In this study, a design strategy employing stress-induced β to hexagonal α′ martensitic transformation to develop a novel Ti-15Nb-5Zr-4Sn-1Fe alloy with ultrahigh strain hardening rate (~6.1 GPa) is ...reported. Microstructural investigations reveal that the superior strain hardening response is contributed by stress-induced β to α′ transformation with accompanying reversion of athermal ω and profuse internal {1¯011}<101¯2> and {112¯2}<112¯3> α′ mechanical twinning which promote twin-twin interactions, imparting high strength and outstanding uniform elongation. This study may serve as a template for developing new class of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) Ti alloys based on β to α′ martensitic transformation.
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Effect of scandium (Sc) additions on the microstructure, mechanical properties and fracture behavior of Al–Si–Mg casting alloy (F357) were systematically investigated. It was found that Sc addition ...caused a multi-refining efficiency on the microstructure of as-cast F357 alloy, including refinement of grains and secondary dendrite arm spacing (SDAS), modification of eutectic Si and harmless disposal of β-Al5FeSi phase. Subsequent T6 heat treatment had further induced the complete spheroidization of eutectic Si and precipitation of fine secondary Al3Sc dispersoids in the Sc modified alloys. Thus the mechanical properties, especially the ductility, were significantly enhanced by the addition of Sc combined with the heat treatment. The highest ultimate tensile strength, yield strength and elongation were achieved in 0.8wt.% Sc modified F357 alloy combined with T6 heat treatment. Furthermore, fractographic examinations indicated that the ductile fracture mechanism served as a dominate role in the modified alloys due to the formation of fine, deep and uniformly distributed dimples.
•Detailed characterization of the multi-refining microstructure of Sc modified F357 alloy was performed.•The multi-refinement was proposed to refine grain and SDAS, modify eutectic Si and β-phase.•Sc modifier combined with T6 treatment is effective in improving tensile properties.•Modification of eutectic Si in F357 alloy with Sc is consistent with the IIT mechanism.
The microstructure and tensile properties of Al–Si–Cu–Mg alloys with different Cu contents and Cu/Mg ratios were investigated. The results show that the Mg2Si, θ-Al2Cu and Q-type intermetallic phases ...form in the as-cast Al–Si–Cu–Mg alloys depending on the Cu content and the Cu/Mg ratio. The Mg2Si is formed in the relatively low Cu/Mg ratio of alloy, while high Cu/Mg ratio promotes the formation of θ-phase and Q-phase. With the same Cu/Mg ratio, the high content of Cu and Mg elements simultaneously enhances the formation of θ-phase and Q-phase. Microstructural observation from the T6 tempered alloys reveals that the Cu content and the Cu/Mg ratio control the dissolution of primary intermetallic phases and the formation of precipitates, thus affecting the strength and the ductility. It is indicated that by controlling the Cu and the Cu/Mg ratio, the Al–6Si–2Cu–0.5Mg aged alloy with few primary compounds and a large number of fine Q′ precipitates exhibits high strength and acceptable ductility.
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•The Cu content and Cu/Mg ratio affect the formation of intermetallic compounds.•The Cu content and Cu/Mg ratio control the dissolution of primary intermetallic phases and the formation of precipitates.•The Al–6Si–2Cu–0.5Mg aged alloy exhibits high strength and acceptable ductility.
In order to investigate the roles of long period stacking ordered (LPSO) structure and Zn solute in the hot deformation behavior of Mg-Gd-Zn alloys, the Mg-12Gd-1Zn-1Mn-0.5Zr alloy containing LPSO ...phase or Zn solute was hot compressed under various deformation conditions in comparison with the Zn-free Mg-12Gd-1Mn-0.5Zr alloy. The results showed that the Zn-containing alloys had stronger softening effects, larger peak stress and higher average activation energy than the Zn-free one within the safe deformation domain. Both LPSO phase and Zn solute in α-Mg matrix increased the efficiency of power dissipation, narrowed down the flow stability regions of processing map and impeded the dynamic recrystallization (DRX), especially when Zn solute was mainly in α-Mg matrix. Furthermore, the LPSO phase or Zn solute in α-Mg matrix promoted the dynamic precipitation. A few Mg5Gd particles can be observed in the Zn-free alloy, while a large number of stacking faults (SFs) and LPSO phase were precipitated in the Zn-containing alloys, and they were eventually transformed into Mg5Gd particles at low strain rate.
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•Two Zn-containing alloys had higher peak hardness and stronger softening effect.•Both LPSO phase and the Zn solute in α-Mg matrix reduced the flow stability regions.•LPSO phase and the Zn solute in α-Mg matrix impeded the dynamic recrystallization.
In order to clarify the possibility of Zr substitution for Sc on the modification of Al-Si casting alloys, the microstructural evolution and tensile properties of Al-Si-Mg based alloys with different ...combinations of Sc and Zr contents (Sc+Zr=0.5wt.%) were systematically investigated. It was found that 0.5wt.% Sc addition could refine the microstructure significantly and modify the eutectic Si from plate-like morphology to fiber, which promotes the spheroidization of eutectic Si during heat treatment. When Zr was added to partly replace Sc, the microstructure was first further refined, but was then slightly coarsened with increasing Zr content. Moreover, high Zr content was found to decrease its modification on eutectic Si. It was observed that Zr can also concomitantly improve strength and ductility compared with the alloy modified by Sc only. The improvement of mechanical properties was attributed to microstructural refinement, particularly the modification of eutectic Si and precipitation of secondary nano-scale Al3(Sc1−xZrx) dispersoids.
