Enhanced Yield Strength Research Articles

Through-Thickness Microstructures and Yield Strength Enhancement for AZ31 Mg Sheets Treated by Ultrasonic Nanocrystal Surface Modification

An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 µm from the surface) and twin-dominated (200~300 µm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} tensile twins (despite compressive strikes by the hemispherical tip), {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1}-{10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} double twins and {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1} compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress.

Mechanical behavior of thin CoCrFeNi high-entropy alloy sheet under laser shock peening

It is a challenge to optimize the mechanical performance of high entropy alloys (HEAs). Here, we obtain yield strength enhancement along with considerable tensile ductility of CoCrFeNi HEA by laser shock peening (LSP). The surface hardness of the material is also increased significantly by 40% after LSP. Based on microstructure analysis, it is observed that the grain refinement and the mixed deformation mode of dislocation slip and deformation twin are the main strengthening mechanisms after being treated by LSP. This study demonstrates an effective method for tailoring the mechanical properties of HEAs.

Interactions of nanoprecipitates, metastable austenite, and Lüders banding in an ultra-low carbon medium Mn steel with abnormal strength dependence of hydrogen embrittlement

A hierarchical microstructure, containing an ultrafine-grained matrix with mixed lamellar and equiaxed morphology, metastable austenite, and intermetallic nanoprecipitate, was designed to improve the hydrogen embrittlement (HE) resistance of ultra-low carbon medium Mn steels. Compared with the warm-rolled (WR) state, the warm-rolled-tempering (WRT) sample shows a simultaneous enhancement of yield strength (YS) and tensile ductility, but lower total elongation loss (∼3%) after hydrogen charging, thus exhibiting an abnormal strength dependence of HE. The microstructural analysis with numerical calculation proves that an additional tempering not only introduces a heterogeneous distribution of Ni(Al, Mn) precipitations, but also results in a reverse γ→α transformation with Mn partitioning, thus contributing to a self-stabilization of austenite with refined grain size and enriched element stabilizer. On one hand, the introduction of intermetallic Ni(Al, Mn) precipitations could be beneficial for the intrinsic hydrogen capture and storage, although its limited number density only contributes little to impeding hydrogen diffusion. Moreover, the introduction of nanoprecipitate can improve the strain compatibility and suppress the strain concentration in ferrite phases, which is consistent with a lower kernel average misorientation (KAM) value of the WRT sample after deformation. Meanwhile, the increased austenite stability leads to a shorter yield point elongation (YPE) and an effective transformation-induced plasticity (TRIP) effect at large strains, and the transformation product of nano-lamellar α’-martensite from self-stabilized austenite contributes little to the hydrogen-accelerated strain localization in weak α/γ interfaces of the WRT sample. In contrast, the low HE resistance of the WR sample is mainly attributed to the enhanced martensitic transformation process during the propagation of Lüders banding, which leads to a strong localized stress concentration in α/α’ interfaces, and the premature failure due to hydrogen-enhanced decohesion effect (HEDE). The interactions of nanoprecipitate, metastable austenite, and Lüders banding provide new insight into the design of high-strength steel with excellent HE resistance.

Synergistic influence of La and Zr on microstructure and mechanical performance of an Al-Si-Mg alloy at casting state

The evolution of microstructure and mechanical properties of Al-7Si-0.4Mg alloys doped with La/Zr/La+Zr was investigated by experimental characterization and thermodynamic calculation. The combination of growth restriction effect by La-rich intermetallics and inoculation effect by peritectic Zr-rich cores led to the most dramatic grain refining. Furthermore, eutectic silicon was observed to be refined from avg. 9.8 µm in length within original alloy to avg. 5.8, 8.7, and 6.4 µm under the effect of La/Zr/La+Zr additions, respectively. The length reduction in eutectic silicon was attributed to the shrunk growth space caused by grain refinement and the presence of La-rich phases, as well as the impeded effect of several microns La-rich phases distribute directly on the growth front of Si phase. The strength and ductility were both elevated in the alloys at the casting state alloyed with La, Zr, or La+Zr. Particularly, the simultaneous introduction of La+Zr led to the optimal combination properties with a yield strength of 103 ± 3.4 Mpa, an ultimate tensile strength of 189.2 ± 4.6 Mpa, and fracture elongation of 6.1 ± 0.8%. The enhancement of yield strength was related to the strengthening from grain refining and dispersed precipitates. The improved ultimate tensile strength was ascribed to the higher and more stable strain-hardening rate, resulting from the combined influence of refined microstructures (e.g. grain and eutectic Si), as well as the emergence of La-rich and/or Zr-rich intermetallics. The elevation of fracture elongation was intimately correlated with the refining in eutectic silicon, which relieved the nucleation and development of crack along the interface between silicon and Al matrix.

