Peak Yield Strength Research Articles

Uniaxial tension deformation study of copper/nickel laminated composites: Effects of lamella number and interlamellar spacing

The lamella number and interlamellar spacing are crucial structural parameters in determining mechanical properties of layered nanocomposites, while they are rarely discussed. Herein, we investigate effects of the two parameters on mechanical properties of copper/nickel (Cu/Ni) nano-laminated composites (NLCs) under uniaxial tension using molecular dynamic simulation. The Cu/Ni NLCs with one single Ni laminate exhibit the largest enhancement in Young’s modulus (E = 121 GPa) and yield strength (σz = 9.8 GPa), attributing to the stronger texture of Ni and interfacial blocking effect on the motion of dislocations. With different Ni layer spacing, the shockley dislocations form a crescent slip plane and enter the Cu matrices along the 45° direction with respect to the Ni laminate. According to the evolution of dislocation length, the flow stress shows a negative relationship with the mean dislocation length. The peak yield strength coincides with the stage of sudden increase of the total dislocation length, indicating decreased mechanical properties with increased dislocation length. These findings provide fundamental relationships between nanoscale laminated structure and macroscopic mechanical properties of Cu/Ni composites, shedding new lights on the rational design of NLCs with enhanced mechanical performances.

Bi-lamellar microstructure in Ti–6Al–4V: Microstructure evolution and mechanical properties

Some characteristic microstructures in Ti–6Al–4V alloy, including lamellar microstructure, martensite microstructure, bimodal microstructure and equiaxed microstructure, have been intensively investigated in the last 30 years. Nevertheless, the bi-lamellar microstructure, which was first defined by G. Lutjering, has not attracted much attention, despite of its significant benefits on various mechanical properties. In the present study, the bi-lamellar microstructure having fine secondary α lamellae (~300 nm) in-between coarse primary α lamellae was obtained after intercritical annealing treatments in the α+β region of a lamellar microstructure. The effects of intercritical annealing temperature (800–960 °C) and cooling rate (water quench~100 °C/min) on the bi-lamellar microstructure as well as their yield strength at room temperature were systematically studied. It was found that the thickness (and volume fraction) of transformed β (βtrans) regions including fine secondary α lamellae with various orientations increased with the increase of the annealing temperature. The thickness of the secondary α lamellae increased with the decrease of the cooling rate. Yield strength of the bi-lamellar microstructure was significantly improved compared with that of the lamellar microstructure composed of coarse primary α plates. Appearance of the peak yield strength of the bi-lamellar microstructure at intermediate annealing temperature was found for the first time. By means of TEM observation, slip line analysis as well as elemental analysis of Al, this peak yield strength was attributed to the combined effects of both increased volume and thickness of βtrans as well as decreased solid solution strengthening of primary α lamellae by Al with the increase of the annealing temperature. The yield strength decreased monotonically with the decrease of the cooling rate. It was believed that the reduced volume and thickness of βtrans and similarity in orientation between primary and secondary α lamellae led to the gradual drop of strength with decreasing the cooling rate.

Contribution of Zr to strength and grain refinement in Cu[sbnd]Cr[sbnd]Zr alloy

The mechanical properties and microstructure transformation of CuCrZr alloys obtained by upward continuous casting with varying Zr contents were studied by optical microscopy (OM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) after cold drawing with different strain and heat treatment. The results show that good strength can be obtained through cold drawing with suitable heat treatment. The peak tensile strength can achieve 587.8 MPa and the peak yield strength can reach 499.6 MPa; The difference of yield strength of CuCrZr alloy during cold drawing deformation becomes larger with the increase of Zr element content when the strain gradually improves; The increase of the Zr element will promote the recrystallization of the alloy at aging after cold drawing, refining the crystal grains. It contributes about 70% of the increase in strength, act as a fine grain strengthening effect, increasing the strength of the alloy.

Aging Response and Precipitation Behavior after 5% Pre-Deformation of an Al-Mg-Si-Cu Alloy.

In this study, Al-1.00 Mg-0.65 Si-0.24 Cu alloy was solution heat-treated, water-quenched, and then pre-deformed for 5% before aging. The peak hardness and yield strength of the pre-deformed sample with subsequent artificial aging were similar to that of a T6 condition sample. It was also found that the pre-deformation treatment could inhibit the negative influence of natural aging to some degree. After seven days of natural aging, the pre-deformed sample obtained better peak hardness and yield strength upon artificial aging than the sample without pre-deformation. In addition, the pre-deformation treatment could reduce 50% of the artificial aging time to reach the peak aging condition compared with T6 treatment. For the peak aged condition in the pre-deformed sample, transmission electron microscopy (TEM) observation found two types of precipitates exhibited along the dislocations besides the β″ precipitates in the Al matrix. Both precipitates had disordered atomic arrangements on the ordered subcell (Si network). The disordered precipitates occupied a number of Mg and Si atoms, resulting in less β″ precipitates formed during artificial aging at 180 °C.

