Microyield Strength Research Articles

Evaluation on dimensional stability of Cf/AZ91D composite based on the micro-yield strength

The dimensional instability caused by irreversible micro-plastic deformation can be evaluated with the micro-yield strength. Cf/Mg composite material prepared by liquid-solid extrusion following vacuum infiltration(LSEVI) process was taken as testing material and its dimensional stability was evaluated. The load-unload cycle test method is used to explore the stress range for the Cf/Mg composite to generate residuals plastic strain in static conditions. The results show that micro-plastic deformation for Cf/Mg composite material occurred in the stress range between 35 and 45 MPa. When the micro-plastic strain reaches 10−5, the micro-yield strength of the Cf/Mg composite material is evaluated as 79MPa. Compared with matrix alloy, the micro-yield strength has increased by more than 89%, which is the same level as beryllium alloy. This paper provides an effective strategy for characterize the dimensional stability of continuous fibre reinforced metal matrix composites.

Effect of creep annealing on the dimensional stability of dispersion strengthened copper alloy

This study aims to enhance the dimensional stability of alumina dispersion strength copper alloy via creep annealing treatment. The microstructure, residual stress and micro-yield strength of alloys after different treatments were investigated. During creep annealing, the residual stress in the alloy was dramatically reduced due to the decrease of dislocation density. The nano-Al2O3 particles in the alloy did not coarsen or agglomerate during the high temperature treatment and retarded the growth of subgrains effectively, which played important roles in high micro-yield strength of creep annealed alloy. The pores were effectively reduced by the creep deformation during creep annealing, resulting in a more uniform residual stress distribution and an increase in micro-yield strength. Compared with extruded alloy, the residual stress of alloy creep annealed at 950 °C for 12 h was reduced by 88%, while the micro-yield strength increased by 9.7%, leading to a significant enhancement in dimensional stability of the alloy.

Effect of pre-strain on microstructure and micro-yield properties of Al-Cu-Li alloy

In this study, Al-Cu-Li alloys were pre-strained to various plastic strains before ageing. The T1 (Al2CuLi) precipitates and dislocation density in various pre-strained Al-Cu-Li alloys were analyzed with transmission electron microscopy (TEM) and X-ray diffraction (XRD) technology respectively. The micro-yield strength (Micro-YS) of tested alloys was measured and the strengthening mechanism was discussed. It is found that the pre-strain increases the dislocation density, promotes the precipitation and inhibits the growth of T1 precipitates. The precipitation strengthening of T1 precipitates is higher than strain strghening of dislocations and it first increase then decrease whearas the strain strenthening continuously increase with pre-strain. The improvement in strength of Al-Cu-Li alloy caused by pre-straining is mainly due to strain strengthening rather than precipitation strengthening. Therefore, although pre-strain improves the macro-yield strength (Macro-YS) as well as the Micro-YS, excessive pre-strains are not conducive to the improvement of the Micro-YS. The pre-strain of 2% is a promising method to simultaneously improve the Micro-YS and Macro-YS of Al-Cu-Li alloys due to its synergistic improvement effect on precipitation strengthening and strain strengthening. Macro-YS and Micro-YS of pre-strained Al-Cu-Li alloys can be estimated with a simple strength mode. However, the larger error between the estimated and measured Micro-YS suggests that the accurate estimation of Micro-YS requires further to consider the adverse effect of mobile dislocations caused by pre-strain on the Micro-YS.

The Effect of Ni Plating on the Residual Stress and Micro-Yield Strength in an Al-Cu-Mg Alloy Under Different Diffusion Treatments

Due to the low density and high fatigue strength, Al-Cu-Mg alloy has become an attractive material for aerospace and navigation applications. However, the relatively high residual stress and low micro-yield strength after the fabrication process restrict its further application in the field of inertial instruments. The objective of this work is to investigate the effect of Ni plating on the residual stress and micro-yield strength in an Al-Cu-Mg alloy under different diffusion treatments. Compared with properties of the alloy without diffusion treatment, the micro-yield strength of the alloy after diffusion treatment increases 9.5%, and the corresponding residual stress decreases > 70%. This can be attributed to the increase of diffusion layer thickness and release of elastic energy.