From waste to high strength alloy – recycling of magnesium chips

Abstract 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.

Plastic deformation behavior of ultrafine grained CoCrFeNiMn high entropy alloy with nanoparticles

High strength ultrafine grained CoCrFeNiMn high entropy alloys (HEAs) containing TiO(C) nanoparticles were fabricated by thermomechanical consolidation of a mechanically milled powder at 950 and 1000 °C, respectively. The sample consolidated at 950 °C with an average grain size of 417 nm had a high yield strength (YS) of 1457 MPa, but exhibited necking immediately after yield drop, leading to a limited elongation to fracture of 1.3%. In contrast, the sample consolidated at 1000 °C with an average grain size of 489 nm had a clearly lower YS of 1236 MPa, but exhibited yield drop, Lüders deformation, work hardening and then necking, with a significant elongation to fracture of 10.0%. Our work shows that by introducing intragranular Ni–Ti nanoparticles in the coarser grained sample through quenching to liquid nitrogen and aging, the YS of the sample was brought back to 1449 MPa, and still kept a substantial elongation to fracture of 4.2%. The transition of the plastic deformation behavior with grain refinement is analyzed based on dislocation dynamics theory. Based on this analysis, it is proposed that to maintain the uniform deformation, the grain sizes should be larger than a critical value, above which maximum flow stress of the material is higher than the upper yield stress. Introducing intragranular Ni–Ti nanoparticles is an effective strategy to further enhance yield strength without losing work hardening ability due to the synergistic effect with intragranular TiO(C) nanoparticles to inhibiting dislocation annihilation at grain boundaries.

Effects of particle size of hydroxyapatite on mechanical property and cytocompatibility of hydroxyapatite/poly(amino acid) composites

AbstractIn this work, three different particle sizes of hydroxyapatites (HA) were synthesized, and these three types of HA were combined with poly(amino acid) (PAA) to form composites (PAA/HA) by phase separation technique. XRD, FT‐IR, SEM, EDS, and XPS analyses were utilized to characterize the structure and morphology of the synthesized three HA particles and composites. Results revealed that HAs with three different particle sizes (short rod (HA1), long rod in nanoscale (HA2), and long rod in micron level (HA3)) were successfully prepared and there was an interface interaction between PAA and HA in composites. Mechanical tests showed that the difference of HA in size played an important role in mechanical performance of the composites. Furthermore, the long rod nano‐HA showed the highest enhancement in flexural yield strength at 81.3% for PAA/HA2 compared with pure PAA. After 28 days of immersion in PBS, PAA, and its composites showed good stability, and the mechanical performance of all composites were remained unchanged. The cell culture results showed all composites were cytocompatible, cells attached on the materials well with excellent morphology. Thus, it could be concluded that particle size influenced mechanical properties of PAA/HA composites, without impacting their cytocompatibility and degradation behavior.

Functionally graded materials for marine risers by additive manufacturing for high-temperature applications: Experimental investigations

In an offshore environment, marine risers are subjected to high environmental loads caused due to axial tension, wave, and current action. The hydrocarbons flowing through these marine risers put the marine riser under a continuous threat of corrosion. In this research, duplex stainless steel, a highly corrosion-resistant alloy, is functionally graded with carbon-manganese steel and the inner layer, providing structural and functional properties to the newly developed material. An experimental investigation on using functionally graded material for a marine riser application is carried out, testing its strength and durability. The mechanical properties and durability aspects are compared with that of the conventional X52 Carbon Manganese steel, a common candidate for marine risers. Wire Arc Additive Manufacturing (WAAM) is used to fabricate the FGM in the lab scale. WAAM is economical and possesses high deposition rates. ASTM E8 tensile test specimens are cut out from the FGM build to determine the mechanical properties. Based on the experimental studies carried out, it is seen that FGM shows about 11% and 25% enhanced yield strength and ultimate strength, respectively. The yield strength and elongation at a high temperature of the FGM material is close to that of X52 at room temperature. In contrast, the ultimate strength is higher, which proves its suitability for high-temperature applications. It also possesses improved durability due to the innate corrosion resistance of duplex stainless steel. The proposed FGM is seen as an effective alternate candidate for marine risers with enhanced structural properties and durability characteristics.

