a result of a slow static recovery at the low aging temperatures (850 and 900 °C); whereas the austenite matrix is fully recrystallized at the high aging temperatures (950 and 1000 °C). It is also found that the amount of δ-phase and the number density of spherical δ-phase particles increase with the increase in the degree of cold rolling both at low and high aging temperatures. With respect to different microstructural changes for the cold-rolled samples at the low or the high aging temperatures, two distinct mechanisms have been, respectively, introduced to interpret the changes in the precipitation behavior and the appearance of δ-phase. [ABSTRACT FROM PUBLISHER]

The microstructural evolution of type 347H heat-resistant austenitic steel during long-term aging at 700–900 °C was investigated by using a transmission microscope, a scanning electron microscope, and electron energy spectrum technology. The microstructural examination showed the typical micrographs of MX carbonitrides and M23C6 carbides after aging. The existence of the Z phase (NbCrN) at the grain boundaries during aging was identified. Meanwhile, the possible precipitation sequence of these particles was also confirmed. In the beginning of aging, fine Nb(C,N) precipitates first, then, M23C6 carbides precipitate along the grain boundaries. Finally, the Z phase is also observed at the grain boundaries. Moreover, the influence of isothermal holding temperature on the precipitation of MX carbonitrides and M23C6 carbides was discussed. The various microstructural characterizations showed that the M23C6 carbides and MX carbonitrides precipitate more easily with the increase of aging temperature. Furthermore, the number and the size of MX particles and M23C6 carbides increase when the isothermal holding time is prolonged. [ABSTRACT FROM AUTHOR]

Purpose -- to investigate the transformations during aging at 200°C for different periods on microstructure and mechanical properties of the high-temperature Zn-4Al-3Mg solders. Design/methodology/approach -- The solder was melted in a resistance furnace and different cooling rates were obtained by changing the cooling medium. Subsequently, all the specimens were aged at 200°C for 20 h and 50 h, respectively. A scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) detector and X-ray diffraction (XRD) were used for the observation of microstructures and the determination of phase composition. Tensile tests and Rockwell hardness (HRB) tests were also performed. Findings -- After aging, Zn atoms precipitated from the supersaturated α-Al and the (α-Al + η-Zn)[sub eutectoid] phase with the original fine lamellar structure coarsened and spheroidized to minimize the system energy. Among these solders, the furnace-cooled alloys exhibited the highest thermal stability, largely retaining their original morphology after aging, whereas the collapse and spheroidization of the η-Zn phase and the coarsening of the η-Zn dendrites took place in the air-cooled and water-cooled samples respectively. Furthermore, a decrease in tensile strength during aging was attributed to the thermal softening effect. The variation of macro-hardness was mainly associated with the microstructural alterations in terms of quantity, morphology and distribution of soft η-Zn phase and hard intermetallic compounds (IMCs) induced by the aging treatment. Originality/value -- The structural stability of eutectic Zn-4Al-3Mg solders solidified at different cooling rates and the effect of aging on mechanical properties were investigated. [ABSTRACT FROM AUTHOR]

Oxide-dispersion-strengthened (ODS) ferritic alloys are fascinating materials for future high temperature energy production technologies. MgAl2O4 ODS alloy incorporating nanoscale oxide particles were produced by mechanical milling (MM) followed by hot isostatic pressing (HIP). The MgAl2O4 nanoscale oxide particles were formed during the HIP process by the addition of MgO and Al2O3 to the Fe–Cr matrix. Microstructural evolution of ODS alloys was structurally characterized at each step of the elaboration processes by means of scanning electron microscope (SEM), transmission electron microscope (TEM), and x-ray diffraction (XRD). The observations of structure of the mixed powders in ODS alloys after MM indicated that the initial powders, coupled with the original MgO and Al2O3 powders, got fractured by severe plastic deformation and ultrafine bcc grains (∼17 nm) of the matrix and amorphous phase composed of Mg, Al, and O were formed during MM. The main driving force for the formation of amorphous phase comes from the increase of volume fraction of bcc Fe grain boundary and the increase of interfacial energy due to the decrease in the size of MgO and Al2O3 powders. The MgAl2O4 nanoscale oxide particles formed at 1173 K which was far below the traditional sintering temperature of the raw material. And the structures of MgAl2O4 nanoscale oxide particles were observed by TEM. [ABSTRACT FROM PUBLISHER]

