Slip behavior and microstructure evolution caused by the indentation pile-up deformation were studied in Mg single crystal with {10–12} pre-twin structure at room temperature. The anomalous ...indentation is formed on the surface of the large-size single crystal. It is attributed to plastic flow induced by the combined effects of dislocation slip, detwinning and {10–12} twinning during indentation. At the early stage of indentation, basal slip dominates the pile-up deformation. And the reaction structure of basal slips, i.e. (0001) slip plane in the twin to correspond to (0001) slip plane in the matrix, is frequently formed on the twin boundary in order to facilitate plastic relaxation of the local stress on the twin boundary. Under pile-up pattern, the stress state favors detwinning. And detwinning behavior is subjected to effects of stress-field magnitude and pre-twin size. For the little-volume and large-volume pre-twins, detwinning behaviors play the modes of twin shrinkage and “local detwinning” respectively. The latter might involve reverse nucleation and growth of matrix. Although pile-up pattern is unfavorable to {10–12} twinning, stress concentration caused by the interaction between slip dislocations and twin boundary induces the local twin growth on the interface. In addition, a pyramidal slip is found in matrix, and its slip trace is corresponding to (0001) slip trace in twin.
The microstructure evolution of the γ′ precipitates in GH4720Li superalloy during the heat treatment processes of water quenching and subsequent aging at temperatures of 760 °C, 800 °C, and 850 °C ...was investigated with particular emphasis on the variation in the morphology, size, and size distribution of the precipitates and related mechanisms. Influence of aging temperature and time on the nucleation, growth, and coarsening behavior of the bimodal dispersion of secondary and tertiary γ′ precipitates were discussed. Results showed that only spherical-shaped secondary γ′ precipitates with sizes ranging from 10 to 50 nm formed during the cooling process of quenching. During aging, the nucleation of tertiary γ′ precipitate, growth of both secondary and tertiary γ′ precipitates follow the typical solid-diffusion-assisted precipitation mechanism, while the abnormal coarsening of the secondary precipitates occurs through coalescing adjacent particles. Aging temperature has a less effect on the precipitation of the tertiary γ′ precipitate but substantially influences the coarsening of the secondary γ′ precipitates. At higher aging temperatures such as 850 °C, the abnormal coarsening of the secondary γ′ precipitates, which always takes place at 760 °C and 800 °C, are significantly suppressed. The aging time mainly influences the growth rate of the secondary γ′ but shows little effect on the evolution of the tertiary γ′ phase. Precipitates strengthen the alloy through strong coupling interaction mechanism. An optimal aging strategy heating at 850 °C for 8 h is proposed for the GH4720Li superalloy, yielding the best uniform dispersion of the precipitates, maximum hardness of the sample, and being considered time and energy-saving.
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•The bio-model of microstructure evolution of the γ′ precipitates in GH4720Li superalloy during the heat treatment processes was investigated.•The nucleation, growth, and coarsening behavior of the bimodal dispersion of secondary and tertiary γ′ precipitates were discussed.•Aging temperature has a lesser effect on the precipitation of the tertiary γ′ precipitate but substantially influences the coarsening of the secondary γ′ precipitates.•The aging time mainly influences the growth rate of the secondary γ′ but shows little effect on the evolution of the tertiary γ′ phase.
The tensile creep behavior of a heat treated β-solidified γ-TiAl alloy has been investigated at 800 °C under 250–300 MPa. Results indicate that the heat treatment procedure proposed in this study can ...improve creep resistance of the alloy. Combined with kinetic analysis (stress exponent n = 3.36) and TEM observation, it can be inferred that creep deformation is dominated by viscous dislocation glide, and assisted by mechanical twinning. It is found that both twin boundary and TiB can hinder dislocation movement and strengthen creep resistance. During creep, there are obvious dynamic recrystallization (DRX) and B2 phase precipitation. On the one hand, DRX volume fraction increases with the increase of creep stress or the extension of creep time. Meanwhile, TiB can act as nucleation sites of recrystallized γ grains. On the other hand, there are two kinds of B2 phase precipitation behaviors: (ⅰ) thin B2 laths are formed in α2 lamellae; (ⅱ) blocky B2 phases are formed at colony boundary. Creep deformation is mainly completed by γ phase, B2 phase can involve in the deformation only under specific creep conditions. Due to poor deformation ability of B2 phase, creep voids prefer to nucleate at the γ-B2 interface and B2-colony interface. When voids are connected, cracks are formed and begin to propagate, eventually leading to creep failure in brittle-ductile mixed fracture.