Heterogeneous structure-induced strength and ductility synergy of α-brass subjected to rapid cooling friction stir welding

The 2 mm-thick α-brass plates were successfully joined using conventional friction stir welding (CFSW) with air cooling and rapid cooling friction stir welding (RCFSW) with liquid CO2 cooling. The microstructure and mechanical properties of the two welds were carefully investigated by electron back-scattered diffraction and transmission electron microscopy. The stir zone of CFSW exhibited homogeneous equiaxed grains, while the stir zone of RCFSW showed a heterogeneous grain structure, i.e. ultrafine grains containing massive dislocations and nano twins. Compared with the CFSW, yield strength and ultimate tensile strength of RCFSW were increased by 31% and 24%, respectively. The enhanced yield strength and improved strain hardening capacity were attributed to grain boundary strengthening and dislocation strengthening. Furthermore, good ductility was achieved due to the released stress concentration of the nano twins caused by the plastic deformation.

Microstructural characterization and mechanical properties of nano-scale/sub-micron TiB-reinforced titanium matrix composites fabricated by laser powder bed fusion

A B4C/Ti-6Al-4V(Ti64) composite powder containing a minor B4C content (0.2 wt%) was developed by a novel technique and was subjected to the laser powder bed fusion (L-PBF) process within a wide range of laser powers and scanning speeds to fabricate titanium matrix composite (TMC) parts. The relative density measurement results revealed the achievement of almost fully dense TMC parts. Compared to the Ti64 case, slightly higher Ev values were required in the TMC system to obtain the highest relative density. Microstructural characterization of the TMC parts revealed the formation of large columnar prior β grains containing in-situ formed nano-scale/sub-micron TiB needles homogeneously dispersed in a martensitic matrix. While having almost the same ductility, the fabricated TMC parts showed 25% and 8% higher nanohardness, compressive yield strength, respectively, and 12% lower wear rate than the Ti64 sample. The improved mechanical properties of the TMC were due to the contribution of several factors, including the incorporation of nano-scale/sub-micron TiB reinforcement, the refinement of the martensite α՛ laths, and the solid solution strengthening effects of carbon atoms. The contribution of TiB presence and solid solution strengthening was ~60% and ~40% in the overall yield strength enhancement of the TMC parts, respectively.

On simultaneous enhancement in local yield strength and plasticity of short-term annealed bulk metallic glasses

The variations in local yield strength and plasticity of a Zr-based bulk metallic glass at different fictive temperatures (Tf) are studied. To probe the size and distribution of microscopic carriers of plasticity, spherical-tip nanoindentation and microindentation are employed. With nanoindentation, events signifying the departure from Hertzian elastic contact, which manifest as pop-ins in the load-displacement response and correspond to incipient plasticity, are recorded and analyzed. Results indicate that with increasing Tf, the incipient plastic event and the number of subsequent pop-ins increases. Using statistical analysis of the scatter in strength, the activation parameters for shear transformation zones (STZs) in all specimens are determined. By considering the linear accumulation of STZs to form a shear band, the increase in shear yield strength along with plasticity, with increasing Tf, is explained.

High-performance copper-matrix materials reinforced by nail board-like structure 2D Ti3C2Tx MXene with in-situ TiO2 particles

Two-dimensional (2D) nanomaterials based on MXenes have gained increasing importance in the field of strength reinforcement. However, the easy oxidation of MXenes has long been deemed unfavorable for reinforcement in high-temperature processing. Herein, high-shear mixing was used in present work for simple and efficient dispersion of MXenes in copper powders. Partial MXene sheets were also in-situ oxidized to TiO2 particles to form nail board-like structure reinforcement (NSR) during spark plasma sintering (SPS) process at 600 °C, which allowed one to significantly improve the yield strength of copper matrix by up to 70.3%. Experimental characterization and modeling simulations enabled one to identify second phase strengthening as the main reason for increased yield strength of the composites. In particular, the mechanical engagement of NSR played an important role in strengthening. Meanwhile, the hardness of TiO2 particles on NSR was found to be higher than that of the copper matrix, leading to enhancement in compressive yield strength. Also, the presence of MXene sheets on NSR slightly reduced the resistance of copper matrix composites. In sum, these findings look promising for designing novel MXene reinforced metal matrix composites with improved mechanical properties.

