Extruded ZK60 Magnesium Alloy Research Articles

Modification of the Tensile Performance of an Extruded ZK60 Magnesium Alloy with the Addition of Rare Earth Elements.

The influence of rare earth (RE) elements on the microstructure and mechanical performance of an extruded ZK60 Mg alloy was studied. Two types of RE elements were added to a ZK60 material and then extruded at a ratio of 18:1. The first new alloy contained 2 wt% Y while the second one was produced using 2 wt% Ce-rich mischmetal. The microstructure, the texture, and the dislocation density in a base ZK60 alloy and two materials with RE additives were studied by scanning electron microscopy, electron backscattered diffraction, and X-ray line profile analysis, respectively. It was found that the addition of RE elements caused a finer grain size, the formation of new precipitates, and changes in the initial fiber texture. As a consequence, Y and Ce-rich RE elements increased the strength and reduced the ductility. The addition of these two types of RE elements to the ZK60 alloy decreased the work hardening capacity and the hardening exponent mainly due to grain refinement.

Ratcheting deformation and fatigue of surface treated ZK60 magnesium alloy

Three surface treatment methods, laser shock penning (LSP), micro-arc oxidation (MAO) and a composition of LSP and MAO, are used for the preparation of modified layers at the surface of two orientation (ED and TD) specimens made by extruded ZK60 magnesium alloy. The ratcheting deformation behavior of different specimens under various stress-controlled loading was investigated. The results show that the ratcheting deformation under different loading conditions comes from the deformation mechanism of four modes: dislocation slip, twinning/detwinning transfer to dislocation slip, residual dislocation-retwinning collaboration, and residual twins-dislocation slip interaction. Considering the multi-mode ratcheting effect, a modified energy-based fatigue model is used to predict the fatigue life under various loading conditions.

Effect of adding Y and Ce on corrosion behaviour of the extruded ZK60 magnesium alloy

ABSTRACT The effect of rare earth elements Ce and Y addition on microstructure and corrosion behaviour of an extruded ZK60 magnesium alloy was investigated. Materials characterisation was performed using a scanning electron microscope and X-ray diffraction. Corrosion behaviour of ZK60 and ZK60-3x (x = Ce, Y) alloys were evaluated using electrochemical impedance spectroscopy and potentiodynamic polarisation test. At the end of evaluations, the best corrosion performance was observed for the ZK60 base alloy, due to its microstructure consisting of larger grains and lower volume fraction of second phase particles. On the other hand, the two other alloys suffered from galvanic corrosion rising from their RE-containing particles and the more fraction of high energy grain boundaries acting as possible areas for corrosion initiation.

Tests and Finite Element Simulation of Yield Anisotropy and Tension-Compression Strength Difference of an Extruded ZK60 Mg Alloy

Multiroute yield experiments were carried out on extruded ZK60 magnesium alloy samples under 200 °C and strain rate of 10−2 s−1 by a thermomechanical simulator. A hypoelastoplastic large deformation constitutive relationship was employed to simulate large deformation yield subjected to uniaxial loading, biaxial loading, loading-reverse loading, and orthogonal loading in the finite element (FE) software with user subroutine. The results showed the following: (1) As the accumulative plastic strain increased, the convex yield surface in the 2D stress space gradually expanded or shrank, rotated, and distorted from the approximate ellipse at low accumulative plastic strain. At the same accumulative plastic strain, uniaxial tensile or compressive yield values along different directions were not the same. In addition, the tensile yield value differed considerably from the compressive one. The large deformation yield of ZK60 magnesium alloy showed significant anisotropy, tension-compression strength difference, and evolution effect. (2) FE simulations based on the hypoelastoplastic constitutive relationship could accurately capture the strongly evolving asymmetric yield behavior under complex loading routes. The stress-strain relationships and hardening rates were in better accordance with the experimental results and reflected the yield behavior more realistically compared to simulations without the evolution effect or with traditional linear interpolation. Deformation at low temperature and high strain rate is important to save process costs and improve processing efficiency, but it may cause evolving asymmetric yield during large deformation. It is believed that the simulation approach used herein is reliable for the prediction and optimization of severe plastic deformation processes of hexagonal close-packed (HCP) alloys.