The New Scope of Frictionless Triaxial Apparatus—Disturbed Sand Testing

Abstract Offshore wind turbine foundations episodically lose and recover stiffness as soil is disturbed during extreme events. Such sand behavior is not accounted for during standard testing procedures. Therefore, the following novel dynamic triaxial testing procedure is explored in this study: specimens are loaded past their peak strength and pulled back to initial length, where attempts to recover their initial stiffness are made by applying more loading cycles. This fundamentally changes how we view our specimens—they are no longer fragile and brittle. Instead, specimens can be axially compressed and pulled back to initial length many times as long as shear rupture and bulging are circumvented. Novel testing procedures were attempted using a frictionless triaxial apparatus. Frictionless triaxial is not itself a new concept, but the novel procedures revealed previously undocumented testing capabilities: (1) Multistage testing, wherein sand specimens can be liquefied (undrained), drained, and reliquefied (undrained again) many times, in one sequence, on one specimen; (2) Specimens can be compressed to large axial strain (15 % strain or more) and pulled back to initial length more than once. In the process, peak yield strength can be measured more than once at more than one density, all while using one specimen.

Single-stage aging behaviour and precipitate evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy

ABSTRACTProperty evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy treated at 120°C, with the microstructure subjected to under-aging (UA), peak-aging (PA) and over-aging (OA), is investigated. Precipitate size distributions and distances between neighbouring precipitates are determined. Results indicate that a certain time is required to reach peak hardness and yield strength, and that peak values can be sustained for a relatively long time. As the aging time increases, the conductivities increase persistently. As the alloy temper varies from UA to PA to OA, the main matrix precipitates change from ‘GPI zone and GPII zone and η′ phase’ to ‘GPII zone and η′ phase’ and then to η′ phase. Meanwhile, the precipitate size distribution becomes broader, and the average precipitate size increases.

Strengthening mechanisms of Mo-La2O3 alloys processed by solid-solid doping and vacuum hot-pressing sintering

The molybdenum alloys doped with different contents (0.3, 0.6, 0.9, 1.5 and 2.5 wt %) La2O3 were prepared via solid-solid doping method followed by vacuum hot-pressing sintering. SEM observation and laser diffraction particle size analyzer demonstrated that the presence of La2O3 particles could availably refine the Mo powders after high energy ball milling for 15 h. For the sinters, the grain size of Mo-La2O3 alloys was greatly decreased and the minimum was 83% less compared with pure Mo. TEM analysis revealed that submicro- and nano-scale La2O3 particles were distributed in Mo matrix. Especially, most of the nano-scale La2O3 particles were located within the grains. Meanwhile, Mo-La2O3 alloys with high relative density were obtained. The uniaxial compression tests results showed that the addition of La2O3 particles also improved the yield strength of Mo alloys. Due to the optimized distribution of La2O3 particles, a yield strength peak that is 78% higher compared with pure Mo is exhibited in Mo-0.9 wt % La2O3 alloy. The quantitative relationships between the yield strength and the size, as well as the volume fraction of La2O3 particles and Mo grain size were presented. Calculation results showed that refinement strengthening contributed more compared with Orowan strengthening.

Precipitation strengthening mechanisms during natural ageing and subsequent artificial aging in an Al-Mg-Si-Cu alloy

An Al-Mg-Si-Cu alloy was water quenched after solute heat treatment at 540 ℃ for 1 h, followed by natural aging and subsequent artificial aging. The hardness measurement and tensile tests show that the hardness and yield strength increased slightly after 7 days natural aging. However, applying natural aging could decrease the peak hardness and yield strength during subsequent artificial aging. For peak aged condition, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observation revealed that the β" precipitates in the natural aged sample are much smaller in size and volume fraction than that of sample without natural aging, resulting in a better elongation and a decline of peak-aging hardness and yield strength. It seems that the formation of Mg-Si clusters induced by natural aging is beneficial for the hardness and yield strength, and the clusters still occupied a mount of Mg, Si atoms and quenched-in vacancies during artificial aging, which inhibited the precipitation and growth of β" phase. Moreover, the precipitation strengthening effect of the β" phase was quantitatively evaluated by Orowan mechanism.

