Adding carbon nanotubes and integrating with AA5052 aluminium alloy core to simultaneously enhance stiffness, strength and failure strain of AZ31 magnesium alloy

New bimetal AZ31-CNT/AA5052 macrocomposite comprising: (a) carbon nanotube (CNT) reinforced magnesium alloy AZ31 shell and (b) aluminium alloy AA5052 millimeter-scale core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterisation revealed more rounded intermetallic particle of decreased size, reasonable CNT distribution, and dominant (1 0 −1 1) texture in the longitudinal and transverse directions in the AZ31-CNT nanocomposite shell. Interdiffusion of Mg and Al across the core-shell macrointerface into each other was also significant. Compared to monolithic AZ31, the AZ31-CNT shell had significantly higher hardness (+30%). In tension, the presence of CNT (in the AZ31 shell) and AA5052 core significantly increased stiffness (+39%), ultimate strength (+13%), failure strain (+17%) and work of fracture (+27%) of AZ31, while yield strength (−2%) was marginally decreased. In compression, the presence of CNT (in the AZ31 shell) and AA5052 core significantly increased yield strength (+35%), failure strain (+42%) and work of fracture (+70%) of AZ31, while ultimate strength (+1%) was marginally increased. The effect of joint presence of: (a) CNT (in the AZ31 shell) and (b) AA5052 millimeter-scale core on tensile and compressive properties of AZ31 is investigated in this paper.     

Evolution of internal strain during plastic deformation in magnesium matrix composites

Synchrotron radiation diffraction during in situ tensile tests has been used to evaluate the internal elastic strains within the grains of magnesium alloy, AZ31, unreinforced and reinforced with 5 and 10% volume of SiC particles. Composites present initial thermal residual stresses, which are positive (tensile) in the matrix and negative (compressive) in the reinforcing particles. Internal elastic strains evolve in a similar behaviour in the unreinforced AZ31 and in both composites. However, the accumulated elastic strains are reduced in the case of the composite because a part of the applied load is borne by the ceramic particles.     

Low-cycle fatigue life of SiC-particulate-reinforced Al-Si cast alloy composites with tensile mean strain effects

The low-cycle fatigue behaviour of a SiC-particulate-reinforced Al-Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains. The composites and the unreinforced matrix alloy showed cyclic hardening behaviour. The composite having a higher volume fraction of the SiC particles exhibited a more pronounced strain-hardening rate. For the tensile mean strain tests, the initial high tensile mean stress relaxed to zero for the ductile Al-Si alloy, resulting in no influence of the tensile mean strain on the fatigue life of the matrix alloy. However, tensile mean strain for the composite caused tensile mean stresses and reduced the fatigue life. The pronounced effects of mean strain on the low-cycle fatigue life of the composite compared to the unreinforced matrix alloy were attributed to the initial large prestrain causing non-relaxing high tensile mean stress in the composite with limited ductility and cyclic plasticity. Fatigue damage parameter using strain energy density accounted for the mean stress effects quite satisfactorily. Predicted fatigue life using this damage parameter correlated fairly well with the experimental life within a factor of 3. Moreover, the fatigue damage parameter indicated the inferior life in the low-cycle regime and superior life in the high-cycle regime for the composite, compared to the unreinforced matrix alloy.     

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al₂O₃ are presented and discussed. The 6061/Al₂O₃ composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.     

Investigation of uniaxial low-cycle fatigue failure behavior of hot-rolled AZ91 magnesium alloy

The uniaxial low-cycle fatigue behavior of hot-rolled AZ91 magnesium alloy was investigated by asymmetric cyclic stress-controlled experiments at room temperature. The effects of the sampling direction, peak stress and stress ratio on the fatigue life were discussed. The fatigue life increases with increasing the stress ratio or decreasing the peak stress. Due to the anisotropic property, the specimen in transverse direction shows superior fatigue resistance. Considering the effects of mean stress on the fatigue strength coefficient and fatigue strength exponent, a modified Basquin model was proposed and validated to evaluate the fatigue life of AZ91 magnesium alloy.     