β-solidified γ-TiAl alloys are popular lightweight structural materials well suited for high temperature applications, however, whose service temperature limit is greatly restricted due to the ...insufficient high temperature strength and creep resistance. In present work, a hybrid reinforced β-solidified γ-TiAl alloy was successfully developed with the hybrid addition of C and Y2O3 by induction skull melting (ISM) technique, accordingly, the microstructure, high temperature tensile and creep properties of the alloy are detailly discussed. The results indicate that the hybrid addition of C and Y2O3 can improve the high temperature tensile strength of this nearly lamellar TiAl alloy with the increase of about 13.7%. More importantly, creep resistance of this alloy is significantly enhanced, which is attributed to multiple strengthening mechanisms: (ⅰ) the second phase strengthening from multi-scale Y2O3 particles and dynamically precipitated H–Ti2AlC carbides; (ⅱ) mechanical twins and twin intersections can act as special obstacles to dislocation movement, which is of great significance in decreasing the steady-state creep rate. Particularly, this alloy exhibits excellent microstructure stability during creep. On the one hand, nano-scale Y2O3 and dynamically precipitated H–Ti2AlC particles could pin the B2/γ interface to strengthen lamellar colony boundary. On the other hand, dispersed B2 phase particles could precipitate along α2 lamellae and grow up with the consumption of the α2 phase, which will inhibit the degradation of α2/γ lamellae. This study offers a new composition design approach to optimize high temperature performance of β-solidified γ-TiAl alloys.
•A hybrid reinforced β-γ TiAl alloy with excellent high temperature strength and creep resistance was developed.•Multiple strengthening mechanisms of the investigated alloy were revealed.•The role of blocky B2 phase existed at colony boundary during high temperature deformation is elaborated.•Ti2AlC tends to nucleate heterogeneously at the interface under higher creep stress.•Precipitation behavior of B2 phase from α2 lamellae during creep is insensitive to stress.
In order to investigate the effects of reinforcements on the flow behavior of two-scale network-structured (TiBw + Ti5Si3)/TA15 composites, hot compression tests were conducted at temperatures ...ranging from 890 °C to 980 °C in α+β region, with strain rates varying from 1 s−1 to 0.001 s−1. The microstructure evolution and softening mechanisms during the hot deformation were analyzed. Results show that the primary α phase (αp) first undergoes rotation and then globularization. The globularization processes of αp include boundary splitting in thin lamella and continuous dynamic recrystallization (CDRX) in thick lamella. TiBw and Ti5Si3 promote the dynamic recovery (DRV) of β phase and globularization of the αp by providing higher storage energy and nucleation sites. Deformation heat, DRV of β phase, rotation (in early stage) and globularization (in late stage) of αp contribute to the flow softening. Additionally, the bending, fracture, and rotation of reinforcements along with their accelerated influence on matrix softening further promote flow softening.
•A comprehensive processing map is proposed for additive manufacturing of metals.•Additively manufactured microstructures are developed during and after solidification of melt pool.•Formation ...mechanisms of multistage microstructures are analyzed, accordingly the microstructure control methods are proposed.
As a revolutionary industrial technology, additive manufacturing creates objects by adding materials layer by layer and hence can fabricate customized components with an unprecedented degree of freedom. For metallic materials, unique hierarchical microstructures are constructed during additive manufacturing, which endow them with numerous excellent properties. To take full advantage of additive manufacturing, an in-depth understanding of the microstructure evolution mechanism is required. To this end, this review explores the fundamental procedures of additive manufacturing, that is, the formation and binding of melt pools. A comprehensive processing map is proposed that integrates melt pool energy- and geometry-related process parameters together. Based on it, additively manufactured microstructures are developed during and after the solidification of constituent melt pool. The solidification structures are composed of primary columnar grains and fine secondary phases that form along the grain boundaries. The post-solidification structures include submicron scale dislocation cells stemming from internal residual stress and nanoscale precipitates induced by intrinsic heat treatment during cyclic heating of adjacent melt pool. Based on solidification and dislocation theories, the formation mechanisms of the multistage microstructures are thoroughly analyzed, and accordingly, multistage control methods are proposed. In addition, the underlying atomic scale structural features are briefly discussed. Furthermore, microstructure design for additive manufacturing through adjustment of process parameters and alloy composition is addressed to fulfill the great potential of the technique. This review not only builds a solid microstructural framework for metallic materials produced by additive manufacturing but also provides a promising guideline to adjust their mechanical properties.
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In this work, Al3BC/Al composite with Al3BC fraction of 50 % (50-Al composite) was fabricated by a reactive hot pressing sintering process. Microstructure evolution of the composite with different ...sintering parameters has been systematacially investigated. Sub-micron Al3BC particles are in situ synthesized and distributed uniformly in the Al matrix. The mechanical properties of the 50-Al composite has been characterized by nanoindentation and micropillar compression tests. The hardness and elastic modulus of the 50-Al composite can reach up to 4.79 GPa and 138.7 GPa, respectively. The micropillar compressive strength of the composite is measured to be 1234 MPa with a shrinkage rate of 17.5 %. The excellent strengthening effect of Al3BC particles on Al matrix is also revealed. The Zigzag slip bands observed on the compressed pillars indicate a impeding effect of Al3BC on the slip process, which contributes to the high mechanical properties of 50-Al composite.
•High fraction Al3BC/Al composite was fabricated by a reactive hot pressing sintering.•The ultrahigh strength and modulus of the composite has been fulfilled.•The deformation behavior of the composite is revealed by micropillar compression.