Effects of Ag content on the micro-yield strength and dimensional stability of Al–Cu–Li–Sc–(Ag) alloys

To improve the dimensional stability of Al–Cu–Li–Sc–(Ag) alloys and identify the underlying micro-yield mechanism, the effects of silver (Ag) content (0, 0.2, 0.4, 0.6 and 0.8 wt%) on the microstructure, micro-yield behaviour and dimensional stability of Al–Cu–Li–Sc–(Ag) alloys were investigated by transmission electron microscopy (TEM), cyclic load–unload test and thermal cycle test. The results showed that increasing the Ag content promoted the precipitation of the T1 and δ′ phases in the α–Al matrix and decreased their diameters, which resulted in the increase of the micro-yield strength and strain hardening rate of the Al–Cu–Li–Sc–(Ag) alloys. Because T1 and δ′ precipitates can effectively pin the mobile dislocations and impede their movement, the critical resolved shear stress (CRSS) resulting from the pinning of mobile dislocations by T1 and δ′ precipitates played a critical role in the micro-yield behavior. There existed a relation between σmys and τCRSS similar as that of macro-yield strength, σmys = α · τCRSS + σ0′. However, the low α value in the micro-yield stage compared with the macro-yield stage indicated that fewer activated slip systems would be enough to produce a residual strain of 1 × 10–6 at the beginning of the micro-yield stage. The thermal cycle decreased the residual relative length (RRL) of the alloys and the stable RRL after the thermal cycle could serve as an indicator of the thermal dimensional stability of the alloys. Increasing the Ag content decreased the stable RRL of the Al–Cu–Li–Sc–(Ag) alloys and hence improved their thermal dimensional stability.

Investigation of micro-yield strength and coefficient of thermal expansion of Al–Cu–Mg–Li–Sc–Ag alloys with various contents of Li

Abstract

Effects of thermal treatments on the residual stress and micro-yield strength of Al2O3 dispersion strengthened copper alloy

The microstructure, residual stress, and mechanical properties of a Cu-0.05wt%Al2O3 dispersion strengthened alloy in different treatment states were investigated. The alloy was cold forged with a reduction of 60%, followed by either normal annealing or creep annealing at 600 °C for 6 h. Compared with the cold forged alloy, the micro-yield strength and the residual stress of the creep annealed alloy were reduced by 8.2% and 80.3%, respectively. The significant decrease in residual stress after creep annealing could result from the elimination of voids and defects and the reduction of dislocation density in the sub-grains during heat treatment. The high micro-yield strength of the creep annealed alloy can be attributed to the well dispersed nano Al2O3 particles effectively pinning the sub-grain boundaries at the elevated temperature.

The effect of SiC addition on the dimensional stability of Al-Cu-Mg alloy

Due to the high specific stiffness and low coefficient of thermal expansion, the SiC particles are introduced to improve the dimensional stability of Al-Cu-Mg (2024 aluminum) alloy in present work. The intergrated effects of SiC and aging precipitates on dimensional stability of 2024 aluminum alloy are investigated during the two-stage aging process. It is observed that, with increasing the addition of SiC, the amount of S′ precipitates decreases, and the micro-yield strength first increases and then decreases, while the thermal strain (εc) first decreases and then increases. When the addition of SiC was 10 wt%, both the micro-yield strength and the thermal strain reach the optimal value. It is attributed to the effect of SiC and aging precipitates on pinning the dislocations, which would consume more stored elastic energy, thus leading to an increase in the dimensional stability of the alloy.

The effect of stress-aging on dimensional stability behavior of Al-Cu-Mg alloy

The stress-aging method is proposed to improve the dimensional stability of Al-Cu-Mg (2024 aluminum) alloy. Both the reduction of residual stress and the acceleration of precipitates are investigated during the stress-aging process. It is observed that some S′ particles precipitate in the alloy during stress-aging. With the external stress increase to 100 MPa, the residual stress and thermal strain (ɛc) decrease, while the micro-yield strength increases significantly. This is attributed to that the dislocations are effectively pinned by the uniformly distributed precipitates and the stored elastic energy diminishes remarkably, which leads to a considerable improvement of the dimensional stability of 2024 aluminum alloy. However, when further increases the external stress to 150 MPa, due to the coarsened precipitates, the residual stress and thermal strain increase, while the micro-yield strength decreases slightly.

Effects of two-stage aging on the dimensional stability of Al-Cu-Mg alloy

Due to the excellent structural and functional properties, such as low density, high fracture toughness and fatigue strength, Al-Cu-Mg (2024 aluminum) alloy has become an attractive light weight material for inertial instruments and aerospace applications. However, it has relatively low dimensional stability. The aim of this investigation is to enhance the dimensional stability of 2024 aluminum alloy via two-stage aging method, and identify the precipitation behavior and the underlying stabilizing mechanisms of 2024 aluminum alloy. When the second aging time was 24 h, the uniformly distributed precipitates could effectively pin the dislocations and impede their movement. This leaded to a significant improvement of the micro-yield strength and the thermal strain in the 2024 aluminum alloy, and thus the dimensional stability of the alloy increased.

