ZK60 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.

Characterization and corrosion behavior of composites reinforced with ZK60, AlN, and SiC particles

In this paper, microstructure, mechanical, immersion and potentiodynamic corrosion behaviours of extruded ZK60 matrix composites reinforced with forty-five µm 15% silicon carbide (SiC) particles and aluminium nitride nanoparticle (AIN) (0.2–0.5% 760 nm) were investigated. The SiC and AlN mixtures, which are the reinforcing elements of the composites, were first mixed with magnesium powder as the main alloy, then pressed under a pressure of 450 MPa and sintered at 420 °C. Second, the sintered compacts are placed in the ZK60 alloy matrix at the semi-solid melting temperature, and the melt is mixed mechanically. After the melts are mixed for 30 min and a homogeneous mixture is obtained, the mixtures are poured into metal moulds and composite samples are produced. After homogenization for 24 h at 400 °C, the composite samples were extruded at 300 °C with an extrusion ratio of 16:1 and a piston speed of 0.3 mm/s. Then, microstructure characterization of all composite samples was performed and potentiodynamic and immersion corrosion behaviours were analyzed in 3.5% NaCl solution. It was seen that the corrosion resistance increased depending on the percentages of SiC and AlN reinforcement elements in the matrix. As a result, it was seen that the potentiodynamic corrosion resistance of reinforced ZK60 + 15% SiC (ZK60SiC15), ZK60 + 15% SiC + 02AIN (ZK60SiC15AlN0.2) and ZK60 + 15% SiC + 0.5% AIN (ZK60SiC15AlN0.5) compounds increased by 1.6, 1.8 and 3.5 times compared to the unreinforced ZK60 alloy. The immersion corrosion rates were calculated as 2090.73, 1748.19, 1479.84 and 1397.79 (mg/year) for the unreinforced ZK60, ZK60SiC15, ZK60SiC15AlN0.2 and ZK60SiC15AlN0.5 reinforcements, respectively. As a result of the SEM and elemental spectrum response analysis of the corrosion surfaces, the presence of a layer rich in Si-O elements on the surface of the AlN + SiC reinforced composites enhanced corrosion resistance. Additionally, the formation of the Mg2Si intermetallics in the structure of the SiC reinforced composites improved corrosion resistance, according to the XRD results.

In vitro cellular biocompatibility and in vivo degradation behavior of calcium phosphate-coated ZK60 magnesium alloy

Calcium phosphate (Ca-P) surface coating is a simple but effective way to enhance both corrosion resistance and biocompatibility of ZK60 magnesium alloy. However, cell compatibility on different Ca-P layers coated on ZK60 alloy has seldom been investigated. In this study, the effects of type, morphology and corrosion protection of several Ca-P coatings formed at pH 6.5, 7.8 and 10.2 on cell behavior were examined by using an osteoblastic cell line MC3T3-E1. Furthermore, in vivo behavior in rabbits of the alloy coated with the optimum Ca-P layer was also studied. It was found that the surface factors governed the cell morphology and density. The coating morphology plays a dominant role in these surface factors. The sample coated at pH 7.8 showed the best cellular biocompatibility, suggesting that the hydroxyapatite (HAp) layer formed at pH 7.8 was the optimum coating. In rabbits, this optimum coating enhanced remarkably the corrosion resistance of the alloy. During implantation, the outermost crystals of the HAp coating were shortened and thinned due to the dissolution of HAp caused by the body fluid of the rabbits. It is indicated that ZK60 alloy coated at pH 7.8 can be applied as a biodegradable implant.

Microstructure and Mechanical Properties of Magnesium Matrix Composites Reinforced by In Situ Reduced Graphene Oxide.

Due to their excellent mechanical properties and large specific surface area, graphene and its derivatives are widely used in metal matrix composites as reinforcements. In this study, the thermal reduction behavior of large-size graphene oxide are investigated systematically, and reduced graphene oxide (RGO) with few residual oxygen groups and good structural integrity is obtained. ZK61 matrix composites with varying content of in situ RGO are fabricated using the semi-powder metallurgy method. The results reveal that the addition of RGO can cause the refinement of the grains and the second phase, which is attributed to the uniform distribution of the RGO throughout the matrix. The formation of nano-MgO particles is beneficial in increasing the interfacial bonding strength between the RGO and the matrix, resulting in simultaneous increments in yield strength and elongation in the RGO/ZK61 composites. The composite containing 0.6 wt.% RGO shows a superior mechanical property, including microhardness of 79.9 HV, yield strength of 203 MPa and excellent elongation of 17.5%, with increases of 20.9%, 8.6% and 7.4%, respectively, when compared with the ZK61 alloy. Quantitative analysis indicates that the main strengthening mechanisms of RGO-reinforced magnesium matrix composites are load transfer strengthening and grain refinement strengthening.