Nano-scale heterogeneity-driven metastability engineering in ferrous medium-entropy alloy induced by additive manufacturing

Selective laser melting (SLM) offers unprecedented advantages in fabrication of metals and alloys with complex geometry and unique microstructural features with hierarchical heterogeneity. The SLM process also induces a unique cell structure with high dislocation density and solute segregation at cell boundaries. Here, we propose an innovative utilization of unique dislocation network to achieve superior mechanical properties through metastability engineering of ferrous-medium entropy alloy (FeMEA). While the high dislocation density at cell boundaries contributes to the improvement of yield strength as additional barriers of dislocation movement, the solute segregation at cell boundaries can beneficially control the phase instability of matrix in materials produced by SLM. Our results demonstrate that solute segregation at cell boundaries decreases the face-centered cubic phase stability in the matrix and activates transition of the deformation mechanism from slip to metastable plasticity (i.e., transformation-induced plasticity). Furthermore, the high density of dislocation at cell boundaries also has an effect on not only yield strength enhancement but also controlling kinetics of metastable plasticity, and it beneficially contributes the high ductility of the SLM-processed FeMEA. This work presents a new microstructural design strategy for beneficially customizing the material performance of high-quality products based on SLM-driven metastability engineering of metallic materials.

Combination of enhanced strength and sufficient tensile ductility in a sintered ultrafine-grained CoFeMnNi medium-entropy alloy

A CoFeMnNi medium-entropy alloy (MEA) was prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The mechanically alloyed powders were sintered to near-full density and the as-sintered material consists of FCC matrix comprising a minor distribution of oxide particles. A high twin density is observed in MEA bulk samples. The twins are not generated during high-energy ball-milling but formed in the recrystallized grains during the sintering process. Compressive and tensile tests were performed on the sintered specimen at room temperature. The tension-compression yield asymmetry is weakened with increasing sintering temperature. An ultrafine-grained microstructure with an average grain size of 535 nm is obtained at a sintering temperature of 1000 °C. The sintered MEA exhibits an enhanced yield strength of 740 MPa, which is by 244% higher than that of its coarse-grained counterpart, meanwhile maintaining a sufficient tensile ductility of 14.4%. These excellent mechanical properties mainly stem from the high relative density, fully recrystallized ultrafine grains, and precipitation of fine particles that significantly strengthen the material while simultaneously providing high strain-hardening capability.

Achieving superior strength-ductility balance in a novel heterostructured strong metastable β-Ti alloy

In metastable β-Ti alloys, the primary deformation mechanisms, including ordinary dislocation plasticity (ODP), deformation twinning (DT), and phase transformation, are primarily controlled by the stability of the β phase. Based on the Bo-Md diagram, here, we prepared a novel metastable β-Ti alloy with a nominal composition of Ti-1.8Al-7.8Mo-3.7Cr-6.7Zr, and further manipulated its microstructural heterogeneities (in terms of the grain size and the dislocation density) via thermomechanical processing to achieve a controllable activation sequence of deformation mechanisms. It was unveiled that in the materials with heterogeneous laminated structures (HLS), ODP was the first deformation mechanism to be activated, whereas, in the gradient equiaxed grained (GEG) alloys, as well as those with heterogeneous bimodal structures (HBS), DT was found to precede ODP. Compared with the GEG alloys, which exhibited an inferior strength-ductility combination (yield strength ∼639 MPa, total elongation ∼18.6%, uniform elongation ∼5%), both alloys with heterostructures, the HLS and HBS samples, manifested simultaneous enhancement in yield strength and tensile ductility. The former, with a high pre-existing dislocation density, showed an outstanding total elongation of ∼34% (uniform elongation ∼18%). The latter, with a large back stress, displayed, on the other hand, ultra-high yield strength of ∼910 MPa. This heterostructure-induced property enhancement was rationalized by a newly proposed strengthening model, which takes into account the grain size distribution and considers both the isotropic and the kinematic hardening. The present results imply the combination of dislocation engineering and heterostructure design being an effective strategy to achieve excellent strength–ductility balance in metastable β-Ti alloys.

Improved Mechanical Properties of Two Twinning‐Induced Plasticity Steels with Novel Grain Morphology

The mechanical properties and microstructures of two high Mn austenitic steels with novel grain morphology prepared by directional solidification and warm forging are investigated and compared with common twinning‐induced plasticity (TWIP) steel with equiaxed grains (EGs). The directional solidified sample (DS) is composed of columnar grains with simplified grain boundaries mainly characterized by low‐angle boundaries (LABs) with a fraction of 0.886. The warm forged sample (WF) contains numerous fibrous‐liked grains and fine nearly equiaxed grains featured with dominate LABs and complicated dislocation substructure. The significantly improved elongation and energy absorption capability of DS compared with EG are ascribed to the larger dislocation mean free path together with better plastic deformation compatibility and synergy between and within grains of DS, respectively. The largest energy absorption efficiency with enhanced yield strength of WF is mainly attributed to numerous fine grains with dominated LABs and complicated dislocation substructure. Inhomogeneous deformation characterized by well‐developed twins structure with dense dislocation pileup occurs in some regions of WF. It will contribute to enhancing work hardening, leading to a relatively high uniform elongation and significant increased strength. On the contrary, it may easily cause severe stress concentration and promote the occurrence of necking followed by plastic instability.