Numerical Analysis of Skin Effect on Microplastic Flow of HPDC Mg Alloys

An axisymmetric finite element (FE) model is established to investigate how “skin effect” affects microplastic flow behavior of hpdc Mg–Al alloys. Due to internal interaction, initiation of microplastic flow requires an externally applied stress higher than microyield strength, while propagation can take place at external stresses lower than yield strength. Degree of internal interaction is quantified by characteristic constraint stress defined, which increases with progression of microplastic flow. Instability of microplastic flow occurs when characteristic constraint stress reaches a critical value, and an increase in Al content delays the occurrence of flow instability. In addition, the size of skin region has been well‐defined based on the present analysis.

Texture effect on microyielding of wrought magnesium alloy AZ31

Uniaxial tension tests and elastoplastic self-consistent (EPSC) simulations were conducted to study the effect of texture on the microyielding of highly textured AZ31 plate. The onset of microyielding is always associated with the activation of basal 〈a〉 slip irrespective of starting texture, while the value of the microyield stress does depend on the texture. The macroscopic yield stress is largely affected by the critical resolved shear stress of the deformation mode with the highest Schmid factor. An inverse relation is found between the microyield strength normalized by the macroscopic yield strength and the average Schmid factor of prism 〈a〉 slip, which is useful for a rough estimate of the microyield strength.

Effect of heat treatment on microstructure and dimensional stability of ZL114A aluminum alloy

The effects of heat treatment process on microstructure, micro-yield strength and dynamic dimensional stability of ZL114A aluminum alloy were investigated by optical microscopy (OM), transmission electron microscopy (TEM), tensile testing and thermal cycling on-line measuring method. Fine dispersed eutectic Si phases are observed, and long strip eutectic Si and massive primary Si phases decrease in ZL114A alloy after high-temperature and long-time solution treatment, which result in the increase of micro-plastic deformation resistance. With the increasing of aging temperature, aging precipitation behaviour of ZL114A alloy transforms from precipitation of GP zone and β' phases simultaneously at lower temperature to precipitation of stable Mg 2Si phases at higher temperature. Because coherent strengthening is the main strengthen mechanism for micro-plastic deformation, precipitation of stable Mg 2Si phases is unfavorable to the improvement of micro-plastic deformation resistance. Micro-yield strength cannot characterize dimensional stability comprehensively, and dynamic dimensional stability under alternative temperature should also be tested cooperatively for better evaluation of dimensional stability.

Elasticity and inelasticity of biomorphic carbon, silicon carbide, and SiC/Si composite produced on the basis of medium density fiberboard

The amplitude and temperature dependences of the Young’s modulus and the internal friction (ultrasonic absorption) of biomorphic carbon, silicon carbide, and SiC/Si composite produced from medium density fiberboard (MDF) by pyrolysis (carbonization), followed by infiltration of molten silicon into the prepared carbon preform have been studied in the temperature range 100–293 K in air and under vacuum. The measurements have been performed by the acoustic resonance method with the use of a composite vibrator for longitudinal vibrations at frequencies of approximately 100 kHz. The data obtained by acoustic measurements of the amplitude dependences of the elastic modulus have been used for evaluating the microplastic properties of samples under study. It has been shown that the Young’s modulus, the decrement of elastic vibrations, and the conventional microyield strength of the MDF samples differ from the corresponding data for previously studied similar materials produced from natural eucalyptus, beech, sapele, and pine woods. In particular, the desorption of environmental molecules at small amplitudes of vibrations, which is typical of biomorphic materials based on natural wood, is almost absent for the MDF samples. The results obtained have been explained by different structures and the influence of pores and other defects, which, to a large extent, determine the mechanical characteristics of the biomaterials under investigation.

INFLUENCE OF REINFORCEMENT SIZE ON MICROPLASTIC DEFORMATION BEHAVIOR OF SiCP/Al COMPOSITES

The microplastic deformation behavior of 20 vol.% SiCp / Al composites with various SiC particle sizes were investigated. The SiC particles are in four nominal sizes of 1, 5, 20 and 56 µm. The experimental results showed that the micro-yield strength is very sensitive to composite microstructure features. As the particle size increases, the micro-yield strength of composites decrease firstly, and then increase. The observed results were attributed to thermal residual stress and dislocation density due to the large difference in coefficient of thermal expansion between the matrix and reinforcement.