Assessing residual stresses in the surface layer of the ZK60 alloy after an exposure to corrosion solution

Magnesium alloys preliminary immersed in a corrosion solution suffer from embrittlement, referred to as pre-exposure stress corrosion cracking (PESCC). It was suggested that PESCC can be attributed to the corrosion product film-induced stress (CPFIS), which is known to be responsible for SCC in many alloys. However, the internal stress associated with the formation of the corrosion products (CP) layer on Mg alloys have not been investigated as yet. Thus, in the present study, the internal residual stresses of the first and second kinds were assessed in the alloy ZK60 exposed to the corrosion solution, using the deflection of the thin plate and by the X-ray diffraction technique, respectively. It is found that the deposition of CP on the surface of the alloy ZK60 creates the compressive internal stresses of both kinds — I and II. The macro residual stress of the kind I in the thin plate is found to be not exceeding 3 MPa, while the micro residual stress of the second type in the surface layer of 20 – 30 µm can be as high as 290 MPa and cause plastic deformation of the bare metal with the internal stress which cannot be relieved by the removal of CP.

Genetic effects of Mg-Zr master alloy in extruded thick sheets of ZK60 magnesium alloy

The genetic impact of Mg-Zr intermediate alloys is demonstrated in this article using ZK60 extrusion plates. The results demonstrate that the fine homogeneous distribution of submicron and nanoscale Zr particles is a critical contributor to the quality of the ZK60 alloy. It demonstrates that intermediate alloys have a genetic impact in the preparation of magnesium alloys.

Effect of prior heat treatment on the microstructure evolution and creep resistance of ZK60 Mg alloy under tensile creep loading along normal direction

Tensile creep tests were performed along the normal direction on the as-solutioned (AS) and as-aged (AA) samples of a commercial ZK60 alloy in this work. The results showed that the AA sample obtained a stronger 0.2% proof stress but a poorer creep resistance in comparison to the AS sample. It was revealed that the creep deformation in the AS sample was attributed to basal slip and twinning while the creep behavior of the AA sample was controlled by basal slip, pyramidal <c+a> slip and twinning. Besides, the reasons for the poorer creep resistance of the AA sample were unveiled. Pyramidal dislocations towards various moving directions were found to accelerate the creep deformation, and basal dislocations kinking at twin boundaries were found to induce heavy stress concentration. Furthermore, massive dynamic precipitates including β1′ and β2′ types were formed in the AS sample during the creep exposure, contributing to the superior creep resistance. But in the AA sample, plentiful β1′ and β2′ phases generated during the prior peak-ageing treatment, dissolved into the matrix fast and only β1′ phases re-precipitated during the creep process. With the decreased area fractions of precipitates, the age-hardening effect slacked off and failed in enhancing the creep resistance.

Synergistic Effects of Carbon Nanotube (CNT) and Reduced Graphene Oxide (RGO) on Mechanical and Thermal Properties of ZK61 Alloy

Synergistic Effects of Carbon Nanotube (CNT) and Reduced Graphene Oxide (RGO) on Mechanical and Thermal Properties of ZK61 Alloy

Effect of Grain Size on Dynamic Compression Behavior and Deformation Mechanism of ZK60 Magnesium Alloy

The dynamic compression deformation of 3 µm, 15 µm and 25 µm grain size ZK60 alloys under high-strain-rate compression is systematically studied. Dynamic compression experiments at a strain rate of 1700 s−1 were conducted using a split Hopkinson pressure bar, and the microstructure of the specimen was characterized via electron backscatter diffraction and transmission electron microscopy, as well as via the calculation of Schmid factors. The results showed that the alloy exhibited the decrease in yield strength and peak stress as the grain size increased under dynamic compression. The grain refinement in the alloy was conducive to the activation of basic slip. In turn, an increase in the grain size caused the transition in the main deformation mechanism from pyramidal &lt;a&gt; slip to {10−12} tensile twinning and pyramidal &lt;a&gt; slip. Based on these deformation mechanisms, the Johnson-Cook constitutive equation with different grain sizes was modified, and the fitting results were in accordance with the experimental data.