In order to better understand the thermal stability and mechanical properties of the nominal Ti–22Al–25Nb alloy with different initial microstructures, microstructural evolution after long-term aging, and mechanical properties at room temperature and 650 °C of four differently heat-treated Ti–22Al–25Nb alloy were investigated. The investigation results indicated that α 2 platelets formed by aging at (α 2 +B2) region were not as stable as previous research claimed and spontaneously decomposed into fine (O+β/B2) mixture during the annealing at 650 °C. As a result, the alloy's steady-state creep strain rate dropped dramatically at the secondary creep stage and tensile strength grew. On the contrary, long time aging exposure resulted in the connection of original dispersed lenticular O precipitation and thus degraded the ductility of all specimens. Besides, the discontinuous precipitation B2→β+O caused by supersaturation of solutes in the matrix also harmed the mechanical properties. Under the condition of 650 °C/250 MPa, the steady-state creep strain rates of the Ti–22Al–25Nb specimens show a significant dependence on their grain size. Crept samples showed that most dislocation motion was restrained in the β channels under a small strain and then extended into O phase when creep strain grew over 10%. Finally, by comparing the room temperature tensile property and creep resistance of four samples, supertransus solution and furnace cooling ensured the alloy most balanced mechanical properties among lamellar samples. The supertransus then furnace-cooled sample did not only have an excellent creep resistance but also kept a moderate room temperature elongation to failure of nearly 6%. • Thermal stability and mechanical properties of four different microstructures were investigated and compared. • Metastability of α 2 phase resulted in its decomposition and promoted the creep resistance of the alloy. • Grain size including equiaxed precipitation dominates creep resistance in intermediate stress regime. • Discontinuous precipitation and piling up of dislocations promoted the interfacial cracks. [ABSTRACT FROM AUTHOR]

Intercritical treatments within the dual-phase (α+γ) region were applied on HSLA pipeline steels, for acquiring a low yield ratio (YR) well balanced with desirable strength. Intercritical cooling treatment (ICT), step cooling treatment (SCT) as well as direct cooling treatment (DCT) after full austenization were designed to obtain an optima multiphase microstructure. Effects of cooling rate in DCT routine and intercritical temperatures in ICT and SCT routines on microstructural evolution and corresponding mechanical properties were also investigated. Especially, the SCT treatment applied with an intercritical temperature of 750 °C produces a microstructure composited of "soft" coarse polygonal ferrite, and "hard" acicular bainite and lath martensite containing large amounts of dislocation tangles or networks generated by deformation. Such multiple phase constituents guarantee the high strength and remarkable ductility on deformation, meanwhile cleavages propagation is hindered by the high-angle boundaries of bainite and martensite sheaves, which leads to the lowest YR ~0.61 combined with highest tensile strength among all. In addition, by using the Swift equation to elucidate the relationship between the phase component and yield ratio, it is found that simply increasing the fraction of low-temperature transformed phases, like high-strength acicular bainite and lathed martensite, or the percentage of soft polygonal ferrite for good ductility, can hardly solve the problem how to achieve ultralow- YR pipeline steels balanced with enhanced strength. The present result proves that, through utilizing the proposed SCT heat treatment on pipeline steels, an ultralow yield ratio ~0.61 achieved is synchronized with a desirable strength, which efficiently overcomes the trade-off limit between the strength and yield ratio when applying conventional heat-treatment routines. The fact indicates that, rationally adjusting the content of multi-phase microstructure through optimizing the intercritical treatment conditions, can enable us of realizing the synchronous improvement of the YR and strength in HSLA pipeline steels for real engineering. [ABSTRACT FROM AUTHOR]

The Ti 2 AlNb and Mo-modified Ti 2 AlNb alloys were fabricated by hot isostatic pressing, and then underwent annealing treatment at 800 °C for 1, 2 and 3 h respectively. To evaluate the tensile properties of the two kinds of alloys, tensile tests were conducted at ambient temperature and 650 °C in this study. For the Ti 2 AlNb alloy, the room-temperature ultimate tensile strength (UTS) was 700–900 MPa, and the highest elongation to failure (ε f) reached 6.2% after aging for 2 h at 800 °C. Since the Mo addition promoted the precipitation of the coarse B2+O colony structure near the grain boundary, the Mo-modified Ti 2 AlNb alloy exhibited decreased UTS and ε f at room temperature. As for the high-temperature performance of both the Ti 2 AlNb and Mo-modified Ti 2 AlNb alloys, the UTS reached approximately 700 MPa and the ε f exceeded 3.0%. Meanwhile, the two kinds of alloys presented the same tendency that both the high-temperature UTS and ε f increased firstly and then decreased as the aging time increased. Among them the specimens after aging at 800 °C for 2 h for both alloys acquired the highest UTS and ε f. Moreover, the Mo-modified alloy exhibited a better high-temperature ductility than that at room temperature, which is associated with the B2↔O phase transformation during the loading at 650 °C. The Mo addition effectively refined the lath of the precipitates in the grain and the grain size of the matrix, and it induced abundant dislocations. Thus, the Mo-modified Ti 2 AlNb alloy aged for 1 and 3 h exhibited superior high-temperature mechanical properties than those of the Ti 2 AlNb alloy. [ABSTRACT FROM AUTHOR]