AZ31镁合金搅拌摩擦焊接头断裂机制

对AZ31镁合金搅拌摩擦焊接头进行力学性能实验.拉伸、疲劳实验结果显示,AZ31镁合金搅拌摩擦焊接头抗拉强度可以达到母材强度的92.9%,断裂位置在前进面的机械热影响区,认为是前进面机械热影响区不均匀的层状组织和应力集中作用的结果.扫描电镜显示:断口有明显的撕裂纹和纤维状组织.     

Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting

Cyclic deformation behavior and fatigue life of squeeze-cast AZ31 magnesium alloy was studied under stress amplitude-control at room temperature. Low and high cycle fatigue tests with engineering stress amplitudes in the range from 40 to 110 MPa were conducted. Analysis of hysteresis curves was performed. Tension–compression asymmetry of hysteresis loops was not observed; the alloy exhibited cyclic hardening in tension and compression. The fatigue life in the low cycle fatigue region was expressed by Wöhler and derived Manson–Coffin curves. Experimental data in both, the low and high cycle fatigue regions were fitted by means of regression functions. S–N curves exhibited a smooth transition from the low to the high cycle fatigue regions and significant scattering of experimental points was observed. Furthermore, metallographic and fractographic analyses were performed. Crack initiation occurred from the specimen surface or on clusters of secondary particles; the region of final fracture was characterized by a transgranular ductile fracture.It can be concluded that the fatigue properties of squeeze cast magnesium alloy AZ31 are significantly improved comparing to materials prepared by common methods of casting. Squeeze casting also enables the cost-effective fabrication of complicatedly shaped parts.     

Tensile properties of carbon nanotube reinforced aluminum nanocomposite fabricated by plasma spray forming

Uniaxial tensile tests were performed on plasma spray formed (PSF) Al–Si alloy reinforced with multiwalled carbon nanotubes (MWCNTs). The addition of CNTs leads to 78% increase in the elastic modulus of the composite. There was a marginal increase in the tensile strength of CNT reinforced composite with degradation in strain to failure by 46%. The computed critical pullout length of CNTs ranges from 2.1 to 19.7 μm which is higher than the experimental length of CNT, leading to relatively poor load transfer and low tensile strength of PSF nanocomposites. Fracture surface validates that tensile fracture is governed strongly by the constitutive hierarchical microstructure of the plasma sprayed Al–CNT nanocomposite. The fracture path in Al–CNT nanocomposite occurs in Al–Si matrix adjacent to SiC layer on CNT surface.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

An investigation on the high stress sensitivity of fatigue life of rolled AZ31 magnesium alloy under constant amplitude fatigue loading

In the present work, an investigation on the high stress sensitivity of the fatigue life of the AZ31 rolled magnesium alloy under constant amplitude fatigue loading has been carried out. Different damage parameters were involved to quantify fatigue damage accumulation at the various scales of material volume corresponding to the changing fatigue damage mechanisms that prevail at the various stages of the fatigue life. The experimental work included mainly nano‐indentation measurements to evaluate hardness evolution at the nano‐scale due to cyclic plasticity, results of micro‐crack monitoring by using the replication technique and fractographic analysis to obtain the fracture characteristics of the fatigue specimens after failure. The hexagonal close‐packed structure of the alloy and the resulting difficulty for the activation of five independent slip systems required for homogeneous plastic deformation were considered to determine the high stress sensitivity of the fatigue life observed for the rolled AZ31 alloy under the investigated loading conditions.     

Effects of annealing treatment on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

The objective of this investigation is to study the effects of annealing treatment on the ratcheting behavior of extruded AZ31B magnesium alloy. First, the microstructures and monotonic tensile properties of the extruded and annealed alloys were assessed. The results showed that the grain size increased slightly with increasing annealing time until an annealing time of 6 h after which abnormal grain growth happened. Accordingly, the ultimate tensile strength of the Mg alloy decreased with increasing annealing time, while the tensile yield strength and elongation percentage of the Mg alloy increased with annealing time until the annealing time reached 2 h. The cyclic softening/hardening behavior of the annealed AZ31B Mg alloy was similar to that of the extruded alloy: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. The annealing treatment delayed the occurrence of the cyclic hardening. It was also shown that the effects of the annealing time on the ratcheting strain of the Mg alloy depended of the loading path.     