Wear behavior of extruded ZK60 magnesium alloy in simulated body fluid with different pH values

Two specimens were prepared along the extrusion (ED) and transverse (TD) directions of the extruded ZK60 magnesium alloy bar, respectively. The wear behavior of ED and TD specimens in air (Dry) and simulated body fluid (SBF) of different pH values (pH 4, 7.4 and 9) was evaluated using ball-on-disk configuration. The wear morphology of specimens was characterized by three-dimensional white light interference profiler, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results indicated that pH value had a significant effect on the wear behavior of specimens. The average friction coefficient of ED and TD specimens in each environment decreased in the following order: pH 7.4 ≈ pH 9 > Dry > pH 4, and the wear volume decreased in the following order: Dry > pH 4 > pH 7.4 ≈ pH 9. Abrasive wear and oxidative wear were the main wear mechanisms of ED and TD specimens in air, and corrosive wear was taken place in SBF of pH 4, 7.4 and 9. The formation of Mg(OH)2 film improved the wear resistance of specimen in SBF. The wear behavior of ED and TD specimens in SBF of pH 4 showed an orientation correlation, and the wear volume of TD specimen was greater than that of ED specimen. The effect of pH value and initial texture on the wear behavior of magnesium alloy was discussed.

Effect of Twin-Induced Texture Evolution on Corrosion Resistance of Extruded ZK60 Magnesium Alloy in Simulated Body Fluid

Different pre-compression deformations are carried out along the extrusion direction of ZK60 magnesium alloy to change the initial texture of the material. The corrosion resistance of ZK60 magnesium alloy associated with twin-induced texture evolution was investigated by immersion and electrochemical experiments in simulated body fluid (SBF). The results showed that a small amount of twins decreased corrosion resistance, while high density twins improved the corrosion resistance of the material.

Micromechanical modeling of cyclic plastic deformation of ZK60 Mg alloy using 3D full-field crystal plasticity coupled with computational homogenization

A spatially resolved crystal plasticity finite element model in conjunction with computational homogenization is proposed to simulate the micromechanisms of cyclic plastic deformation of an extruded ZK60 magnesium alloy. Dislocation slipping, twinning and detwinning are included through an integrated framework. The ability of the proposed model to reproduce experimental macroscopic responses has been evaluated through the numerical solution of a real representative volume element (RVE) of microstructure under strain-controlled cyclic loading. The proposed model can capture the main macroscopic cyclic plastic characteristics of the ZK60 extruded Mg alloy including asymmetry in stress–strain curve, mechanical anisotropy, strain amplitude dependency of response, mean stress evolution and Bauschinger effect. Simulations were carried out up to 1% macroscopic strain. In agreement with experimental texture, both basal as well as off-basal grains, whose c-axes deviate from being normal to extrusion direction (ED), are included in the RVE. To assess the micromechanisms of cyclic plastic deformation three distinct grain ensembles termed as microtextures have been implanted in the bulk texture. The mixed grain cluster consists of both basal and off-basal grains. Off-basal microtexture is comprised of off-basal grains. Twinned microtexture is formed when a few contiguous grains are in a favorable position for twinning. The simulation results show that under certain conditions, off-basal and twinned microtextures can act as a channel for long-range flow softening and strain localization which evolves with loading cycles. Additionally, severe incompatible plastic deformation stemming from the presence of micro-textures induces short or long-range evolving intergranular stress concentration.

Evolution of microstructure, texture, and mechanical properties in a multi-directionally forged ZK60 Mg alloy

An extruded ZK60 magnesium alloy with the nominal composition of Mg–6Zn–0.5Zr was processed by multi directional forging (MDF) for 2, 4, 6 and 8 passes at 473 K. Microstructural studies showed that the uniform microstructure of the as-extruded material transforms to a duplex structure consisting of fine recrystallized grains surrounding some patches of unrecrystallized grains after 2 and 4 passes of MDF. Further MDF passes, however, resulted in a homogeneous equiaxed structure in such a way that the as-extruded grain size of 12.7 μm was refined to 1.9 μm after 8 passes. Evaluation of crystallographic texture revealed that deformation by MDF changed the conventional fiber texture of the extruded condition to a new texture, in which the basal planes tend to align almost 45° to the transverse direction. Mechanical properties of the extruded and MDFed samples were studied by shear punch testing (SPT) at room temperature. The results indicated that the 182.6 MPa shear yield stress (SYS) and 184.9 MPa ultimate shear strength (USS) of the extruded condition were, respectively, reduced to 170.0 and 172.3 MPa after 2 passes of MDF. This drop in strength is attributed to the presence of coarse patches of unrecrystallized grains. Further pressing of the alloy was accompanied by a decrease in the volume fraction of coarse grains and an increase in the number density of fine grains. Textural softening that occurred at higher strain levels of the final MDF passes, however, offset the strengthening effect of these fine grains.