To investigate the effect of initial microstructure on the hot deformation behavior of a Ni 3 Al-based alloy, the isothermal compression tests were conducted on the studied alloy with two different microstructures at 1100 °C and 1200 °C with strain rate range of 0.01–1 s−1. The results show that the hot deformation behavior of the Ni 3 Al-based alloy is sensitive to the initial microstructure, particularly under low deformation temperature and high strain rate. The flow curves signified that the peak stress and the magnitude of the stress drop for the alloy with coarse γ′ precipitates are higher than those for the alloy with fine γ′ precipitates at a given temperature and strain rate. Besides, it is found that the critical conditions are affected by deformation temperature, strain rate and initial microstructure. Both the critical stress and strain of dynamic recrystallization significantly decreases with the increase of strain rate and decrease of deformation temperature. For different initial microstructures, the critical stress and strain for the alloy with coarse γ′ precipitates are higher than that for the alloy with fine γ′ precipitates. The critical ratios of the studied alloy were also determined. The dissolution of coarse γ′ precipitates easily occurs and the dissolution rate can be accelerated at the sufficiently high temperature, which leads to that the effect of initial microstructure on hot deformation behavior of the Ni 3 Al-based alloy is weakened. • Hot deformation behavior of Ni 3 Al-based alloy with two microstructures was studied. • Hot deformation behavior is sensitive to initial microstructure. • Critical stress and strain were determined at various deformation conditions. • Critical conditions affected by initial microstructure, strain rate and temperature. [ABSTRACT FROM AUTHOR]

Oxide Dispersion Strengthened (ODS) steel with a composition of Fe-14Cr-2W-0.2V-0.07Ta-1Al-0.3Y2O3 was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). All of the investigations were performed on the as-SPSed ODS sample without further heat treatment. The microstructure, particles, and tensile properties of the ODS sample were analyzed. According to the results, both submicron-sized and micron-sized grains existed in the microstructure, showing a bimodal structure. The nanoparticles were homogeneously distributed in the matrix, and the nanoparticles in the ODS sample were mainly Y-Al-O composite oxides. Two kinds of Y-Al-O nanoparticles were identified: Y3Al5O12 particles with body-centered cubic structures and Y4Al2O9 particles with monoclinic structures. The size of the nanoparticles ranged from 2 nm to 61.5 nm, and the average size of the nanoparticles was 11 ± 2.7 nm. Except for the nanoparticles, large M23C6 particles that were detrimental to the ductility of the ODS sample were also identified. These large M23C6 particles possessed a long strip or rod-like morphology, and were generally distributed along the grain boundaries. The generation of cracks generally occurred in the region where the large M23C6 particles were located during the tensile test. The as-SPSed ODS steel possessed a ductile/brittle mixed fracture mode according to the fracture surfaces analysis. [ABSTRACT FROM AUTHOR]

Hot deformation tests of 14Cr oxide dispersion strengthened (ODS) steel fabricated by mechanical alloying and hot isostatic pressing (HIP) were performed on a Gleeble-1500D simulator at temperatures ranging from 1050 to 1200 °C with the strain rate range of 0.001−1 s−1. The relationship between the rheological stress and the deformation condition was studied, and a processing map at the true strain of 0.5 was proposed. Microstructure evolution during the deformation process and the effects of deformation conditions on microstructures were also investigated, as well as the stability of nanoparticles. Results show that the 14Cr ODS steel possesses positive strain rate sensitivity. The flow stress increases with the decrease of deformation temperature and the increase of strain rate. The recrystallization process is promoted by the increase of deformation temperature and the reduction of strain rate. Nanoparticles possess excellent stability during the deformation process and are coherent with the matrix. [ABSTRACT FROM AUTHOR]

