Enhanced Yield Strength Research Articles

Heat treatment behavior and strengthening mechanisms of CNT/6061Al composites fabricated by flake powder metallurgy

The carbon nanotubes reinforced 6061Al composites (CNT/6061Al) with CNTs uniformly dispersed were fabricated by flake powder metallurgy process. The mechanical properties of composites, with and without artificial heat-treatment, were compared to the unreinforced 6061Al alloy. It was found that higher content of oxygen was introduced with increasing CNTs and consumed the magnesium element, which had a negative effect on the precipitation strengthening mechanism. Thus, the strength improvement of CNT/6061Al composites by heat treatment was less significant than the monolithic 6061Al alloy. However, the T6-aged CNT/6061Al composites still exhibited enhanced yield strength and ultimate tensile strength than the monolithic 6061Al alloy. This was attributed to the synergistic strengthening of multi-mechanisms, such as precipitation strengthening, grain refinement strengthening, load transfer behavior of CNTs and especially, CNT-induced back stress strengthening and the strain hardening. As results, the aged CNT/6061Al composites with 1.5 wt% CNTs had balanced tensile strength of 469 MPa and uniform elongation of 6%.

Deformation microstructure and tensile properties of Alloy 709 at different temperatures

Alloy 709 austenitic stainless steel is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of deformation mechanisms on Alloy 709 and of tensile response of aged Alloy 709 is lacking. In this study, thus, the tensile behaviour of as-received and aged Alloy 709, their deformation microstructures and failure mechanisms, have been investigated at room temperature (RT), 550, 650 and 750 °C. Aging brought about the formation of particles at grain boundaries and interior of grain, thus leading to enhancement of yield strength but reduction in ductility. The ultimate strength of both materials is strongly temperature dependent, which clearly decreases with temperature. It is caused by the decreasing strain hardening ability, dynamic strain aging and dynamic recovery together with dynamic recrystallisation at different temperatures.

Exceptional combination of soft magnetic and mechanical properties in a heterostructured high-entropy composite

High-entropy alloys (HEAs) represent a class of metallic materials that provide new pathways to attain heretofore unachievable combinations of physical and mechanical properties. However, published studies are focused on either physical properties or mechanical properties, rather than combinations thereof. Here, a “heterogeneous microstructure” was designed, via in situ formation of nano-sized TiC particles, to achieve enhancements in both mechanical and soft magnetic properties in a FeNiCo-based HEA. When compared to the HEA matrix counterpart material, a dramatic enhancement in tensile yield strength, with a negligible loss in fracture strain was obtained in the heterostructured high-entropy composite (HS-HEC). In addition, its soft magnetic properties were also enhanced as evidenced by a remarkable improvement in coercivity. Our results confirm that the HS-HEC displays a heretofore unattainable combination of soft magnetic and mechanical performance when compared to those of previously studied HEAs. This exceptional combination of soft magnetic and mechanical properties was primarily attributed to the presence of a heterogeneous microstructure containing dispersed TiC particles. The results presented here validate the hypothesis that a heterogeneous microstructure provides a powerful strategy to tailor the soft magnetic and mechanical response of HEAs.

Hydrogen embrittlement in metallic nanowires

Although hydrogen embrittlement has been observed and extensively studied in a wide variety of metals and alloys, there still exist controversies over the underlying mechanisms and a fundamental understanding of hydrogen embrittlement in nanostructures is almost non-existent. Here we use metallic nanowires (NWs) as a platform to study hydrogen embrittlement in nanostructures where deformation and failure are dominated by dislocation nucleation. Based on quantitative in-situ transmission electron microscopy nanomechanical testing and molecular dynamics simulations, we report enhanced yield strength and a transition in failure mechanism from distributed plasticity to localized necking in penta-twinned Ag NWs due to the presence of surface-adsorbed hydrogen. In-situ stress relaxation experiments and simulations reveal that the observed embrittlement in metallic nanowires is governed by the hydrogen-induced suppression of dislocation nucleation at the free surface of NWs.