Investigation on the corrosion behavior of single-phase and binary-phase Mg-Sc alloys: An experimental and first-principles study

Microstructural characteristics and corrosion behaviors of the single-phase and binary-phase Mg-xSc (wt%, x = 5, and 15) were investigated. Both single and binary phases Mg-Sc alloys display typical peritectic microstructure, where Sc-depleted areas are surrounded by Sc-rich zones. The MgSc phases are prone to precipitate in Mg-rich zones of Mg-15Sc, which is beneficial to the enhancement of yield strength. To analyze the corrosion behavior, experimental and first-principles studies were conducted. The single-phase Mg-5Sc alloy shows the lowest weight loss rate with 0.6 mg/cm2/day, which is one order of magnitude smaller than that of binary-phase Mg-15Sc alloy. The potential difference, the cathode H2 evolution kinetics, and the surface layer containing Sc2O3 are believed to be the main reasons for the corrosion behavior difference of single-phase and binary-phase Mg-Sc alloys.

Grain-size dependent elastic-plastic deformation behaviour of inconel 625 alloy studied by in-situ neutron diffraction

The effect of grain size on elastic-plastic deformation and twinning behaviour of Inconel 625 alloy was studied. Alloy samples were investigated using compressive deformation analysis, in-situ neutron diffraction, electron backscatter diffraction, and transmission electron microscopy. The alloy was found to exhibit strong elastic and plastic anisotropy. Grain refining was found to have several advantages, including the increased grain-specific diffraction elastic moduli, improved compatibility and homogeneity of polycrystalline deformation, and enhanced yield strength at room temperature. Two strong preferred orientations were present in coarse-grained samples: the Copper orientation of {112}<111>, in deformed grains because of dislocation slip, and the Brass orientation of {110}<112>, in elongated grains owing to deformation twinning. The coarse-grained samples also showed large quantities of stacking faults, multiple-slip lines, and dislocations. In contrast, stacking faults were not observed in fine-grained samples, however, a dense presence of slip lines, dislocations, and dislocation pile-ups at grain boundaries were observed. The fine-grained samples exhibited a higher density of dislocations than the coarse-grained samples given the same applied load during compressive deformation. The deformation mechanisms of the coarse-grained alloy were dominated by dislocation slipping and the formation of stacking faults, while the deformation of the fine-grained alloy showed only dislocation slipping.

Gradient nanotwinned CrCoNi medium-entropy alloy with strength-ductility synergy

In this study, a high-strain rate ultra-precision machining technology named single point cubic boron nitride turning is developed to fabricate a gradient nanotwinned CrCoNi medium entropy alloy layer. The grain size of the ~ 150 µm-thick gradient layer is gradually refined from the original ~ 17 µm to ~ 25 nm in the topmost surface, exhibiting a significantly enhanced yield strength (from ~ 450 MPa to ~ 1100 MPa) and well-retained ductility of ~ 27%. High-resolution transmission electron microscope and atomistic simulations were mainly performed to unveil the size-dependent twinning mechanisms governing the gradient refinement process from the core to the topmost surface, i.e. transiting from the parallel twins segmenting ultrafine grains, twin-twin intersections refining rhombic blocks and rotating the intersected nanograins, and finally to the zero-macrostrain deformation nanotwinning in the refined nanograins. The machining process provides sufficient equivalent stress to activate the twinning partials for forming the gradient nanotwinned structure.

Performance analysis of TIG welded dissimilar aluminium alloy with scandium added ER5356 filler rods

ABSTRACT The use of aluminum alloys has become far greater than ever with increasing needs for lighter and stronger structures leading to new aluminum alloys with rare earth admixtures. Scandium (Sc) is one such rare earth mineral used as an admixture for aluminum alloys. This study investigates the effect of using Standard ER5356 filler rod and Sc-added ER5356 filler rods in tungsten inert gas (TIG) welding dissimilar aluminum alloys AA5083-H111 and AA5754-H111. The investigations found that the presence of Sc during the welding process is a significant contributor toward enhanced grain refinement and producing dissimilar aluminum alloy with defect-free joint with improved tensile strength and hardness. In comparison, the TIG-welded joint using Sc-added filler rod exhibited enhanced yield strength of 128 MPa to the 117.5 MPa obtained for TIG weld without Sc in the filler rod. The microhardness evaluated by Vicker’s Hardness test displayed improved hardness of 124 HV1 due to the presence of Sc and 76 HV1 without presence of Sc during TIG welding.