Mo-modified Ti-22Al-25Nb alloys are sintered at 900 °C, 1000 °C, 1070 °C, and 1150 °C, followed by quenching and furnace cooling, respectively. We made comparisons of phase composition and microstructure between quenched and furnace-cooled alloys to interpret the phase transformation and microstructure evolution during cooling. The phase transformation was B2→O for the alloy cooled from B2+O region, and was B2→B2+O for those cooled from α 2 +B2+O, α 2 +B2, and B2 phase regions. Moreover, the morphology was B2+O colonies for those cooled from B2+O and α 2 +B2+O phase regions, and was Widmanstätten structure for those cooled from α 2 +B2, and B2 phase regions. Kinetics analysis indicated that the activation energy for the phase transformation of α 2 →B2 was 170 kJ mol−1 for Ti-22Al-25Nb, while it was 330 kJ mol−1 for Mo-modified Ti-22Al-25Nb. The retarded phase transformation in Mo-modified Ti-22Al-25Nb further led to curvy Widmanstätten structure when the alloy was furnace-cooled from single B2 phase region. The hardness of the Mo-modified Ti-22Al-25Nb alloys is up to 488 HV, while that of the Ti-22Al-25Nb alloys without Mo addition is less than 400 HV. • Diverse B2+O structures are observed in Mo-modified Ti 2 AlNb alloys. • Mo-rich kinky Widmanstätten structure corresponds to high Vickers hardness. • Kinetic analysis shows Mo addition inhibits the phase transformation of α 2.→B2. • The phase transformation during different cooling routes is reported. [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]

In this paper, the effects of hafnium (Hf) addition on the thermal stability of the microstructure and tensile properties of Al-containing 16Cr-ODS steels were investigated. High temperature aging tests of the ODS steels with and without Hf were conducted at 750 °C, 950 °C and 1150 °C for 100 h respectively. And the microstructure evolution after thermal aging was investigated using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The results indicated that Hf addition could improve the coarsening resistance of the oxide particles and the grains during aging treatment due to the formation of stable Y 2 Hf 2 O 7 particles. Tensile tests on the aged-steels showed that the ultimate tensile strength and yield strength of 16Cr-Hf-ODS steel were higher than those of 16Cr-ODS steel. It was considered that the excellent thermal stability and tensile properties of 16Cr-Hf-ODS steel were related with the refined microstructure. [ABSTRACT FROM AUTHOR]

Microalloying of an Al–15%Mg 2 Si alloy (wt%) with Ni and Cu produced an enhanced strengthening response, increasing hardness whilst also improving tensile strength due to the formation and evolution of intermetallic phases. With the increase of Cu/Ni ratios, intermetallic phases translate from Al 3 Ni or Al 3 CuNi to Al 7 Cu 4 Ni, and the total volume fraction of Al 3 CuNi and Al 7 Cu 4 Ni increases to 3.51% (1%Ni and 5%Cu, Cu/Ni ratio = 5). The thermally stable Al 3 CuNi and Al 7 Cu 4 Ni phases are connected to each other forming chains and tend to build up a network. As a result, the ultimate tensile strength (at 350 °C) of the alloy (Cu/Ni ratio = 5) reached up to 132 MPa, 131.6% higher than that of Al–15%Mg 2 Si alloy. [ABSTRACT FROM AUTHOR]

The transformation behavior of Nb–V–Ti microalloyed ultra-high strength steel during continuous cooling was investigated by Gleeble-1500 thermal mechanical simulator. Thermal dilation curves were measured at different cooling rates, based on which continuous cooling transformation (CCT) curve was established. The microstructure at slow cooling rate, primarily consists of polygonal ferrite, pearlite and lower bainite, while at intermediate cooling rate besides dominated lower bainite, small amount of polygonal ferrite can also be observed. At higher cooling rate, martensite lath is obtained. The nano-sized precipitates at slow cooling rate include M 23 C 6 , M 2 C, M 3 C, rich-Nb MC and rich-Ti MC and at intermediate cooling rate and higher cooling rate, the type of precipitates were similar, M 3 C and two types of MC. At cooling rates ranging from 0.2 to 1 °C/s, Vickers hardness of the steel decreases significantly. While at intermediate and higher cooling rates, the Vickers hardness increases gradually with increasing cooling rate. In addition, the polygonal ferrite transformation start temperatures (Ar 3 ) decreases with increasing cooling rate, which is related to the carbide precipitation. [ABSTRACT FROM AUTHOR]

In this paper, the influence of compressive deformation in metastable austenite zone (at 700, 600 and 500 °C, respectively) on microstructural characteristics and martensitic transformation behavior for the high Cr ferrite/martensite steel is investigated by using high-resolution dilatometry and transmission electron microscopy (TEM) observation. The results show that compressive deformation in metastable austenite zone results in decrease of martensite lath width and increase of martensite starting temperature ( M s ) due to increase of defection density for martensitic nucleation. The martensitic transformation time is extended by the pre-stress loading above M s , which is ascribed to decrease of interfacial migration rate during martensitic transformation by means of strain hardening. The results of tempering experiments indicate that decrease of deformation temperature leads to increase of recovery fraction. This is ascribed to that decrease of deformation temperature raises the residual strain energy and thus lowers the apparent activation energy for recovery. Hence, phenomenon of recovery occurs prematurely in the tempering specimen with the low deformation temperature. [ABSTRACT FROM AUTHOR]