Defects in graphene nanoplatelets and their interface behavior to reinforce magnesium alloys

Defects in graphene nanoplatelets (GNPs) and their interface behavior to reinforce magnesium alloys was studied by using two types of GNPs, one with high integrity and the other with multiple defects, as reinforcements in ZK60 alloy. Herein, GNPs prepared by chemical exfoliation exhibited multiple defects, such as carbon atom vacancies, dangling carbon bonds, oxygen-containing functional groups and wrinkles. During the composite fabrication, Mg atoms restored the sp2 structure of graphene and reacted with the oxygen-containing groups on graphene, leading to a strong interfacial bonding. Meanwhile, graphene defects scattering in nanoscale triggered nucleation of Mg nanograins, which connect the “hard” GNPs and “soft” Mg matrix for effective transfer of stress and strain. The defective GNPs (0.05 wt%) reinforced ZK60 alloy with large tensile yield strength enhancement (65%) and especially remarkable elongation improvement (44%). This work highlights the structural feature of GNPs to reach their full strengthening potential as reinforcements in metal matrix composites.

Strength-toughness improvement of martensite-austenite dual phase deposited metals after austenite reversed treatment with short holding time

To obtain a better strength-toughness balance of martensite-austenite dual phase deposited metals, an austenite reversed treatment (ART) with short holding time (namely S-ART), combining with the advantage of quenching & partitioning (Q&P) is proposed. The dual phase deposited metal is heated to the intercritical region (620 °C) for austenite reversion with a short holding time of 0.5 h. Compared to the conventional ART (C-ART) with the holding time of 2.0 h, S-ART provides significant improvement in yield strength, elongation and impact toughness. Due to the stronger partitioning effect of carbon, the carbon concentration of the retained austenite (RA) during S-ART is higher than that of C-ART and as-welded. Therefore, the stability of the RA in S-ART state increases dramatically and induces sustained transformation induced plasticity (TRIP) effect, which contributes to the enhancement of yield strength, ductility and toughness. Moreover, after S-ART, the volume fraction of RA increases and the dislocation density in martensite decreases significantly, further improving the ductility and toughness.

Microstructure Analysis and Mechanical Properties of LM25 alloy with AC4B Nano-Composite using Gravity Die Casting Method

Casting of Aluminium alloys with zero defects would be the ultimate aim of any manufacturer. In the present work, the Alloy LM25 which was being used for the manufacturing of intricate Automobile components is compared to Alloy AC4B Nano-composite using Gravity Die Casting method. In order to investigate the mechanical properties of the concerned materials, Tensile followed by Hardness, and Compressive tests was implemented. Microstructure investigation is examined using Scanning Electron Microscope. The outcome from micro structural study confesses the uniform distribution, grain refinement, and low porosity in Nano-composite specimen. Mechanical results affirm that addition of B4C Nano-composite leads to enhancement of yield strength, ultimate strength and hardness. The uplift in mechanical properties manifested that the type of fabrication process and particle size involved in it.

Effect of Asymmetric Rolling on the Microstructure, Mechanical Properties and Texture Evolution of Mg–8Li–3Al–1Y Alloy

Asymmetric rolling was applied to dual phase Mg–8Li–3Al–1Y alloy with different speed ratio and rolling reduction. With increasing reduction at the same speed ratio the population of β-Li grains decreased with a feature of fibered grains along the rolling direction. As for different speed ratio under the same reduction, β-Li grains kept equiaxed, but α-Mg phase was first elongated and fibered and then transited to equiaxed small grains. The enhanced yield strength and tensile strength were achieved with the increase of rolling reduction and speed ratio. In particular, when speed ratio reached to 1:1.3 under the same rolling reduction (30%), the optimal mechanical properties were obtained. As for texture evolution, we found the basal texture was obviously weakened and some new weak texture was formed after asymmetric rolling. Interestingly, it was noted that the texture on the side of slow roll is distinct from that on the side of the fast roll due to the different rolling speed.