The Effect of Aging on the Microstructure of Selective Laser Melted Cu-Ni-Si

Precipitation hardening copper alloy C70250 was selectively laser melted to successfully produce components around 98 pct dense with high mechanical strength and electrical conductivity. Aging heat treatments were carried out at 723 K (450 °C) directly on as-printed samples up to 128 hours. Mechanical testing found that peak yield strength of around 590 MPa could be attained with an electrical conductivity of 34.2 pct IACS after 8 hours of aging. Conductivity continues to increase with further aging while the peak strength appears to be less sensitive to aging time exhibiting a broad range of time where near-peak properties exist. After aging for 128 hours, there is a drop in yield strength to 546 MPa with an increase in conductivity to 43.2 pct IACS. Electron microscopy analysis revealed nanometer-scale silicon-rich oxide particles throughout the material that persist during aging. Deformation twinning is observed in the peak-age condition after tensile testing and several strengthening mechanisms appear to be active to varying degrees throughout aging which account for the broad range of aging time where nearly the peak mechanical properties exist.

Semi-solid slurry preparation, rheo-die casting and rheo-squeeze casting of an AZ91–2Ca–1.5Ce ignition-proof magnesium alloy by gas-bubbling process

Abstract

Effects of annealing on microstructure and mechanical properties of nano-grained Ni-based alloy produced by severe cold rolling

Nano-grained (NG) Ni-based alloy with average grain size about 50nm was produced via severe cold rolling at room temperature. Compared to the solution-treated alloy prepared for rolling, the cold-rolled (CRed) NG alloy exhibited approximately 475%, 130% and 180% increase in the average yield strength (YS), ultimate tensile strength (UTS) and microhardness, respectively. Effect of the annealing on mechanical properties and microstructure of this NG Ni-based alloy were investigated systematically. The annealed-CRed alloys possessed NG (90nm) and ultrafine-grained structure (200nm) after 60min annealing at 700 and 800°C, respectively, suggesting high thermal stability of the nanostructure. Peak YS and UTS of 2047.5 and 2142MPa, respectively, were obtained with appropriate annealing temperature and time. The mechanisms of variations in mechanical properties were analyzed based on phase composition and microstructure evolution of the annealed NG Ni-based alloy. Both the high thermal stability and strength are due to the formation of the γ′ precipitates and slight grain growth of the NG matrix.

Effect of aspect-ratio alterations on plastic-deformation-behavior of a large-plastic bulk metallic glass

Because very limited plastic strain can be developed in bulk metallic glasses (BMGs) before failure under uniaxial loading, the plastic-flow behavior is difficult to investigate. In present paper, through optimizing preparation processes and regulating alloy composition, we prepared a 3 mm-in-diameter Zr-based BMG rod with superplasticity at room temperature by using copper-mold vacuum suction casting. The effect of specimen’s aspect ratios on room-temperature superplastic deformation behavior in the BMG under uniaxial compression was studied at a strain rate of 4.2 × 10−4 s−1. The results showed that there is no sign of fracturing after the BMG specimens have suffered from a nominal strain of 85.5%, i.e., a true strain of 193.1%, indicating the superplasticity of the BMG. With the increase of aspect ratio, compressive strength, plastic strain, true yield stress, end stress and stress fluctuation increase, while elastic modulus, disproportionate stress, stress peak interval, yield strength, elastic strain and shear band sliding interval reduce. The higher the aspect ratios, the larger the densities of shear bands in the deformed side surfaces. During the plastic flow, the pseudo strain-hardening effect becomes more obvious with the increasing aspect ratios, while the real strain-softening effect becomes more serious with the descending aspect ratios.

Evolution of the structure and the properties of the Ni-29Cr-9Fe alloy during high-temperature plastic deformation: Experiment and simulation

The effect of torsional deformation conditions at temperatures of 900–1150°C on the kinetics of dynamic recrystallization of a Ni-Cr-Fe alloy has been studied. The activation energy of hot deformation is determined, and a mathematical model is constructed for calculating the dynamically recrystallized grain size. An estimation is performed for critical degree of deformation ɛc required for dynamic recrystallization to be initiated, and a correlation is found between ɛc and deformation ɛp corresponding to peak yield strength σp observed in the stress-strain curve.

PRECIPITATION KINETICS AND YIELD STRENGTH MODEL FOR NZ30K-Mg ALLOY

Age-hardening effect is considerably strong in magnesium alloys containing Nd, making it possible to develop magnesium alloys with low cost and high strength. Although there have been massive researches about the precipitation product sequence and strengthening models in magnesium, aluminum and other light alloys during their ageing processes, those of NZ30K-Mg alloy, a newly-developed magnesium alloy, has not been carefully investigated. The present work mainly focuses on the model of precipitation kinetics and strengthening of NZ30K-Mg alloy. The precipitation kinetics has been investigated using electrical resistivity testing during continuous heating with different heating rates and formulated based on the isoconversional method. Two related model parameters, modified pre-exponential factor A'αrand activation energy Eαrwere respectively determined. The precipitation behavior of NZ30K-Mg alloy during ageing processes can also be intrinsically explained from the variations of Eαrand ln A'αr with the precipitation fraction. This kinetics model with two above-mentioned parameters can accurately describe the precipitation of strengthening phase during different ageing processes. The yield strength of under-aged and peak-aged NZ30K-Mg alloy have been tested and the results show that the testing samples isothermally aged at different temperature from 180 to 250 ℃ have almost the same peak yield strength of about 150 MPa, indicating that the strengthening effect of under-aged and peak-aged NZ30K-Mg alloy is only determined by the precipitation fraction within a certain range of temperatures. The precipitation strengthening model of NZ30KMg alloy has been carefully derived, and the parameter C in the model has then been determined by least squares method based on the tested yield strength data. The value of C is about 93 MPa. The prediction of yield strength of under-aged and peak-aged NZ30K-Mg alloy has been performed and fit well with the tested ones, demonstrating the effectiveness of precipitation strengthening model and its engineering application prospects.