Powder metallurgic Ti 2 AlNb alloys were sintered at 900 °C, 990 °C, 1060 °C, and 1100 °C (i.e. in the O + B2, α 2 + B2 + O, α 2 + B2, and B2 phase region, respectively) for 12 h, followed by water quenching and furnace cooling. Quenching was employed to reserve the high-temperature phase and microstructure, and furnace cooling aimed to regulate the room-temperature microstructure for the enhancement of hardness. Widmanstatten B2 + O structure, which contributes to the properties, was induced from B2 crystals by sintering, unless the alloy was treated in the α 2 + B2 phase region. With the elevation of the sintering temperature, the content of α 2 phase became lower in the furnace-cooled alloys, and the hardness was improved accordingly. The highest hardness performance, 389 ± 23 HV, was obtained in the alloy solution treated in the single B2 region, and the alloy was comprised of complete O + B2 Widmanstatten structure. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) steel is one of the most promising candidate structural materials for the high-temperature nuclear reactor application. In this study, two compositions of ODS steels (14Cr-ODS and 14Cr-Zr-ODS) were prepared to investigate the influence of Zr addition on the microstructures and mechanical properties of ODS steels. The microstructures, including dispersion morphology and crystal structures of oxide particles, particle-matrix interface coherency and particle-dislocation interactions, were characterized using TEM, HRTEM, and SEM, and the mechanical properties at room and high temperatures were measured using uniaxial tensile tests. Results show that Zr addition leads to the formation of finer precipitated particles, which was identified as rhombohedral Y 4 Zr 3 O 12 , with denser dispersion in the matrix. The calculation results reveal that the lattice misfit, δ , at the interface between particle and matrix increases as the particle size increases. In addition, the strength and elongation of ODS steels are improved with Zr addition due to the stronger interface bonding force between fine particles and matrix as well as the larger pinning effect of small particles to dislocation movements. [ABSTRACT FROM AUTHOR]

Microstructure evolution and mechanical performances of the Nb–V–Ti microalloyed ultra-high strength steel subjected to quenching–partitioning–tempering (Q–P–T) treatment are investigated by SEM, TEM, XRD examination and mechanical tests. After Q-P treatment, the microstructure consists of multiple phases, including lath martensite, retained austenite and small amount of twin martensite. Moreover, three types of carbides have been identified, namely M 3 C, Nb-rich MC and Ti-rich MC. By conducting tempering treatment, martensite lath is recovered, and the volume fraction of retained austenite is decreased, as well as the carbon concentration in retained austenite. M 2 C, M 7 C 3 and M 23 C 6 are precipitated at the expense of M 3 C in tempering process. In addition, part of M 7 C 3 are transformed to M 23 C 6 with the increase of carbon partitioning time, and the other two types of MC precipitates are found to be the stable phases existing during tempering treatment. In both of the samples after Q-P and tempering process, martensite lath width, the volume fraction of retained austenite and the carbon content in retained austenite increased as the partitioning time is prolonged. After tempering process, the impact toughness first increased and then decreased, and the strength, reduction of area and elongation with increase in partitioning time. [ABSTRACT FROM AUTHOR]

The microstructure, precipitates and mechanical properties of Nb-V-Ti microalloyed ultra-high strength steel under different tempering temperatures are investigated. With the increasing tempering temperature, the width of martensitic laths gradually increased with reduced dislocations within them, and the volume fraction of retained austenite first increased and then decreased. Furthermore, the Charpy impact toughness, reduction of area and elongation are enhanced at the sacrifice of the strength. Four kinds of precipitates are identified at different tempering temperatures, namely M 3 C, M 2 C, M 23 C 6 and M 7 C 3 . M 3 C precipitated when tempering at 200–400 °C, and it was replaced partially by M 2 C when the temperature is elevated to 500 °C, at which M 23 C 6 also precipitated. For a higher temperature, above 600 °C, M 3 C disappeared, and some M 2 C transformed into M 7 C 3 . The other two types of MC precipitates are found to be the stable phases existing over different tempering temperatures. The correlation between the mechanical properties and microstructure was established, and the precipitation mechanisms of carbides at the different tempering temperatures are explained. [ABSTRACT FROM AUTHOR]