Enhanced tensile and bending yield strengths of 304 stainless steel and H62 brass by surface spinning strengthening

In this work, the tensile and bending yield strengths of 304 stainless steel and H62 brass are effectively enhanced by surface spinning strengthening (3S) that can introduce a strengthening layer with gradient microstructure on the samples. The results show that the two metals exhibit distinct grain refinement in the top layer as well as severe grain deformation in the subsurface layer with obvious gradient characteristics. For the tensile yield strength, it increases from 336 MPa to 372 MPa for 304 stainless steel, and 280 MPa–294 MPa for H62 brass, respectively. For the bending yield strength, it increases from 581 MPa to 667 MPa for 304 stainless steel, and 496 MPa–523 MPa for H62 brass, respectively. A two-phase (soft and hard phases) structure for the surface strengthened metals is proposed to analyze the enhancement of the tensile yield strength, and the enhancement of tensile yield strength is determined by the maximum microhardness and the thickness of the hardened layer. The enhancement of bending yield strength may be attributed to the maximum microhardness and the corresponding nano-scale grains in the topmost layer. Based on the results above, the tensile and bending yield strengths can be improved and optimized by constructing the gradient microstructure with different grain refinement and microhardness distributions.

Microstructure refinement of Mg-Al-RE alloy by Gd addition

The present study investigated the effect of Gd on the microstructures and mechanical properties of permanent mould cast Mg-4Al-5RE (wt%) (where RE represents La-Ce mischmetal) alloy. Coarse grains and acicular Al11RE3 phases in Mg-4Al-5RE alloy are successfully refined by the addition of 0.66 wt% Gd. The average grain size is dramatically decreased from 493.9 μm to 205.8 μm, and the acicular Al11RE3 phase is modified to rod-like sharp with a pronouncedly decreased average length from 22.0 μm to 3.8 μm. The modified microstructures are attributed to the formation of Al2(Gd, RE) particles, which act as nucleation sites for both α-Mg and Al11RE3. Eventually, the modified alloy exhibits dramatically enhanced yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) about 12%, 23% and 48%, respectively.

Influence of Heat-Treatment on Enhancement of Yield Strength and Hardness by Ti-V-Nb Alloying in High-Manganese Austenitic Steel

To deal with the problem of poor yield strength and hardness in the initial use of high-manganese austenitic steel, we investigated the alloying design, microstructure, precipitates, mechanical properties, and comprehensive strengthening mechanism of high-manganese austenitic steel through two novel heat-treatment processes, namely continuous heating process (CHP) and segmented heat preservation process (SHPP). In this work, austenitic Fe-0.9C-17Mn-0.8Si-2.0Cr-0.3Ni-0.5Cu-0.7Mo steels alloyed with Ti, V, and Nb were designed. The grain size of SHPP steels was smaller than that of CHP steels due to the smaller size of precipitates. The results of mechanical experiments showed that the yield strength and impact toughness of SHPP steel were obviously higher than those of CHP steel, but the Brinell hardness of CHP steel was higher than that of SHPP steel. The higher Brinell hardness and poorer impact toughness of CHP steel were mainly due to the larger-sized precipitates. Finally, solid-solution strengthening played the most effective role of increasing the yield and tensile strengths of the two steels.

Analytical and experimental investigation on magnetorheological behavior of CoFe2O4-rGO-incorporated epoxy fluid composites

CoFe2O4-reduced graphene oxide (CoFe2O4-rGO) composites were synthesized by one-pot solvothermal technique. Different amount of well-dispersed nanosized cubical grains of CoFe2O4-rGO were incorporated into the epoxy resin for the preparation of fluid composite samples. The magnetorheological properties of the prepared samples showed significant enhancement in yield strength as well as viscoelastic properties. The flow curves of the samples showed non-Newtonian behavior at lower strain rate (γ˙ < 10 s−1) under high magnetic field strength, thereafter with increasing the value of γ˙, the flow curves shifted Newtonian regime. The composite samples also possessed a yield stress (τy) under a magnetic field at adequately high strain rates and followed Bingham relation $$ \tau ={\tau}_{\mathrm{y}}+\overset{\hbox{'}}{\gamma }{\eta}_0 $$ , where, τ is shear stress perpendicular to the applied magnetic field and η0 is the plastic viscosity. The experimentally obtained values of τy for different values of B0 and φ correspond well to the values predicted theoretically by Rosensweig.