Damage Tolerant Magnesium Metal Matrix Composites: Influence of Reinforcement and Processing

A growing impetus to enhance our understanding of the behavior of magnesium-based alloys for use in weight critical applications resulted as a consequence of the low density of magnesium. In an attempt to enhance the applicability of magnesium for a wide spectrum of performance-critical applications, the addition of reinforcement to the alloy was considered as an economically affordable and potentially viable scientific alternative. In this paper are reported the results of a study aimed at understanding the influence of saffil alumina short fiber reinforcement on microstructural development of a squeeze-cast magnesium alloy. Preliminary results confirm promise of the reinforced alloy, which retains hardness, strength, and stiffness better at elevated temperatures compared to the unreinforced counterpart. However, impact strength and toughness of the reinforced alloy are inferior. The importance of the matrix alloy in governing the overall mechanical response of the composite microstructure is discussed based on fractographic observations. The importance of volume fraction of the reinforcing phase on properties of the composite microstructure is highlighted.     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

Improving low-cycle fatigue properties of rolled AZ31 magnesium alloy by pre-compression deformation

The effect of pre-compression deformation on the low-cycle fatigue properties and cyclic deformation behavior of as-rolled AZ31 alloy was investigated by performing the stress-controlled low-cycle fatigue tests at room temperature. Fatigue properties and cyclic damage process should be closely related to the twins. The present work aimed to investigate the deformation mechanism and fatigue life caused by the introduced {1 0−1 2} twinning–detwinning from the viewpoint of stress amplitude. The results reveal that the twins contribute to the fatigue properties and cyclic damage process of AZ31 alloy. There were noticeable changes in hysteresis loops, microstructures and fatigue lives when the stress amplitude increased from 120 to 150 MPa. The fatigue life of pre-compressed samples was more superior to that of the as-rolled sample under different stress amplitudes, especially under the stress amplitude close to the tensile yield strength of the as-rolled sample.     

Effect of Particulate Silicon Carbide on Cyclic Plastic Strain Response and Fracture Behavior of 6061 Aluminum Alloy Metal Matrix Composites

In this paper, the cyclic stress response and cyclic stress–strain response characteristics, cyclic strain resistance and low-cycle fatigue life, and mechanisms governing the deformation and fracture behavior of aluminum alloy 6061 discontinuously reinforced with silicon carbide (SiC) particulates are presented and discussed. Two different volume fractions of the carbide particulate reinforcement phase in the aluminum alloy metal matrix are considered. The composite specimens were cyclically deformed using fully reversed tension–compression loading under total strain-amplitude-control. The stress response characteristic was observed to vary with strain amplitude. The plastic strain-fatigue life response was found to degrade with an increase in carbide particulate content in the metal matrix. The fracture behavior of the composite is discussed in light of the interactive influences of composite microstructural effects, cyclic strain amplitude and concomitant response stress, deformation characteristics of the composite constituents and cyclic ductility.     

Fatigue performance of alumina reinforced metal matrix composites

Abstract

The room temperature fatigue performance of two Saffil reinforced metal matrix composites manufactured by squeeze forming is assessed. For the composite with an LM 13 matrix, introduction of Saffil does not result in an increase in the ultimate tensile strength, and the fatigue performance is inferior to the unreinforced alloy. By contrast, the composite with a 6082 type matrix exhibits a markedly superior ultimate tensile strength and stiffness compared with the unreinforced equivalent and this is coupled with an improved overall fatigue performance.

MST/767     

Processing and Characterization of In Situ (TiC–TiB2)p/AZ91D Magnesium Matrix Composites

In this paper, a practical and cost‐effective processing route, in situ reactive infiltration technique, was utilized to fabricate magnesium matrix composites reinforced with a network of TiC–TiB₂ particulates. These ceramic reinforcement phases were synthesized in situ from Ti and B₄C powders without any addition of a third metal powder such as Al. The molten Mg alloy infiltrates the preform of (Ti_p + B₄C_p) by capillary forces. The microstructure of the composites was investigated using scanning electron microscope (SEM)/energy dispersive X‐ray spectroscopy (EDS). The compression behavior of the composites processed at different conditions was investigated. Also, the flexural strength behavior was assessed through the four‐point‐bending test at room temperature. Microstructural characterization of the (TiB₂–TiC)/AZ91D composite processed at 900 °C for 1.5 h shows a relatively uniform distribution of TiB₂ and TiC particulates in the matrix material resulting in the highest compressive strength and Young's modulus. Compared with those of the unreinforced AZ91D Mg alloy, the elastic modulus, flexural and compressive strengths of the composite are greatly improved. In contrast, the ductility is lower than that of the unreinforced AZ91D Mg alloy. However, this lower ductility was improved by the addition of MgH₂ powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates.     

循环道次对高温叠轧AZ31镁合金板材组织与性能的影响

目的通过高温累积叠轧工艺制备出高强度的镁合金,并研究该过程中循环道次对AZ31镁合金板材的微观组织与性能的具体影响。方法对累积叠轧1～5次板材进行微观组织观察,并进行显微硬度的测试,得到不同板材的硬度值,通过X射线衍射分析得到不同板材的取向结果,最终进行力学性能实验,并对比分析。结果随着循环道次的增加,板材抗拉强度有明显改变。从260 MPa先增加至310 MPa,最后稳定在350 MPa左右;非基面织构比重增加;断裂伸长率先降低后升高并稳定在10%左右。结论累积叠轧工艺使得AZ31镁合金板材产生了加工硬化,并显著细化了晶粒。循环道次的增加、孪晶产生和晶界数量显著增多导致强度进一步提高。     

Compressive behaviour of SiC/ncsc reinforced Mg composite processed through powder metallurgy route

In this present work nano coconut shell charcoal (ncsc) and silicon carbide (SiC) particulates were reinforced with AZ31B Mg alloy and suitable magnesium composite was developed by using the powder metallurgy technique followed by hot extrusion. Density measurement of the Mg composites revealed that the addition of ncsc significantly improved the density of the composites and porosity measurement showed minimal porosity. The microstructure of the composites showed even distribution of the ncsc in the AZ31B/3SiC Mg composite. The compressive and impact behaviour of the samples were characterized, the results showed that on increasing the weight percentage of ncsc in AZ31B/3SiC/0.5ncsc Mg composites the mechanical properties such as ultimate compressive strength, 0.2% yield strength, ductility and impact strength decreased. The scanning electron microscope (SEM) analysis of fractured surface of AZ31B Mg alloy and AZ31B/3SiC/0.5ncsc Mg composites showed quasi-cleavage fracture. The presence of ncsc above 0.5 wt% composites revealed mixture of quasi cleavage planes and some dimples.     

Enhanced mechanical response of hybrid alloy AZ31/AZ91 based on the addition of Si3N4 nanoparticles

The main aim of this study was to simultaneously increase tensile strength and ductility of AZ31/AZ91 hybrid magnesium alloy with Si₃N₄ nanoparticles. AZ31/AZ91 hybrid alloy nanocomposite containing Si₃N₄ nanoparticle reinforcement was fabricated using solidification processing followed by hot extrusion. The nanocomposite exhibited similar grain size to the monolithic hybrid alloy, reasonable Si₃N₄ nanoparticle distribution, non-dominant (0 0 0 2) texture in the longitudinal direction, and 13% higher hardness than the monolithic hybrid alloy. Compared to the monolithic hybrid alloy (in tension), the nanocomposite simultaneously exhibited higher yield strength, ultimate strength, failure strain and work of fracture (+12%, +5%, +64% and +71%, respectively). Compared to the monolithic hybrid alloy (in compression), the nanocomposite exhibited higher yield strength and ultimate strength, lower failure strain and higher work of fracture (+35%, +4%, −6% and +6%, respectively). The beneficial effects of Si₃N₄ nanoparticle addition on the enhancement of tensile and compressive properties of AZ31/AZ91 hybrid alloy are investigated in this paper.