Effect of Compressive Deformation on Wear Property of Extruded ZK60 Magnesium Alloy

The present study investigates the dry sliding wear behavior of precompressed ZK60 alloy using a ball-on-disc wear test machine under normal loads of 20 and 40N at a sliding speed of 0.5m s−1 for a constant sliding distance of 1,500m. Microstructures and worn surfaces were characterized by optical microscopy and scanning electron microscopy. Microhardness measurements were taken to evaluate mechanical properties. Results showed that both the microhardness and wear resistance of uncompressived alloy can be improved with precompression. This is attributed to the grain refinement via subdivision of twin lamellae generated by compressive deformation. Furthermore, the worn surface studies showed a mixed type of wear mechanisms: oxidative and delamination wear took place under higher applied loads.

Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

AbstractThe effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED−9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED−9.4% specimens are similar, and they experience twinning exhaustion → <c + a> slip and detwinning exhaustion → <c + a> slip during each loading cycle. At a medium strain amplitude of 1%, twinning‐detwinning is involved in the cyclic deformation, but different deformation mechanisms were observed in the 3 different specimens. Partial twinning‐complete detwinning mode dominates the cyclic deformation in the ED specimen, while partial detwinning‐retwinning mode occurs in the ED−9.4% specimen. For the TD specimen, both <a> basal slip and tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED−9.4% specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens.

Effects of solution and quenching treatment on the residual stress in extruded ZK60 magnesium alloy

The quench induced residual stress in extruded ZK60 magnesium alloy is investigated. X-ray diffraction and layer removal technique are employed to determine the residual stress in extrusion and long transverse direction through the thickness of the extruded plate after solution and quenching treatment. FEM simulation is employed to clarify the origin of the stress state. The residual stress is not uniform through thickness, it is compressive on the surface and first decreases and then increases in absolute magnitude with increasing depth and becomes tensile in the center. Anisotropic residual stress can be detected, with larger stress in long transverse direction and smaller stress in extrusion direction. This is rationalized by the anisotropic flow stress caused by the tilted basal texture. Anisotropic residual stress can be replicated in simulation using an isotropic yield function at high temperature and an anisotropic yield function at low temperature during quenching, the smaller flow stress in extrusion direction facilitates plastic flow at low temperature and results in smaller compressive residual stress in extrusion direction. The fact that no twins can be found after quenching demonstrates that the asymmetric yield strength associated with twinning does not affect the residual stress.

Impression creep behavior of magnesium alloy ZK60

Abstract Creep behavior of the ZK60 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress at high temperatures (175, 200 and 225 °C) under low stresses (25, 50, 75 and 100 MPa) and at low temperatures (100, 125 and 150 °C) under high stresses (100, 125, 150 and 175 MPa). The creep mechanism of the extruded ZK60 magnesium alloy is grain boundary sliding by dislocation climbing obtained at low temperature and high pressure. At high temperature and under low pressure, the creep mechanism of the extruded ZK60 magnesium alloy is grain boundary diffusion by grain boundary sliding. The initial microstructure of the extruded ZK60 magnesium alloy was composed of mainly Mg phase and a little MgZn phase. Under the conditions of low temperature and high pressure, MgZn2 phase precipitated during creep deformation. Under the conditions of high temperature and low pressure, MgZn2 phase and a small amount of Zr-Zn phase precipitated during creep deformation. The creep resistance of magnesium alloy ZK60 under the conditions of low temperature and high pressure is better than under the conditions of high temperature and low pressure.