Effect of TiB2 particles on microstructure and crystallographic texture of Al-12Si fabricated by selective laser melting

Pronounced crystallographic texture, characteristic for the parts manufactured by selective laser melting, is eliminated upon addition of TiB2 microparticles to Al-12Si powder. Due to the wetting of TiB2 ceramic by Al-12Si melt and a high inoculation efficiency, a homogeneous microstructure consisting of fine equiaxed grains with random crystallographic orientation is formed by SLM processing of the Al-12Si/TiB2 powder mixture. The Al-12Si/TiB2 samples exhibit enhanced yield strength and microhardness, compared to the TiB2-free Al-12Si samples produced at the same SLM conditions.

Formation of the elliptical texture and its effect on the mechanical properties and stretch formability of dilute Mg-Sn-Y sheet by Zn addition

All the time, it is difficult for magnesium alloys to equilibrate the relationship among strength, ductility and stretch formability at room temperature. Here, we added 0.6 wt% Zn into Mg-0.4Sn-0.7Y (TW00, wt%) alloy attempting to achieve a good combination among strength, ductility and stretch formability at room temperature. After extrusion, we found that Mg-0.4Sn-0.7Y-0.6Zn (TWZ000, wt%) alloy sheet exhibited a weak elliptical texture, accompanied with significant grain and second phase refinement. The formation of this weak elliptical texture was mainly originated from the joint contributions of multiple dynamic recrystallization (DRX) mechanisms consisted of twin-, compound- and dislocation slip-induced DRX behaviors. Grain boundary strengthening and second phase strengthening were mainly dedicated to the enhanced yield strength (~ 150.0 MPa) and ultimate tensile strength (~ 235.1 MPa), while non-basal slip, grain boundary cohesion, texture modification and twins commonly guaranteed the high ductility (~ 39.8%). Besides texture modification and non-basal slip, a large number of twins altered the initial microstructure along with the formation of new twinning textures during stretch forming. These new twinning texture facilitated the higher Schmid factor (SF) for basal slip beneficial to the enhancement in stretch formability (~ 7.0 mm). Therefore, TWZ000 alloy sheet provided an enormous potential to broaden magnesium alloys applications.

Ultrastrong Medium-Entropy Single-Phase Alloys Designed via Severe Lattice Distortion.

Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium- and high-entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid-solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation-mediated plasticity effectively enhances the strength-ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra-strong materials for structural engineering applications.

On the microstructure, mechanical properties and wear resistance of an additively manufactured Ti64/metallic glass composite

Selective laser melting (SLM) provides flexibility in creating novel metal-matrix composites (MMCs) with unique microstructures and enhanced mechanical properties over conventionally manufactured MMGs. In this study, a Zr-based metallic glass (MG) decorated Ti6Al4V (Ti64) composite with a unique hybrid nanostructure and enhanced mechanical properties and wear resistance was fabricated using SLM. The results revealed that a near-full dense and crack-free Ti-based composite was produced, with its reinforcements consisting of ultrafine β dendrites set with partially crystallized MG nanobands uniformly distributed along the boundaries of the melt pool. The addition of MG significantly affected the solidification behavior of the Ti-liquid because of its higher dynamic viscosity and density as well as compositional effect on the phase stability. With such a unique nanostructured reinforcement, the Ti64/MG composite exhibited an enhanced yield strength (>1 GPa) with reasonable ductility and fracture toughness. On the basis of the result of a theoretical analysis, we attributed the main strengthening mechanism to Orowan strengthening. The wear resistance was also much improved in the Ti64/MG composite, arising from the higher hardness of the nanostructured reinforcement and the formation of a more protective tribo-oxide layer during sliding. The confinement of the 3D distributed reinforcement phase played a crucial role in preventing the delamination of the tribo-layer on the matrix. This work opens a pathway to the design of novel additively manufactured MMCs with outstanding mechanical properties.