In Vivo Degradation Behavior of Magnesium Alloy for Bone Implants with Improving Biological Activity, Mechanical Properties, and Corrosion Resistance.

This study aimed to establish a surface modification technology for ZK60 magnesium alloy implants that can degrade uniformly over time and promote bone healing. It proposes a special micro-arc oxidation (MAO) treatment on ZK60 alloy that enables the composite electrolytes to create a coating with better corrosion resistance and solve the problems of uneven and excessive degradation. A magnesium alloy bone screw made in this way was able to promote the bone healing reaction after implantation in rabbits. Additionally, it was found that the MAO-treated samples could be sustained in simulated body-fluid solution, exhibiting excellent corrosion resistance and electrochemical stability. The Ca ions deposited in the MAO coating were not cytotoxic and were beneficial in enhancing bone healing after implantation.

Electrochemical behaviour and surface treatment of magnesium alloy ZK60

Magnesium alloys, specifically ZK60, have emerged as a promising material for use in modern and safe transportation due to their lightweight and high strength properties. To enhance the corrosion resistance of ZK60, the PEO (Plasma Electrolytic Oxidation) process was applied to form a ceramic-like oxide layer on the surface of the alloy. Additionally, a PVA polymer coating was applied on top of the PEO layer. The effect of the PEO layer and the combined PVA polymer coating on the corrosion resistance of ZK60 was evaluated using electrochemical impedance spectroscopy (EIS). The results showed that the PEO layer combined with the PVA polymer coating significantly improved the corrosion resistance of the ZK60 alloy. This makes ZK60 alloy a suitable material for use for many applications not even in the transportation industry.

Скоростная чувствительность механических свойств сплава ZK60 с высокой степенью коррозионных повреждений

According to the stable opinion, hydrogen absorbed by magnesium alloys during corrosion can cause their stress corrosion cracking. One of the characteristic markers indicating the involvement of diffusible hydrogen into the fracture mechanism of metals is the negative strain rate dependence of the embrittlement degree. Recent studies show that the loss of ductility of the ZK60 alloy specimens subjected to a short-term (1.5 h) pre-exposure in a corrosive medium actually decreases with the increasing strain rate. However, after the removal of corrosion products from the surface of specimens, the strain rate dependence of the ductility loss becomes positive, which indicates the absence of hydrogen in the bulk of a metal. At a short-term exposure in a corrosive environment, the deep penetration of hydrogen into a metal could be limited due to the insufficient time for hydrogen diffusion. The paper studies the mechanical behavior of the ZK60 alloy subjected to a longer (12 h) pre-exposure in a corrosive medium followed by tensile testing in air at various strain rates. The authors consider the effect of strain rate, long-term pre-exposure in a corrosive medium, and subsequent removal of corrosion products on the strength, ductility, stages of work hardening, and localized deformation, as well as on the state of the side and fracture surfaces of specimens. The study identified that the ductility loss of specimens pre-exposed in a corrosive medium for 12 h decreases with the increasing strain rate, regardless of whether the corrosion products have been removed from their surface or not. It is shown that in this case, the negative strain rate dependence of the ductility loss is associated not with hydrogen dissolved in the bulk of a metal but with the presence of severe corrosion damage of the specimens’ surface. The authors proposed an explanation for the effect of corrosion damage on the mechanical properties and their strain rate sensitivity.