Deformation of extruded ZK60 magnesium alloy under uniaxial loading in different material orientations

The deformation behavior of an extruded ZK60 magnesium (Mg) alloy was investigated experimentally under uniaxial loading in the extrusion direction (ED) and the transverse direction (TD). X-Ray diffractometer (XRD) and electron back scattered diffraction (EBSD) were employed to examine the microstructure evolution using companion specimens. The results indicated that the flow stress and strain hardening rate were strongly affected by the loading directions relative to the initial material orientations. Strong tension-compression asymmetry was exhibited when the loading was applied along the ED. Practically identical stress-strain responses were obtained when the tension/compression was applied in the TD. The difference in tension-compression responses between the ED and TD specimens arises from the distinctive twinning activities, which are inherently related to the initial basal texture. When loading was applied in the ED, copious tension twinning can attained only under compression. However, in the TD, either tension or compression can impose similar amounts of grains under c-axis tension, thus activating tension twinning in both directions. Detailed analyses of strain hardening rate responses under different loading paths were made. The deformation mechanisms associated with different stages of strain hardening rate were discussed with a particular emphasis on the roles of deformation twinning.

Orientation imaging microscopy and microhardness in a ZK60 magnesium alloy processed by high-pressure torsion

Abstract An extruded ZK60 magnesium alloy was processed by high-pressure torsion (HPT) at room temperature for up to 5 turns under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. This processing produced an average grain size of ∼700 nm. The grain size distributions and textures were examined by electron backscatter diffraction (EBSD) and this revealed some multi-modality in the microstructure at different stages of straining with fractions of both coarse grains and ultrafine grains. EBSD analysis at the mid-radius positions of unprocessed and HPT-processed materials revealed a gradual evolution from a prismatic { 10 1 ¯ 0 } fiber to an ultimate basal { 0001 } fiber texture with the c-axis parallel to the normal direction. The majority of grain boundaries had misorientations larger than 15° throughout the processing. The strain hardening tended towards a reasonable hardness homogeneity with a hardenability exponent, η, of 0.07 up to strains of ∼20 and with a subsequent hardness saturation at Hv ≈ 125.

Characteristic cyclic plastic deformation in ZK60 magnesium alloy

A detailed analysis of the plastic deformation characteristics was performed for an extruded ZK60 magnesium alloy under uniaxial cyclic loading along the extrusion direction. The experiments used for the analysis were performed under single-step strain-controlled loading, two-step strain-controlled loading, and stress-controlled loading. An elastic limit with an offset of 10−5 plastic strain is used for the demarcation of elastic and elastic-plastic deformation. An inflection point is used to signify a transition of the dominated deformation mechanism from twinning-detwinning to dislocation slips. The macroscopic stress-strain response of the material is intrinsically related to the microstructures of the material during cyclic loading. The elastic limit range is closely related to the microstructure of the material at the peak stress prior to the loading reversal. If the microstructure at the peak stress displays a strong basal a-texture, yielding is dominantly associated with the activation of basal slips. The elastic limit range to activate basal slips for the ZK60 magnesium alloy under investigation is 100 MPa. If the microstructure at the peak stress contains tension twins, the elastic limit range during subsequent loading reversal reflects the activation stress of detwinning/retwinning process, which can be interpreted as the critical stress to activate the gliding of twin boundaries. The stability of twin boundaries is influenced by twin volume fraction, twin morphology, and cyclic hardening. Dependent on the twin volume fraction and loading history, the elastic limit range varies from 20 MPa to 100 MPa for the material under investigation.

Role of compression direction on recrystallization behavior and texture evolution during hot deformation of extruded ZK60 magnesium alloy

Hot deformation behavior of as-extruded ZK60 magnesium alloy was investigated using uniaxial hot compression tests over the temperature range of 300–450 °C under strain rates of 0.001–1.0 s−1 using a Gleeble® 3500 available at the University of Waterloo. In order to take in account the effect of initial texture and microstructure on the hot deformation behavior of the alloy, compression tests were conducted in both the extrusion and radial directions (designated as ED and RD, respectively). The as-received alloy exhibited an inhomogeneous microstructure featured by unrecrystallized elongated grains stretched along the extrusion direction, surrounded by equiaxed grains, with a typical fiber texture. Visual observation of the hot compressed samples showed that deformation along the radial direction led to pronounced anisotropic flow which reduced by increasing the deformation temperature or decreasing the strain rate. Processing maps determined from the compression tests predicted possible dynamic recrystallization (DRX) for both directions, specifically over the temperature regime of 400–450 °C which was confirmed by microstructural studies. It was observed that DRX grain size for both deformation directions is very close under identical deformation conditions; however, the final texture was strongly dependent on the deformation direction. While a weak and close to random texture was developed after deformation along the extrusion direction (ED), RD samples exhibited either similar texture to the starting texture or a sharper texture. Such a behavior was attributed to the initial texture and DRX mechanism.