The release and transformation mechanism of residual stress contributes to the improvement of metal materials' processing and performance, enhancing their structural stability, surface integrity, and ...fracture resistance. This paper focuses on the Ultrasonic Impact Treatment-Solid Particle Entrainment by Waterjet (UIT-SPEWJ) composite surface modification of 7075-T6 aluminum alloy, investigating the effects of UIT-SPEWJ process parameters (jet pressure and target distance) on the alloy's surface quality, roughness, microhardness, residual stress, and the evolution mechanism of microstructure. The results indicate that the 7075-T6 aluminum alloy modified by UIT-SPEWJ mainly exhibits a "meteorite crater" effect, accompanied by significant smearing and ploughing phenomena. Surface roughness shows a trend of first decreasing and then increasing with the rise of jet pressure and target distance. The minimum surface roughness, 0.852 μm, is achieved at a jet pressure of 25 MPa and a target distance of 7.5 mm. The microhardness of the samples modified by UIT-SPEWJ gradually decreases from the surface to the substrate with increasing depth. The hardened layer depths of the samples UIT-SPEWJ-1–6 after modification are approximately 174 μm, 226 μm, 200 μm, 196 μm, 176 μm, and 172 μm, respectively. After UIT, SPEWJ, and UIT-SPEWJ surface modifications, the surface of 7075-T6 aluminum alloy mainly exhibits compressive residual stress (CRS). The surface residual stress σ_srs of samples modified by UIT and SPEWJ are approximately 195.161 and 215.752 MPa, respectively. The surface residual stresses of UIT-SPEWJ-1–6 samples are significantly improved to 277.089, 529.499, 393.615, 459.261, 368.084, and 220.635 MPa, respectively, with UIT-SPEWJ-2 sample having the highest surface residual stress, which is 2.7 and 2.4 times that of UIT and SPEWJ modified surfaces. Samples modified by UIT-SPEWJ present significant dislocation phenomena, and the precipitated phases mainly include the needle-like metastable phase η', as well as the equilibrium phase η (MgZn2) with a rod-like structure. At lower jet pressures and larger target distances, the 7075-T6 aluminum alloy exhibits a continuous PFZ with a size of approximately 12–23 nm. As jet pressure increases and target distance decreases, discontinuous grain boundary precipitate bands appear, sized around 8–17 nm. Moreover, when the jet pressure is 25 MPa and the target distance is 7.5 mm, the grain boundary precipitated phases are more dispersed.
•The synergistic strengthening mechanism of ultrasonic impact and water jet is studied.•The release and transformation mechanism of residual stress induced by ultrasonic impact during jet strengthening process are analyzed.•The microstructure evolution mechanism under the composite strengthening process was studied.
The corrosion and passive behavior of the as-cast AlxCrFeNi3−x (x = 0.6, 0.8, 1.0 in molar ratio) high entropy alloys in 3.5 wt% NaCl solution at room temperature was investigated by the ...electrochemical impedance spectroscopy, potentiodynamic polarization measurement, Mott-Schottky measurement, X-ray photoelectron spectroscopy tests, and scanning Kelvin probe force microscopy. The increase of the Al content resulted in the decrease in corrosion resistance, which might be attributed to the reduction of stability and protection of passive films caused by composition difference. Furthermore, the p-type and n-type semiconductor properties of passive films were discussed.
•The corrosion and passive behavior of the as-cast eutectic high entropy alloys in 3.5 wt.% NaCl solution was investigated.•The relationship between the composition of alloys and corrosion resistance was presented.•Semiconductor properties and chemical compositions of passive films were studied.
TiAl-based alloy is an excellent high-temperature structural material, but poor hot workability has been the bottleneck hindering its development. In this study, we take into account the hot ...workability and mechanical properties of Ti-48Al alloy through microstructure adjustment. Powder metallurgy Ti-48Al alloy prepared by vacuum sintering is used as the raw material, which has a near-γ microstructure composed of equiaxed grains. Although the composition is simple, the alloy achieves high mechanical properties after forging with 75 % deformation. Controlling the forging temperature to 1300 °C can make the microstructure transform into a duplex structure composed of γ/α2 lamellar colonies and γ equiaxed grains. As-forged alloy has an ultimate tensile strength of 650 ± 17 MPa and a yield strength of 644 ± 16 MPa at room temperature. It still maintains high strength at high temperature, with ultimate tensile strength of 684 ± 19 MPa at 500 °C, 700 ± 19 MPa at 700 °C, and 320 ± 10 MPa at 900 °C. In addition, the alloy has good ductility with elongation of 58.2% due to dynamic recrystallization at 900 °C.
•Mechanical property is significantly improved by forging with simple composition.•The alloy has ultrafine equiaxed grain of 2.45 µm and lamellar spacing of 20–50 nm.•Duplex microstructure without remnant lamellae forms in PM Ti-48Al alloy.•Ti-48Al alloy exhibits great plasticity of 58.2 % at 900 °C.