Effects of reduced extrusion temperature on microstructure and mechanical properties of Mg–6Zn–0.5Zr alloy

Improving the strength of the commercial ZK60 alloys plays a key role in promoting their wide applications. In this work, a ZK60 alloy was prepared by conventional direct extrusion at an obviously reduced temperature of 180 °C. Compared with ZK60 alloys directly extruded at high temperatures (>300 °C), the studied alloy has finer dynamic recrystallized (DRXed) grains with the average size of ∼1.7 μm, stronger typical fiber texture and more profuse lath-like sub-grains. In addition, the ultra-low extrusion temperature increased dynamic precipitates (rod-like Mg4Zn7 and MgZn2) in DRXed grains. After aging, a great many of nanosized precipitates formed in both DRXed grains and non-recrystallized grains, and relatively coarse MgZn2 particles precipitated on sub-grain boundaries. As a result, the aged ZK60 alloy exhibit significantly high ultimate compressive strength of ∼675 MPa on the basis of high yield strength of ∼310 MPa, indicating a significant strain hardening capability. Meanwhile, it has excellent strength-ductility balance and a weakened tension-compression yield strength asymmetry under owning a high TYS in order of ∼400 MPa.

Kinetics, stages and fracture modes of stress-corrosion cracking in ZK60 and AZ31 alloys

The kinetics and fractographic features of stress corrosion cracking (SCC) in ZK60 and AZ31 alloys was investigated. It is established that ZK60 alloy possesses higher SCC resistance and fracture toughness compared to those of AZ31 alloy as is evidenced by the higher threshold stress intensity factors K1SCC and Kc. However, the lower average velocity of the stable SCC is observed in AZ31 alloy than in ZK60 one. Stable SCC occurs in two sub-stages, the first of which is featured by the lower crack growth rate and brittle fracture mode, while the higher crack velocity and quasi-cleavage fracture mode herald the second sub-stage. The crack growth velocities at specific SCC stages as well as stress intensity factor Kf associated with the alternation of the fracture mode are evaluated.

Microstructure and corrosion behavior of Y-modified ZK60 Mg alloy prepared by laser powder bed fusion

The microstructure and corrosion behavior of Y-modified ZK60 Mg alloy prepared by laser powder bed fusion (LPBF) are studied. By controlling the content of Y, the quantity, distribution, and type of precipitates are regulated, and the corrosion resistance is improved. Specifically, the ZK60 alloy with 1 wt% Y exhibits excellent corrosion resistance, which is mainly due to the formation of a continuous network of rare earth-containing precipitates at the grain boundaries that form corrosion barriers to impede corrosion and induce grain refinement. In addition, the passivation and stability of the surface film are improved after the addition of Y.

Study of microstructure evolution of magnesium alloy cylindrical part with longitudinal inner ribs during hot flow forming by coupling ANN-modified CA and FEA

Hot flow forming (HFF) is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs (CPLIRs) made of magnesium (Mg) alloys, which has wide applications in the aerospace field. However, due to the thermo-mechanical coupling effect and the existence of stiffened structure, complex microstructure evolution and uneven microstructure occur easily at the cylindrical wall (CW) and inner rib (IR) of Mg alloy thin-walled CPLIRs during the HFF. In this paper, a modified cellular automaton (CA) model of Mg alloy considering the effects of deformation conditions on material parameters was developed using the artificial neural network (ANN) method. It is found that the ANN-modified CA model exhibits better predictability for the microstructure of hot deformation than the conventional CA model. Furthermore, the microstructure evolution of ZK61 alloy CPLIRs during the HFF was analyzed by coupling the modified CA model and finite element analysis (FEA). The results show that compared with the microstructure at the same layer of the IR, more refined grains and less sufficient DRX resulted from larger strain and strain rate occur at that of the CW; various differences of strain and strain rate in the wall-thickness exist between the CW and IR, which leads to the inhomogeneity of microstructure rising firstly and declining from the inside layer to outside layer; the obtained Hall-Petch relationship between the measured microhardness and predicted grain sizes at the CW and the IR indicates the reliability of the coupled FEA-CA simulation results.

Simultaneous improvement of strength and ductility in ZK60 magnesium alloy by constructing the bimodal grain structure