Micro-yield behaviors of Al 2-O 3-SiO 2(sf)/Al-Si metal matrix composites

Effects of the volume fraction and the size of crystallized alumina silicate short fibers as well as heat treatment processes on micro-yield strength(MYS) of Al 2-O 3-SiO 2(sf)/Al-Si metal matrix composite(MMC) that was fabricated by squeezing cast, were investigated by using continuous loading method on an Instron 5569 tester with a special extensometer with an accuracy of 10 −7. The results show that MYS of MMC decreases with the increase of volume fraction and length of the alumina silicate short fibers in the metal matrix composite, respectively. MYS of quenched Al 2-O 3-SiO 2(sf)/Al-Si MMC is the lowest, MYS of the MMC through peak-aging treatment is higher than that through other heat treatment methods. And before the peak-aging, MYS of MMC aging treated gradually increases with the increase of the aging time. Aging treatment after solution treatment is a preferred way that enhances micro and macro-yield strength of Al 2-O 3-SiO 2(sf)/Al-Si MMC.

Micro-yield property of sub-micron Al 2O 3 particle reinforced 2024 aluminum matrix composite

The microstructure and micro-yield strength of sub-micron Al 2O 3 particle reinforced 2024Al composites and the effect of the thermal-cold cycling treatment on the microstructure and properties were studied. The results show that the dislocations are rare in the microstructure of the sub-micron Al 2O 3p/2024Al composite in the squeeze casting condition. Aging and thermal-cold cycling treatment does not change this phenomenon. The Al 2O 3 particles are fine, so the thermal misfit between particles and the matrix is very small during the temperature change, resulting in decreased dislocations. The tiny and uniformly dispersed S′ precipitates and sub-micron particles can effectively pin dislocations, therefore, the micro-yield strength of the composite increases. Depending on the condition of the thermal-cold cycling treatment after aging, both the size and distribution of the S′ precipitates in the composite change, and they have great effect on the micro-yield strength of the composite.

Study on isotropy treatment process of hot extruded LY12 Al alloy

Anisotropy of micro-deformation due to textural structure in hot extruded LY12 Al alloy bar was investigated by means of measuring the notch change of ring specimens and testing micro-yield strength (MYS). The results show that the notch change in transverse (perpendicular to axis of the extruded bar) direction is about 13 times than that in longitudinal (parallel to axis of the extruded bar) direction, MYS in longitudinal direction is about 1.8 times than that in transverse direction. Thermal-cold cycling treatment (TCT) has no effect on changing anisotropy of micro-deformation. Isotropy treatment process was studied by means of quantitative metallographic analysis and X-ray diffraction test. Treated by the optimum process, the uniform and fine equiaxed grains can be acquired. MYS test shows that the values in longitudinal and transverse directions are close to each other indicating that anisotropy of micro-deformation has been eliminated.

Studies on microyield behavior of SiCp/2024Al composite materials

Microyield behavior of silicon carbide particles reinforced 2024 aluminum alloy (SiCp/2024Al composites) with three volume fractions of SiCp were investigated in this paper. The results of the studies showed that the composites exhibited some types of notable strain relaxation before microyielding when subjected to an applied stress far lower than conventional yielding stress. The microyield mechanism did not conform to the classical linear relation between the applied stress (σ) and square root of plastic strain (εp ½), proposed by Brown and Lukens. Microyield strength could be strongly affected by heat treatment routes. The thermal processing with large temperature change, such as high temperature quenching or annealing, rapid cooling to liquid nitrogen temperature, thermal cycling with large upper-lower cyclic temperature interval, etc., would lead to deterioration of microyield strength, whereas an appropriate aging treatment would significantly increase microyield strength of the SiCp/2024Al composites. TEM and X-ray diffraction analyses revealed the relationship between microyield behavior and microstructures. In general, dispersed strengthening phase S′, lower residual stresses and lower population of movable dislocations would be beneficial to microyield strength of the SiCp/2024Al composite materials. The effect of SiCp on microyield strength was dependent to thermal processing history.

Improved Microyield Strength in Beryllium Produced by Vapor Deposition

Beryllium is a metal of major importance in the space and nuclear industries. It has a low density, high elastic modulus, elevated melting point, high heat capacity, and good nuclear hardness. However, Be also has some intrinsic problems such as low ductility and poor fabricability, which limits it from wider applications. Be has a hcp crystalline structure and is thus highly anisotropic. Several years ago, Nieh et al. used a physical vapor deposition technique to produce thick Be deposits and found that such deposits have unique grain and crystalline texture characteristics. Specifically, the deposit consisted of columnar grains with their major axis parallel to the direction of incident adatoms. The self-shadowing mechanism was not observed to have a prevalent contribution to the development of the microstructure during the growing process. The deposits were found also to be highly anisotropic with the major axis of their columnar grains corresponding to the [001] crystallographic direction and always pointing towards the evaporation source. In this paper, the authors describe these microstructural and crystallographic characteristics, and present a technique to improve the microyield strength of Be.