Microstructure and enhanced strength of laser aided additive manufactured CoCrFeNiMn high entropy alloy

The CoCrFeNiMn high entropy alloy (HEA) bulk alloy was successfully manufactured using laser aided additive manufacturing (LAAM). The microstructure and mechanical behaviors of the as-fabricated HEAs were investigated. The as-built HEAs exhibit directional solidification at regions close to the melt-pool boundaries, forming dendritic columnar grains and transiting to equiaxed grains further away from the boundaries. Compared with the conventionally cast HEAs, the CoCrFeNiMn fabricated using LAAM possesses significantly higher yield strength (518 MPa) and ultimate tensile strength (660 MPa). The strengthening effect is attributed to finer grains and could be explained quantitatively through grain boundary strengthening. The as-built HEA shows a simultaneous enhancement in yield strength and ductility with decreasing testing temperature. The improved low temperature tensile properties could be ascribed to the formation of deformation twins at low temperature, which results in a steady strain hardening behavior. This work demonstrates the potential of using LAAM technology to extend the application of HEAs with fabrication of larger and more complex parts with good mechanical properties.

Microstructure-property relationships of 420 stainless steel fabricated by laser-powder bed fusion

In this study, we report the mechanical properties, microstructure and corrosion behavior 99+ % dense 420 stainless steel parts fabricated by laser-powder bed fusion (L-PBF). As-printed L-PBF parts exhibited an ultimate tensile strength of 1050 ± 25 MPa, yield strength of 700 ± 15 MPa, an elongation of 2.5 ± 0.2% and a hardness of HRC 55 ± 1. After the heat treatment operation at 315 °C, the ultimate tensile strength improved significantly to 1520 ± 30 MPa, yield strength increased to 950 ± 20 MPa, and elongation increased to 6.3 ± 0.2% respectively whereas hardness remained at HRC 53 ± 1. These properties are higher than previously reported literature values for 420 stainless steel fabricated by L-PBF, metal injection molding and powder metallurgy routes. The microstructure indicated a mixture of martensite and retained austenite phases in the as-printed parts. After heat treatment, the microstructure was richer in tempered martensitic laths that is consistent with the enhancement of ultimate tensile strength, yield strength, and elongation without appreciable change in hardness. The corrosion behavior of the as-printed L-PBF parts exhibited a corrosion current of 2.85 ± 0.4 mA.cm−2, a polarization resistance of 17,100 ± 520 Ω.cm−2 and a corrosion rate of 28 ± 2 μm/year which were comparable to the corrosion properties of wrought 420 stainless steel. A significant increase in pitting potential (170 mV) was obtained after the heat treatment.

Plastic deformation behavior and microscopic mechanism of metastable Ti-10V-2Fe-3Al alloy single crystal pillars orientated to β in submicron scales Part II: Phase transformation dependence of size effect and deformation mechanism

Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along <011>β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity.

Strain‐Induced Martensite and Reverse Transformation in 2304 Lean Duplex Stainless Steel and its Influence on Mechanical Behavior

Duplex stainless steels (DSS) combine the mechanical properties of ferrite (α) with the corrosion resistance of austenite (γ), and is widely used in industrial applications. The γ can transform into martensite (SIM) during a deformation. When annealed, these types of steel undergo a reverse transformation, namely, SIM reversed to austenite (SIMRT). The aim of this study is to evaluate SIM and SIMRT in 2304 lean DSS (LDSS) as well as its influence on the mechanical properties after a 60% reduction in thickness. The annealing is conducted within a range of 600–900 °C with a soaking time of 1800s. The samples undergo X‐ray diffraction, EBSD, and a tensile test. The results show that, after cold rolling, the amount of α′‐martensite formed is made up 24%. The α′‐martensite collaborates with the enhanced yield strength and reduces the total elongation. The orientation among γ and α′‐martensite, and (111)γ//(110)α [110]γ//[111] Kurdjumov‐Sachs and (111)γ//(110)α [110]γ//[001]α Nishiyama–Wassermann, is observed after deformation and during the reversion process. The EBSD shows a high misorientation inside the γ after the cold rolling. The SIMRT shows diffusional and shear reversion characteristics, at 900 °C, yield strength is 468 MPa, and the elongation is 32%.