Effect of severe plastic deformation on an extruded ZK60 magnesium alloy

ABSTRACTEqual channel angular pressing (ECAP) was carried out on extruded ZK60 magnesium alloy until an equivalent strain of ∼4 (corresponding to 4 passes) at 523K following route Bc. The effect of the deformation on both microstructure and texture was investigated by analysis of inverse pole figures and pole figures respectively, determined by EBSD. Additionaly, the mechanical properties were evaluated through tensile tests. The ECAPed processed ZK60 alloy showed the presence of dynamic recrystallization (DRX) process, correlated with a strong basal fiber texture. The presence of pyramidal slip was also observed, which can be correlated with the presence on twin-oriented boundaries in the material. Both the grain size reduction and the existence of texture influenced the ductility of the magnesium alloy processed by ECAP, as the ductility of the ZK60 alloy after four ECAP passes increased two times in comparation with the initial (un-processed) material.

Microstructural properties, thermal stability and superplasticity of a ZK60 Mg alloy processed by high-pressure torsion

An extruded ZK60 magnesium alloy was used to investigate microstructure, hardness and tensile properties after processing by 5 turns of high-pressure torsion (HPT) at room temperature. EBSD results confirmed the successful production of an ultrafine-grained structure with a mean grain size of ~700 nm with reasonable homogeneity and a majority of grains oriented parallel to the shear direction. This material also reached a homogeneous microhardness across the disk with an average hardness value saturated at Hv ?124 from the as-received hardness value of Hv ?74. The obtained high value is due to a high density of dislocations, the very small grain size and texture strengthening. The microhardness retained homogeneity after annealing samples processed by HPT for 40 hours at 448 K. However, the hardness value dropped to Hv ?85 while the mean grain size increased to ~2.1 ?m. These changes may be a result of restoration processes and consequent texture softening. Specimens processed by 5 turns of HPT exhibit excellent superplastic properties with a maximum elongation of 940% at 523 K and an optimum strain rate of 1.0×10-4 s-1. Significant superplasticity was observed at 448 K due to the stability of the bimodal structure at lower temperatures. This can assist the microstructure to accommodate grain boundary sliding and intragranular slip simultaneously and postpone any necking.

Effect of Artificial Aging on Microstructures and Mechanical Properties of Extruded Mg Alloy ZK60

An extruded ZK60 magnesium alloy was subjected to artificial aging at 180 oC for an investigation of the effect of aging time on its precipitation behavior and mechanical properties. Uniaxial tensile tests were conducted to obtain the mechanical properties. Optical microscopy and transmission electron microscopy (TEM) were employed to observe microstructure change before and after aging treatment. It is shown that, both tensile yield strength and ultimate tensile strength increases with aging time. The fracture elongation after aging for 20 h reaches up to 21.0%, and the yield strength increases to 269.5 MPa, 19.4% higher than that of extruded specimens (un-aged), showing a good match of strength and ductility. Three newly-formed precipitates were observed after aging for over 20 h, among which particulate and dispersive precipitates should be responsible for the good combination of strength and ductility.

Electron backscatter diffraction observations of twinning–detwinning evolution in a magnesium alloy subjected to large strain amplitude cyclic loading

An extruded ZK60 magnesium alloy was subjected to fully-reversed strain-controlled cyclic loading at a strain amplitude of 4.0% in the extrusion direction in ambient air. Electron backscatter diffraction (EBSD) analyses were conducted on samples taken from companion specimens terminated at different loading cycles to study the twinning–detwinning process and the evolution of the twin structures at different stages of cyclic deformation. It is observed that the twin nucleation sites are increased whereas twin growth/shrinkage is inhibited due to repeated twinning–detwinning. The enhanced twin nucleation sites are responsible for the observed increase in the number of twin lamellae and the increased twin volume fraction with loading cycles. Cyclic loading enhances formation of compression and double twins which do not result in immediate fracture of the material. With increasing number of loading cycles, more and larger sized residual tension twin lamellae can be detected by EBSD, but the total volume fraction of the residual twins is trivial.