The engineering tensile stress–strain curves of T6 tempered F357 based alloys with different combinations of Sc and Zr additions. Note that the F357 alloy with 0.3wt.% Sc and 0.2wt.% Zr additions has the best mechanical properties in terms of strength and ductility. Display omitted
•The combined Sc and Zr additions refined the microstructure and modified the eutectic Si.•Tensile properties of F357 alloy were enhanced by Sc and Zr additions.•Enhanced ductility is due to modification of eutectic Si and microstructural refinement.•The fracture mechanism of Al–Si–Mg modified by Sc and Zr is established.
It has been well accepted that the martensites in quenched carbon steels exhibit two typical morphologies which are closely dependent on the carbon content, i.e. lath martensite in low carbon steels ...and lenticular martensite in high carbon steels. Based on conventional belief, the lath martensites in low carbon steels are with high density dislocations as the substructure, in contrast to twin substructure in lenticular high carbon martensite. In the present work, an intensive transmission electron microscopy investigation was made to characterize the microstructures of the lath martensite in a low carbon steel of 0.2 wt%C. It was found that lots of lath martensites consist of twin as their substructure, rather than high density dislocations. In addition, nanoscale precipitates cohering with ferrite matrix were found at the twin interfaces. The orientation relationships between the precipitates and the ferrite matrix are in good agreement with that of primitive hexagonal ω phase in titanium alloys and other bcc metals or alloys.
In this study, an equal-atomic FeNiCoCu high entropy alloy (HEA) and a Ti and Al added (FeNiCoCu)86Ti7Al7 HEA were subjected for high pressure torsion (HPT) up to 10 rotations. Microstructure ...observation and mechanical properties test revealed that significant grain refinement as well as enhanced strength could be obtained in both HPT processed alloys. The HPT processed FeNiCoCu HEA alloy shows nanocrystalline structure consisting of FCC matrix (grain size ∼100 nm) and FeCo-riched BCC phase. The HPT processed (FeNiCoCu)86Ti7Al7 HEA alloy shows nanocrystalline structured FCC matrix (mean grain size ∼50 nm) and refined NiCoTiAl-riched particles (mean particle size ∼0.71 μm). The ultimate tensile strength of the HPT processed FeNiCoCu and (FeNiCoCu)86Ti7Al7 alloys are 1402 MPa and 1849 MPa, respectively. The microstructure evolution during HPT and strengthening mechanisms of the HPT processed specimens were discussed.
•Nanocrystalline (FeNiCoCu)1−xTixAlx HEAs can be prepared by high pressure torsion.•The HPT FeNiCoCu HEA has a high tensile strength and a high ductility.•Minor Ti and Al additions significantly strengthen the as-cast and HPT alloys.
A high-strength Mg-7Y-5Sm-0.5Zn-0.3Zr alloy was developed successfully by hot extrusion and subsequent aging treatment. The results showed that the as-solutionized alloy mainly contained α-Mg matrix ...and 14H LPSO phase distributed at grain boundaries. Lamellar LPSO and γ′ phases were dynamically precipitated during hot extrusion and retained after artificial aging treatment at 200°C. After peak-aged treatment, the alloy exhibited excellent mechanical properties with ultimate tensile strength of 465 MPa, 0.2% proof stress of 413 MPa and elongation to failure of 6.5% at room temperature. The high yield strength was attributed to the combined action of 14H-LPSO phase, basal γ′ precipitates and dense distribution of β′ precipitates at prismatic plane.
An abnormal texture with c axis of the grains parallel to extrusion direction (ED) was found in extruded Mg–Y-Sm-Zn-Zr alloy. The mechanisms for the formation of this abnormal texture were ...investigated based on the dynamic recrystallization (DRX) mechanisms and deformation modes during extrusion using electron backscatter diffraction (EBSD) and a viscoplastic self-consistent (VPSC) model. The microstructure evolution during extrusion indicated that the abnormal //ED texture was dominated by DRX grains. With the strain increasing, the intensity of this texture enhanced. Based on the EBSD results analysis, discontinuous dynamic recrystallization (DDRX) played the dominated role in nucleation of the new grains at the initial stage and then continuous dynamic recrystallization (CDRX) was activated at high strain. The formation of abnormal //ED texture was attributed to the activation of <c+a> slips and it could promote the rotation of c-axis of grains to ED, which contributed to the formation of //ED texture. The simulated texture predicated that the CRSS of basal slip was higher than that of pyramidal <c+a> slip, which played an important role in contributing the formation of the //ED texture component.
•The extruded Mg–Y-Sm-Zn-Zr alloy exhibited an abnormal texture with c-axis of most grains parallel to the extrusion direction(ED).•The formation of abnormal //ED texture was attributed to the activation of <c+a> slips, which promoted the rotation of c-axis of grains to ED.•The activation of <c+a> slips resulted from the reduction of the differences between various critical resolved shear stress (CRSS) slips and suppression of basal slip by precipitates.•The simulated texture predicated that the CRSS of basal slip was higher than that of pyramidal <c+a> slip.
Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical ...properties was investigated. The alloy annealed at 673 K or 773 K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.
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•This study systematically investigated the effect of annealing temperature on microstructure and mechanical properties.•This study analyzed the kinetics of β grain growth in Ti–38Nb-0.2O alloy.•The dependence of critical stresses of SIM and dislocation slip on grain size was comparatively analyzed.
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