An Experimental Investigation of Edge Strain and Bow in Roll Forming a V-Section

V-sections were roll formed from two grades of steel, and the strain on the top and bottom of the strip near the edge was measured using electrical resistance strain gauges. The channels were bent to a radius of 2 and 15 mm along the centerline. The steel strips were of mild and dual phase steel of yield strength 367 MPa and 597 MPa respectively. The longitudinal bow was measured using a 3-dimensional scanning system. The strain measurements were analysed to determine bending and mid-surface strains at the edge during forming. The peak longitudinal edge strain increased with material yield strength for both profile radii. For the 15 mm radius, the bow was larger in the dual phase steel than in the mild steel. For the 2 mm profile radius, the bow was smaller compared with the 15 mm profile radius and it was similar for both steels. It was observed that the difference between the peak longitudinal edge strain and yield strength to Youngs modulus ratio of the material is an important factor in determining longitudinal bow.

Real time indicators of material refinement in an attritor mill

Power, torque, and rotation speed of the impeller of an attritor mill were measured during preparation of metal matrix composite powders. Compositions of Al·MgO and Al·B4C were selected to include ductile (Al) and brittle components. Brittle components, MgO, and B4C, were respectively softer and harder than the milling media (hardened steel). Yield strength of pressed powder compacts and x‐ray diffraction peak width were treated as indicators of the achieved structural refinement. Both peak width and yield strength increased for both materials at longer milling times. Torque and power increased for Al·MgO but decreased for Al·B4C at longer milling times. The results were interpreted considering differences in hardness of MgO and B4C. Amplitudes of rapid oscillations of the milling process parameters were also monitored and were observed to correlate with the material refinement. Utility of real‐time milling progress indicators for different types of materials is demonstrated. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1088–1095, 2013

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.

Precipitation hardening and grain refinement in an Al–4.2wt%Mg–1.2wt%Cu processed by ECAP

The precipitation and the strength evolution during equal channel angular pressing performed at 180 °C in an Al–4.2wt% Mg–1.2wt%Cu alloy have been studied by room temperature compression tests and transmission electron microscopy. The age hardening behaviour of these AlMgCu alloys, in which the precipitation sequence involves the S-phase and its precursors, was investigated and revealed a yield strength peak after 8 days at 180 °C. The influence of the Severe Plastic Deformation on the microstructure and mechanical properties of under-aged and peak-aged samples are presented. Notably, in the under-aged sample, a gradual increase of the strength after each ECAP pass is obtained while, the peak-aged samples loose much of their strength during the first ECAP pass. TEM characterization of the microstructure before and after ECAP is presented and linked to the evolution of the mechanical properties.

The structure and mechanical properties of Fe2AlMn single crystals

The microstructure and mechanical properties of off-stoichiometric single-crystal Fe2AlMn have been investigated. After casting, the alloy contained two sets of thermal antiphase boundaries, (a/4)⟨111⟩ and (a/2)⟨100⟩, which are attributed to the fact that the compound solidifies into a bcc structure and subsequently orders to a B2 structure and then a L21 structure respectively upon further cooling. Crystals strained under tension at room temperature in air at 1s−1 showed 6% elongation, whereas specimens strained at 1 × 10−5 s−1 showed no elongation, indicating that the compound is sensitive to the testing environment. Fracture occurred on {100} in both cases. Compression tests showed that a yield anomaly was present at intermediate temperatures, with the peak yield strength occurring at about 800 K, which is slightly below the L21–B2 transition temperature of 898 K. The slip systems were found to be ⟨111⟩{110} at room temperature and 800 K. Transmission electron microscopy observations showed fourfold-dissociated ⟨111⟩ dislocations in specimens strained at room temperature but only paired ⟨111⟩ dislocations in specimens strained at the peak temperature. The room-temperature yield strength of quenched specimens increased with increasing quench temperatures from 700 to 1100 K.

Effect of carbon addition on the strength and creep resistance of FeAl alloys

We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100°C and 1300°C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacanies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300°C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300°C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700°C, and yield strength up to 800°C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys.