Mechanical and fracture properties of an AZ91 Magnesium alloy reinforced by Si and SiC particles

A commercial AZ91 magnesium alloy (nominal composition Mg–9%Al; 1%Zn; 0.3%Mn, balance Mg in weight percent) reinforced with SiC particles and modified by the addition of Si has been used in this study. Formation of an “in situ” composite (Mg–Mg₂Si) results in strong bonding between Mg₂Si and the matrix interface. Samples were deformed in compression in the temperature interval from room temperature up to 300 °C. Stress relaxation tests were performed with the aim to reveal the thermally activated processes. Reinforcing effect of SiC and Mg₂Si particles decreases with increasing temperature. The estimated values of the activation volume as well as the activation enthalpy indicate that the main thermally activated process is connected with a rapid decrease of the internal stress. Fracture properties were studied in impact tests at various temperatures. A ductility enhancement was found at 200 °C and temperatures above 200 °C.     

Effects of SiCp size and volume fraction on the high cycle fatigue behavior of AZ91D magnesium alloy composites

High cycle fatigue tests (i.e., stress-controlled, axial) were conducted on monolithic AZ91D and AZ91D magnesinm alloy composites processed via squeeze casting and extrusion to contain either 15 gm or 52 gm size SiC particles, at both the 20% and 25% volume fraction reinforcement level. The effects of changes in SiC particle size and volume fraction on the high cycle fatigue behavior have been determined. In addition, the number of cracked particles on the fatigue fracture surfaces, as well as the level of damage beneath the fatigue fracture surfaces were quantified in order to determine the effects of particle size on the evolution of damage during fatigue and during overload failure. Commercial purity Mg specimens containing a large grain size were also tested in fatigue for comparison with the alloy and composite data.     

Corrosion fatigue behavior of extruded magnesium alloy AZ80-T5 in a 5% NaCl environment

The corrosion fatigue behavior of extruded AZ80-T5 magnesium alloy has been investigated in three different environments: (1) a low humidity environment (35-40% relative humidity), (2) a high-humidity environment (80% relative humidity), and (3) a 5 wt.% NaCl environment. Fatigue tests were conducted under axial loading at a stress ratio of −1 and at a frequency of 20 Hz. It was found that in both the high-humidity environment and in the 5 wt.% NaCl environment the fatigue strength was reduced relative to the low humidity environment, especially in the NaCl environment: the reduction rates of fatigue limit under high humidity and NaCl environments were 18%, and 78%, respectively. The reduction of fatigue strength under the corrosive environments was attributed to the pit formation and growth. At low stress amplitudes, multiple pits were formed and coalesced to form a large pit under NaCl environment. A fatigue crack nucleated when the pit grew to the critical size.     

Microstructure investigation and first-principle analysis of die-cast AZ91 alloy with calcium addition

In order to get improved mechanical properties of die-cast AZ91 alloy under elevated temperatures, Ca element was added as a cost-effective alloying constituent. It appeared that minor Ca addition less than 0.5 wt% would result in no apparent change in microstructure, but the tensile strength at elevated temperatures was improved considerably. When increasing Ca addition to more than 1.0 wt%, Al₂Ca phase will precipitate during solidification, no Mg₂Ca phase was discovered. Homogeneous microstructure and high temperature stability in tensile strength of die-cast AZ91 alloy with Ca addition was mainly attributed to the precipitation of Al₂Ca phase, which considerably refined the bulky β-Mg₁₇Al₁₂ phase distributed originally at the grain boundaries of die-cast AZ91 alloy with no Ca addition. The priority of Al₂Ca phase compared to Mg₂Ca phase in precipitation sequence was verified by first-principle calculation of their cohesive energy and formation enthalpy, and can also be associated with more bounding electrons between Al and Ca atoms.     

Characterization of the fatigue behaviour of the magnesium alloy AZ91D by means of mechanical hysteresis and temperature measurements

Stress-controlled load increase and constant amplitude tests were performed on the cast magnesium alloy AZ91D. Rather small values of the plastic strain amplitude reflect the very localised plastic deformation predominant in the _I-phase. The cyclic deformation and temperature curves of the load increase tests exhibit cyclic yield strength values which allow a good estimation of the endurance limit. Stress amplitudes above the endurance limit lead to cyclic hardening. As compared to the behaviour under monotonic tensile loading, the alloy exhibits cyclic hardening. Microscopic investigations illustrate the different influences of the - and β-phase in respect to crack initiation.     

Corrosion fatigue behavior of extruded magnesium alloy AZ31 in sodium chloride solution

In the present study, corrosion fatigue experiments were done using the extruded magnesium alloy AZ31 in the 3% sodium chloride solution to clarify the corrosion fatigue characteristics of the material. Corrosion fatigue lives greatly decreased as compared with those in laboratory air. It was also clarified that most of the corrosion fatigue life (70–80%) at the lower stress amplitude is occupied with the period of the corrosion pit growth. Corrosion fatigue lives were evaluated quantitatively by dividing the corrosion fatigue process into the following two periods, i.e. (1) the corrosion pit growth period preceding the crack initiation from the pit and (2) the crack growth period before the specimen failure. In the analysis, the law of the corrosion pit growth proposed by authors was used to deal with the above first period. The evaluated results corresponded well to the experimental results.     

Influence of Pd addition on the creep behavior of AZ61 magnesium alloy

The effect of Pd addition (0, 2, and 4 wt%) on the microstructure and creep properties of permanent mold AZ61 (Mg-6Al-1Zn) alloy has been studied. The results indicate that Pd addition introduces a lamella-shaped Al₄Pd phase at the grain boundary, in addition to the Mg₁₇Al₁₂ (β) phase. The addition of Pd also suppresses the precipitation of the Mg₁₇Al₁₂ phase and residual Al at grain boundaries during solidification. These effects lead to an improvement in the creep behavior of AZ61. Moreover, extended steady-state creep and reductions in both the minimum creep rate and total creep strain are also observed in the case of 4 wt% Pd addition.     

Self-lubricate and anisotropic wear behavior of AZ91D magnesium alloy reinforced with ternary Ti2AlC MAX phases

The dry sliding wear behavior of Ti₂AlC reinforced AZ91 magnesium composites was investigated at sliding velocity of 0.5 m/s under loads of 10, 20, 40 and 80 N using pin-on-disk configuration against a Cr15 steel disc. Wear rates and friction coefficients were registered during wear tests. Worn tracks and wear debris were examined by scanning electron microscopy, energy dispersive X-ray spectrometry and transmission electron microscopy in order to obtain the wear mechanisms of the studied materials. The main mechanisms were characterized as the magnesium matrix oxidation and self-lubrication of Ti₂AlC MAX phase. In all conditions, the composites exhibit superior wear resistance and self-lubricated ability than the AZ91 Mg alloy. In addition, the anisotropic mechanisms in tribological properties of textured Ti₂AlC-Mg composites were confirmed and discussed.     

Effect of submicron size SiC particles on microstructure and mechanical properties of AZ31B magnesium matrix composites

The magnesium matrix composites reinforced with three volume fractions (3, 5 and 10 vol.%) of submicron-SiC particles (∼0.5 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. With increasing the volume fraction of the submicron SiC particles (SiCp), the grain size of matrix in the SiCp/AZ31B composites was gradually decreased. Most of the submicron SiC particles exhibited homogeneous distribution in the SiCp/AZ31B composites. The ultimate tensile strength and yield strength of the 10 vol.% SiCp/AZ31B composites were simultaneously improved. The study of interface between the submicron SiCp and the matrix in the SiCp/AZ31B composite suggested that submicron SiCp bonded well with the matrix without interfacial activity.     

Damping properties of porous AZ91 magnesium alloy reinforced with copper particles

Abstract

The well distributed open porous AZ91 magnesium alloy reinforced with copper particles was successfully prepared through powder metallurgy route based on space holder method. Its damping properties were characterised by internal friction and the internal friction measurement in a temperature range from room temperature to 100°C was performed by a multifunction internal friction apparatus. Experimental results revealed that the damping capacity of the porous AZ91 magnesium alloy was increased as a result of the addition of copper particle and the increase was further enhanced with increasing volume fraction of copper particle. Attempt is performed to correlate the increase in damping with the microstructural change arising due to the addition of copper particles. It was suggested that the increase should be related to the appearance of plastic zone and thus an increase in dislocation density at the matrix particles interfaces.     

Low cycle fatigue properties of friction stir welded joints of a semi-solid processed AZ91D magnesium alloy

A semi-solid processed (thixomolded) Mg–9Al–1Zn magnesium alloy (AZ91D) was subjected to friction stir welding (FSW), aiming at evaluating the weldability and fatigue property of the FSW joint. Microstructure analysis showed that a recystallized fine-grained microstructure was generated in the nugget zone (NZ) after FSW. The yield strength, ultimate tensile strength, and elongation of the FSW joint were obtained to be 192 MPa, 245 MPa, and 7.6%, respectively. Low-cycle fatigue tests showed that the FSW joint had a fatigue life fairly close to that of the BM, which could be well described by the Basquin and Coffin-Manson equations. Unlike the extruded magnesium alloys, the hysteresis loops of FSW joint of the thixomolded AZ91D alloy were basically symmetrical, while the non-linear or pseudoelastic behavior was still present. The FSW joint was observed to fail in the BM section rather than in the NZ. Fatigue crack initiated basically from the pores at or near the specimen surface, and crack propagation was mainly characterized by fatigue striations along with the presence of secondary cracks.     

Wear behaviour of an Al–12% Si alloy reinforced with a low volume fraction of SiC particles

Aluminium–silicon alloys reinforced with low volume fractions of SiC particles were prepared by the compocasting process. The wear behaviour of the unreinforced Al–12Si alloy and metal-matrix composites (MMCs) was investigated by using a block-on-ring test at room temperature under dry conditions. The results showed that the addition of a low volume fraction of SiC particles (2–8 vol%) is a very effective way of increasing the wear resistance of the matrix alloy. Metallographic examinations revealed that the wear zone of the Al–12Si alloy consists of both hardened and deformation layers. The depth of the hardened layer depended on the applied load and was in the vicinity of 10–50 μm. The formation of the hardened layer was related to the alignment and redistribution of fragmented eutectic phase to the surface region during sliding wear. Furthermore, the delamination of debris from the hardened layer was responsible for a higher wear loss observed in the Al–12Si alloy. The thickness of the hardened layer formed on the MMC specimens was reduced considerably by the incorporation of fragmented SiC particles. This layer exhibited higher hardness and wear resistance than that developed in the unreinforced alloy.     

Machining characteristics,wear and corrosion behavior of AZ91 magnesium alloy - fly ash composites produced by friction stir processing

In the present work, fine grained AZ91 magnesium alloy – fly ash composite has been successfully fabricated by friction stir processing. Microhardness measurements show marginally higher hardness with uniform distribution compared with the base material. No significant difference in the mean cutting force was observed during drilling of the base metal and the composite. However, lower cutting forces were recorded in the sub-surface region of the composites. Interestingly, decreased corrosion resistance was noticed for the composite compared with the base material. Lower mass loss has been observed for the composite during reciprocating wear experiments. The results strongly suggest that the surface composite of AZ91 magnesium alloy – fly ash exhibits better mechanical and wear properties. However, decreased corrosion resistance is a significant observation that warns the applicability of these composites in corroding environment.     

Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (σ<30 MPa), both the apparent stress exponent (n_app≈2) and the apparent activation energy (Q_app≈48 kJ mol⁻¹) are low, while at high stresses (σ>34 MPa), these parameters are much higher (n_app=9–15 and Q_app=230–325 kJ mol⁻¹) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y₂O₃ interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.     

In situ observation of ageing process and new morphologies of continuous precipitates in AZ91 magnesium alloy

An in situ observation of the precipitating process of γ-Mg₁₇Al₁₂ phase in die-cast AZ91 magnesium alloy, was carried out with a transmission electron microscope equipped with a heating stage maintained at 473 K for 8 h. In addition to the thin plate-shaped continuous precipitates, continuous precipitates with rod-shaped and the Potter orientation relationship were observed and analyzed with transmission electron microscopy including high-resolution transmission electron microscopy techniques. It was also observed firstly that there exist plate-shaped continuous precipitates with the Pitsch-Schrader orientation relationship in the die-cast AZ91 magnesium alloy.     

High temperature deformation behavior of permanent casting AZ91 alloy with and without Sb addition

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10^–4s^–1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg₃Sb₂ precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process.     

Influence of nano-alumina and sub-micron copper on mechanical properties of magnesium alloy AZ31

Magnesium composites of AZ31–Al₂O₃–Cu formulations were produced using the disintegrated melt deposition technique following by hot extrusion. Microstructural characterization showed reasonable distribution of secondary phases up to 1 vol.% of copper. A tendency to form clustered agglomeration and longer shape of secondary phases was observed when the amount of copper was increased to 1.5 vol.%. Mechanical tests indicated remarkable improvements in 0.2%YS, UTS and microhardness when nano-alumina and sub-micron copper were added into AZ31. The ductility was increased up to 9.3% in the case of AZ31–1.5Al₂O₃–1.0Cu sample and significantly reduced (5.5%) when the amount of copper was increased to 1.5 vol.%. Heat treated sample of AZ31–1.5Al₂O₃–1.0Cu showed overall improvement in both tensile strength and ductility. The results suggest that the judicious selection of composition and heat treatment has the capability to enhance overall tensile response of Mg–Al₂O₃–Cu nanocomposites.     

Hot deformation behavior and processing map development of AZ110 alloy with and without addition of La-rich Mish Metal

In order to compare the workability of AZ110 alloy with and without addition of La-rich Mish Metal(MM), hot compression tests were performed on a Gleeble-3500 D thermo-mechanical simulator at the deformation temperature range of 473-623 K and strain rate range of 0.001-1 s-1. The flow stress, constitutive relation, DRX kinetic model, processing map and microstructure characterization of the alloys were investigated. The results show that the flow stress is very sensitive to deformation temperature and strain rate, and the peak stress of AZ110 LC(LC = La-rich MM) alloy is higher than that of AZ110 alloy.The hot deformation behavior of the alloys can be accurately predicted by the constitutive relations. The derived constitutive equations show that the calculated activation energy Q and stress exponent n for AZ110 alloy are higher than the calculated values of AZ110 LC alloy. The analysis of DRX kinetic models show that the development of DRX in AZ110 LC alloy is earlier than AZ110 alloy at the same deformation condition. The processing maps show that the workability of AZ110 LC alloy is significantly more excellent than AZ110 alloy and the microstructures are in good agreement with the calculated results.The AZ110 LC alloys can obtain complete DRX microstructure at high strain rate due to its higher stored energy and weak basal texture.     

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.     

Studies on squeeze casting of Al 2124 alloy and 2124-10% SiCp metal matrix composite

Aluminium 2124 alloy and its composite with 10% SiC particles of average particle size of 23 μm were squeeze cast at different pressures. The effect of squeeze pressure during solidification was evaluated with respect to microstructural characteristics using optical microscopy and image analysis and mechanical properties by tensile testing. The microstructural refinement, elimination of casting defects such as shrinkage and gas porosities and improved distribution of SiC particles in the case of the composite were resulted when pressure is applied during solidification. A pressure level of 100 MPa was found to be sufficient to get the microstructural refinement and very low porosity level in both the alloy and the composite. The improved mechanical properties observed in the squeeze cast alloy and the composite could be attributed to the refinement of microstructure within the material.