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.
In this study, nanostructured WC-10Co4Cr coatings were prepared by an improved HVOF (high velocity oxy-fuel) process, with the average grain size of WC grains in as-sprayed coatings being less than ...0.26 μm. It was observed that the segregation of the two phases of Co/Cr and dislocation slip of WC along the {101¯0} crystallographic planes were present in the sprayed coatings. The Co phase underwent a martensitic transformation during the heat treatment process, with the orientation of the two phases following the SN (Shoji-Nishiyama) criterion. The 900 °C heat-treated FCC-Co structure exhibits the presence of twinning boundaries within the grains, which serves to facilitate the absorption and movement of dislocations. During the cooling process, the fcc structure does not undergo complete transformation into the hcp structure, resulting in the coexistence of FCC-Co and HCP-Co at different heat treatment temperatures. During heat treatment, the Cr phase in the coating forms a carbide of Cr23C6 with carbon, which decomposes at higher temperatures and precipitates again during cooling. The amorphous phase of the coating was transformed into nano carbides and twins by heat treatment, and the woven network of three-dimensional carbides effectively enhanced the micromechanical properties of the coating.
•Nanostructured WC-10Co4Cr coatings with a low degree of decarburisation were prepared.•Heat treatment promotes amorphous phase transformation in the coatings and leads to the generation of annealed twins.•Two structures of crystalline Co phase exist in the coating after heat treatment with a special orientation relationship.•The Cr element generates Cr23C6 carbides along the WC grain boundaries.•Heat treatment enhances the micromechanical properties of the coating.
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.
A single-phase or simple-structured alloy does not always possess outstanding combinations of strength and ductility over a wide range of temperatures and strain rates for engineering applications. ...In the present work, a high-entropy alloy with multi-heterogeneous microstructures was in-situ fabricated via powder-plasma-arc additive manufacturing. The compressive behavior of the additive manufactured high-entropy alloy over a wide range of temperatures and strain rates was studied, using an improved split Hopkinson bar system and electronic universal testing machine. It shows exceptional combination of strength and ductility within the selected temperature and strain rate ranges. Microstructural evolution was characterized at various temperatures and strain rates, providing insight into the intricate relationship between microstructure and property. The multicomponent Laves phase is hard yet deformable, while the multicomponent FCC phase is soft and ductile. The deformation twins observed all over the selected temperature and strain rate ranges and dynamic recrystallization appearing at high temperatures in the FCC phase enhance the ductility of the FCC phase and rise the crack-arresting capability. The third-type strain aging occurs at different strain rates, which shifts to a higher temperature range as strain rate increases. Ta and impurity atom, Si, acting as “solute atoms” form atom atmosphere and silicide, pinning the moving dislocations in the FCC phase. Finally, a deformation mechanism map was proposed over a wide temperature and strain rate range. The study explored a potentially new avenue to design alloys with exceptional combinations of strength and ductility over a wide range of temperatures and strain rates.
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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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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.
It has been a challenge to efficiently make titanium alloy thin-walled components by sheet metal forming. Incremental sheet forming (ISF) method provides a promising way to increase the formability ...of low-ductility metallic sheets, but making titanium alloy sheet panels still needs heating by electricity or laser. The present work investigates the deformation mechanism in fabricating truncated cones of titanium alloy with tooth features realized by flexible free incremental sheet forming (FFISF) at room temperature. Experimental investigations on auxiliary sheets and tool path selections, analytical modeling, finite element simulation, and microstructure characterization have been conducted to evaluate the deformation mechanism in terms of the geometric deviation, thickness distribution and microstructure evolution of TA2 and TC4. Results indicate that the I-O loading path coupled with an optimized auxiliary sheets selection can ensure the successful fabrication of designed panel without defects. An analytical model is proposed to predict the free edge dependent law of thickness distribution in FFISF, indicating the material thinning is positively correlated with the distance from the free edge, while the thinning rate is inversely proportional to the yield strength. A more uniform thickness of TC4 panel is obtained compared with TA2. Finally, the improved formability of titanium alloys by FFISF can be attributed to grain subdivision, decreasing of intragranular deformation, and optimization of dislocation movement.
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•Obtained truncated cone with tooth features of titanium alloy by FFISF at room temperature.•Determined optimal process set with feasible auxiliary sheet set and I-O loading path.•Found out free edge dependent thickness distribution against Cosine law.•Revealed homogenous deformation with less intragranular deformation for improved formability.
