We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys ...(TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe50Mn30Co10Cr10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe20Mn20Ni20Co20Cr20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects.
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Magnesium is the lightest constructional metal, which makes it an important material for different applications like automotive, transportation, aviation and aerospace. There are several studies ...about developing properties of existing Mg alloys and introducing new alloy systems to industrial producers. An important way to improve properties of metallic materials is to decrease grain size that results almost in increasing all kind of properties of the material. This review paper aims to summarize the literature about grain refining of magnesium alloys. The text is consisting of three sections, which focused on the (1) grain refining methods used in the past, which are not used today, (2) grain refining methods currently being used in the industry and (3) novel and newly developed methods that may find usage in the industry in future. Before explaining grain refining methods of magnesium alloys a general summary about grain refinement of metals is also provided.
The formation of the coarse columnar crystal structure of Ti-6Al-4V alloy in the process of additive manufacturing greatly reduces the mechanical performance of the additive manufactured parts, which ...hinders the applications of additive manufacturing techniques in the engineering fields. In order to refine the microstructure of the materials using the high intensity ultrasonic via the acoustic cavitation and acoustic flow effect in the process of metal solidification, an ultrasonic vibration technique was developed to a synchronous couple in the process of Laser and Wire Additive Manufacturing (LWAM) in this work. It is found that the introduction of high-intensity ultrasound effectively interrupts the epitaxial growth tendency of prior-β crystal and weakens the texture strength of prior-β crystal. The microstructure of Ti-6Al-4V alloy converts to fine columnar crystals from typical coarse columnar crystals. The simulation results confirm that the acoustic cavitation effect applied to the molten pool created by the high-intensity ultrasound is the key factor that affects the crystal characteristics.
A permanent magnetic stirring (PMS) facility which was fabricated for investigating the influence of PMS on the solidification of Al-4Cu and 2024 Al alloys is described. It was found that PMS can ...effectively refine the grain structures of these alloys with the finest grain structure about 100μm in grain diameter. PMS at 600rpm has a similar grain refining effect to adding grain refiner. At the stirring rates over 600rpm, too big gas holes are induced into the ingots. The cracks on the ingot side surface and elongated holes on the ingot cross section caused by unidirectional PMS can be avoided by applying a direction-reversing PMS. PMS can lead to a slightly more uniform eutectic phase distribution in 2024 Al alloy without changing eutectic phase types. A new nucleation mechanism based on the big-bang nucleation theory is proposed to explain the grain refining effect and limit of PMS.
The present work focuses on the friction stress and the Hall-Petch relationship in CoCrNi equi-atomic medium entropy alloy (MEA). The CoCrNi equi-atomic MEA and a Ni-40Co alloy were processed by ...high-pressure torsion and subsequent annealing. The specimens with fully-recrystallized microstructure with different grain sizes ranging from 199nm to 111μm were obtained. The Hall-Petch plot of the yield strength of the present specimens indicated that the friction stress of the CoCrNi MEA was much higher than Ni-40Co and pure metals, suggesting that the local lattice distortion in the equi-atomic alloy played an important role for the dislocation activity.
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Ultrafine grained magnesium alloy was synthesized via mechanical milling of AZ91 chips. Mechanical property measurement revealed enhanced yield strength of 470 MPa after mechanical milling. The ...increase in yield strength is associated with reduction in grain size which restricts twining and dislocation gliding. The present investigation demonstrates that magnesium alloys can be cost-effectively recycled through the process of mechanical milling.
The indentation of bearing raceways frequently contributes to bearing failures, as in gearboxes. The dents act as surface stress raisers through the Rolling Contact Fatigue (RCF) cycles, leading to ...crack initiation and spalling. The microstructural evolution mechanism occurring is not completely understood. Hence, to enhance bearing lifetime, it is crucial to identify microstructural parameters that influence crack initiation and propagation. Therefore, this study presents a multi-scale characterization of martensitic 100Cr6 bearings before and after RCF. It includes SEM, SEM/FIB, EBSD, TKD and ACOM-ASTAR observations. It showed that, before RCF, the bearing raceway features a 0.5 µm thick surface layer consisting in refined martensite and spread primary carbides, resulting from finishing operations. After RCF, no further grain refinement was observed along the raceway, except beneath the dent shoulder where a material plastic flow, nano-grains and ultra-fine grains of martensite, and sheared and spread primary carbides were detected up to 1.4 µm deep. Below and up to 3 µm deep, a fine grain layer is observed. The martensite morphology, size and disorientation suggest that it undergoes refinement through continuous Dynamic Recrystallisation and twinning. Crack initiation was preferentially observed in the nano-grains layer, at primary carbides/matrix interfaces. Above the crack, another refined region is observed resulting from high deformation and stresses through crack propagation. Finally, even in severely deformed areas, nanosized retained austenite islands were detected, indicating that it is not completely transformed.
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The grain refinement mechanism of Al-7Si-0.7Mg alloy by Sc and Ti have been systematically investigated at different cooling rate. The Si-poisoning effect have been avoided by synergistic inoculation ...of Sc and Ti at special where slow cooling rate before solidification is required. The inoculation of Sc and Ti avoid the Si-poisoning effect by forming a new phase of (Al, Si)3(Ti, Sc). The gradient solidification is adopted to make the (Al, Si)3(Ti, Sc) phase reappear at fast cooling rate to avoid Si-poisoning. The average grain size of Al-7Si-0.7Mg-0.15Ti-0.5Sc alloy solidified through gradient solidification is refined to 131 µm decreased 45 % compared with traditional cast. The mechanism for anti Si-poisoning is also investigated using first-principles calculation and High-Angle Annular Dark-Field STEM.
Data will be made available on request.
•The poisoning effect of Si on Al3Ti have been tackled.•D022-(Al, Si)3(Ti, Sc) are observed in Al-Si-Mg-Sc-Ti alloy.•D022-(Al, Si)3(Ti, Sc) can act as the heterogeneous nucleus of α-Al.•A new casting technology, gradient solidification, makes grain finer.•The mechanism for anti Si-poisoning is investigated.
Grain refining can improve the mechanical properties and solidification-cracking resistance of the weld. Ultrasonic grain refining was conducted by dipping an ultrasonic probe in the weld pool to ...stir it at a distance behind the arc. This new approach produced effective grain refining in arc welds of Mg alloys AZ31 Mg and AZ91 Mg. Grain refining increased when the probe was positioned farther behind the arc. This suggests the initial crystallites or dendrite fragments generated by ultrasound in a cooler melt farther behind the arc were better able to survive. This also suggests dendrite fragmentation was more likely to occur because the probe was closer to the mushy zone. However, a probe too far behind the arc ended up being inside the mushy zone and grain refining, though highly effective, was restricted to only near the weld centerline. At the same probe position, grain refining increased with increasing ultrasound amplitude. Grains were significantly finer in AZ91 Mg welds than AZ31 Mg welds. This suggests grain refining increased with increasing constitutional supercooling caused by the higher solute content of AZ91 Mg than AZ31 Mg.
Grain refining can improve the mechanical properties and solidification-cracking resistance of the weld. Ultrasonic grain refining with a probe dipped in the weld pool at a distance behind the arc produced effective grain refining in Mg alloys AZ31 (∼Mg–3Al–1Zn) and AZ91 (∼Mg–9Al–1Zn). Grain refining was found to increase with increasing offset, oscillation amplitude, and alloying content. However, with an excessive offset the probe can be located inside the mushy zone and grain refining, though very effective, can be limited to near the weld centerline. Dendrite fragmentation was the most likely grain refining mechanism. Display omitted
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