In this study, the microstructure and mechanical properties of the commercial magnesium (Mg) ZK60 alloy were conducted to promote the application of Mg alloys. Scanning electron microscopy, tensile and compression tests, electron back scattering diffraction, transmission electron microscope and visco-plasticity self-consistent methods were employed to analyze the microstructure evolution and the strengthening and toughening mechanisms. By direct extrusion in the as-cast state, a bimodal microstructure was obtained, and the mechanical properties were enhanced. The ZK60 alloy, with a bimodal grain structure, exhibited a good combination of strength and ductility. The tensile yield strength (TYS), ultimate tensile strength (UTS), compressive yield strength (CYS) and fracture elongation (FE) reached 272 MPa, 347 MPa, 289 MPa, and 36.2%, respectively. Compared with the traditional extrusion ZK60 alloy, the increment in TYS, UTS, CYS, and FE was 15 MPa, 31 MPa, 25 MPa and 8.6%, respectively. The formation of the bimodal structure in the ZK60-CE sample is attributed to the combination of the PSN and Zener pinning effects. The improvement in strength is mainly attributed to the residual dislocations in the un-DRXed grains and the good ductility is ascribed to the activation of the non-basal slip. This study provides a low-cost and efficient strategy for the preparation of the bimodal structure by regulating the pre-extrusion microstructure to improve mechanical properties.

Dry Wear Behaviour of the New ZK60/AlN/SiC Particle Reinforced Composites.

This study deals with the microstructure, mechanical, and wear properties of the extruded ZK60 matrix composites strengthened with 45 µm, 15% silicon carbide particle (SiC) and 760 nm, 0.2-0.5% aluminium nitride (AlN) nanoparticle reinforcements. First, the reinforcement elements of the composites, SiC and AlN mixtures were prepared in master-magnesium powder, and compacts were formed under 450 MPa pressure and then sintered. Second, the compacted reinforcing elements were placed into the ZK60 alloy matrix at the semi-solid melt temperature, and the melt was mixed by mechanical mixing. After the melts were mixed for 30 min and a homogeneous mixture was formed, the mixtures were poured into metal moulds and composite samples were obtained. After being homogenized for 24 h at 400 °C, the alloys were extruded with a 16:1 deformation ratio at 310 °C and a ram speed of 0.3 mm/s to create final composite samples. After microstructure characterization and hardness analysis, the dry friction behavior of all composite samples was investigated. Depending on the percentage ratios of SIC and AlN reinforcement elements in the matrix, it was seen that the compressive strength and hardness of the composites increased, and the friction coefficient decreased. While the wear rate of the unreinforced ZK60 alloy was 3.89 × 10-5 g/m, this value decreased by 26.2 percent to 2.87 × 10-5 g/m in the 0.5% AlN + 15% SiC reinforced ZK 60 alloy.

In-situ aging treatment by preheating to obtain high-strength ZK60 Mg alloy processed by laser powder bed fusion

In this study, a ZK60 Mg alloy fabricated by laser powder bed fusion (LPBF) is subjected to in-situ aging treatment by preheating the substrate. The effects of preheating temperature on the microstructure, mechanical properties, and corrosion resistance of the LPBF-processed ZK60 alloy are systematically analyzed. The results show that at the preheating temperatures of 35, 90, and 180 °C, the average grain size of the samples does not change much, ranging between 7.0 and 8.2 μm. As the preheating temperature increases, the mechanical properties of the alloy increase. The specimens treated at a preheating temperature of 180 °C exhibit a yield strength (YS) of 201 ± 5 MPa, ultimate tensile strength (UTS) of 291 ± 7 MPa, and elongation (EL) of 14.7 ± 0.8%. This is attributed to the precipitation of a large amount of β’ 1 phase in the alloy during the aging state in the LPBF process. However, the corrosion resistance first improves and then deteriorates with the increase in the preheating temperature. Under a preheating temperature of 90 °C, the corrosion resistance is improved due to the reduction in the galvanic corrosion area by the precipitated phases. When the preheating temperature is increased to 180 °C, the precipitated phases and α-Mg form a large number of microgalvanic cells, which increases the active center for corrosion and deteriorates the corrosion resistance. These findings indicate that the desired mechanical properties and corrosion resistance of the LPBF-processed ZK60 alloy can be obtained by selecting an appropriate preheating temperature. • The cracks significantly reduce in LPBF-processed ZK60 alloy by substrate preheating. • The mechanical properties increase with increasing preheating temperature. • The corrosion resistance first improves and then deteriorates. • β’ 1 phase is beneficial to mechanical properties but deteriorates corrosion resistance.

Effect of Addition of Rare Earth Elements on the Microstructure, Texture, and Mechanical Properties of Extruded ZK60 Alloy

Effect of Addition of Rare Earth Elements on the Microstructure, Texture, and Mechanical Properties of Extruded ZK60 Alloy