Effect of microstructure on fatigue behaviour of cast Al–7Si–Mg alloy

Abstract

The fatigue behaviour of a cast Al–7Si–Mg alloy, conforming to A356, has been studied. Specimens of this material were tested in both the as cast condition and a solution treated and aged condition. It was observed that the size, number, and position of casting defects influenced the fatigue life very strongly. This marked effect nearly hides that of the heat treatment. Nevertheless, if the analysis is carried out considering only results obtained from sound specimens it is revealed that the heat treatment causes an improvement in the fatigue resistance of the alloy.     

Effect of pulse-current heating multiple forging on mechanical properties of Mg–Nd–Zn–Zr alloy

The effects of pulse current heating on microstructure evolution and mechanical property of Mg–Nd–Zn–Zr magnesium alloy during multiple forging are investigated. The results of uniaxial tensile test show that tensile strength, yield strength and elongation of the alloy are improved with the increasing of forging passes. Moreover, under the condition of pulse current heating, this promotion effect is more obvious. The results of microstructure analysis show that recrystallisation occurs during the deformation, and the recrystallisation is obviously accelerated by pulse current. The magnesium alloy billet with fine grains is obtained eventually. Theoretical analysis shows that pulse current can increase the recrystallisation nucleation rate and reduce the rate of grain growth, which is helpful to refine the grains of alloy.     

In vitro degradation behavior and biocompatibility of Mg–Nd–Zn–Zr alloy by hydrofluoric acid treatment

In this paper, Mg–Nd–Zn–Zr alloy (denoted as JDBM) coated with hydrofluoric acid (HF) chemical conversion film (MgF₂) was researched as a potential biodegradable cardiovascular stent material. The microstructures, in vitro degradation and biocompatibility were investigated. The field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) showed that a compact MgF₂ film was formed on the surface of JDBM. The corrosion rate decreased in artificial plasma from 0.337 to 0.253 mm·y^? 1 and the electrochemical measurement demonstrated that the corrosion resistance of JDBM alloy could be obviously improved due to the protective MgF₂ film on the surface of the substrate. Meanwhile, the hemolysis ratio of JDBM decreased from 52.0% to 10.1% and the cytotoxicity met the requirement of cellular application after HF treatment. In addition, JDBM and MgF₂ film showed good anti-platelet adhesion, which is a very favorable property for implant material in contact with blood directly.     

Influences of heat treatment on microstructural evolution and tensile behavior of squeeze-cast Mg–Gd–Y–Zr alloy

The effects of solution heat treatment and aging on the microstructural evolution and mechanical behavior of a squeeze-cast (SC) Mg–10Gd–3Y–0.5Zr (GW103K) alloy, processed using various applied pressures (e.g., 0.1, 40, 80 and 160 MPa) were systematically investigated. Our results show that, after solution heat treatment, secondary phases and pressure-induced dislocations are dissolved in the matrix of the squeeze-cast alloys. Moreover, subsequent aging heat treatment leads to an increased age-hardening response relative to that in squeeze-cast GW103K and this trend increases with increasing applied pressure. The room temperature tensile test results show that the yield strength (YS) for the squeeze-cast alloy in the as-cast, the as-T4 heat-treated and the as-T6 heat-treated states increases with increasing applied pressure, from 0.1 to 80 MPa, and remains relatively constant when the applied pressure is increased to 160 MPa, whereas the ultimate tensile strength (UTS) and elongation-to-failure (E _f) increases continuously with increasing applied pressure. The measured increases in YS and UTS (or E _f), are discussed in terms of the mechanisms that govern the evolution of microstructure in squeeze-cast GW103K, paying particular attention to gain size and porosity.     

The relation between heat treatment and corrosion behavior of Mg–Gd–Y–Zr alloy

The corrosion behavior of Mg–7Gd–3Y–0.4Zr (GW73K) was investigated in as-cast (F), solution-treated (T4) and peak-aged (T6) conditions using immersion tests and electrochemical measurements in NaCl solution (5 wt.%). Microstructure analyses were carried out on GW73K after different heat treatments by optical microscope (OM), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction. It is found that GW73K alloy exhibits higher corrosion resistance in T4 than in F and T6 conditions due to the fully dissolution of cathodic coarse (Gd + Y) rich eutectic compound. The corrosion products of GW73K have different morphologies for F, T4 and T6 conditions. The product for F is less uniform and compact than T4 and T6, and it has been founded that GW73K-T6 had two different morphologies owing to the presence of β′. The results of polarization curves also confirm that proper heat treatment is beneficial to improve the corrosion resistance of GW73K alloy by transforming the microstructures.     

Influence of fluoride treatment on surface properties,biodegradation and cytocompatibility of Mg–Nd–Zn–Zr alloy

Fluoride treatment is a commonly used technique or pre-treatment to optimize the degradation kinetic and improve the biocompatibility of magnesium-based implant. The influence of changed surface properties and degradation kinetics on subsequent protein adsorption and cytocompatibility is critical to understand the biocompatibility of the implant. In this study, a patent magnesium alloy Mg–Nd–Zn–Zr alloy (JDBM) designed for cardiovascular stent application was treated by immersion in hydrofluoric acid. A 1.5 μm thick MgF₂ layer was prepared. The surface roughness was increased slightly while the surface zeta potential was changed to a much more negative value after the treatment. Static contact angle test was performed, showing an increase in hydrophilicity and surface energy after the treatment. The MgF₂ layer slowed down in vitro degradation rate, but lost the protection effect after 10 days. The treatment enhanced human albumin adsorption while no difference of human fibrinogen adsorption amount was observed. Direct cell adhesion test showed many more live HUVECs retained than bare magnesium alloy. Both treated and untreated JDBM showed no adverse effect on HUVEC viability and spreading morphology. The relationship between changed surface characteristics, degradation rate and protein adsorption, cytocompatibility was also discussed.     

High cycle fatigue properties of cast Mg–xNd–0.2Zn–Zr alloys

The tensile and fatigue strength of cast Mg–xNd–0.2Zn–0.45Zr alloys (x = 0, 1, 2, 3 wt%) in both solution-treated (T4) and solution + 200 °C peak-aged (T6-PA) conditions were investigated in the present study. The results indicate that Neodymium (Nd) is an effective element to improve both the tensile and fatigue properties of cast Mg–0.2Zn–Zr alloys. The strengthening effect depends on its content in a way of power function (σ = σ₀ + K C _Nd ⁿ ), where the power exponent n is about 0.52–0.54 for yield strength (YS) and 0.59–0.61 for fatigue strength. The yield strengthening effect of Nd element in the form of precipitates (T6-PA) is about three times of that as solution atoms (T4), while the fatigue strengthening effect of Nd element in the form of precipitates is only about 50 % higher than that as solution atoms. The improved strength (both YS and ultimate tensile strength) can lead to the same amount improvement of the fatigue strength in T4-treated alloys, while only can cause less than half improvement of the fatigue strength in T6-PA-treated alloys.     

Low cycle fatigue of an extruded Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

The purpose of this study was to evaluate strain-controlled cyclic deformation behavior of an extruded Mg–3Nd–0.2Zn–0.5Zr (NZ30K) magnesium alloy. The microstructure of this alloy consisted of a bimodal microstructure with equiaxed recrystallized grains and unrecrystallized coarse grains along with a large number of smaller second-phase particles present inside the grains and larger particles along the grain boundaries alongside a characteristic precipitate free zone (PFZ). The average grain size was about approximately 5–7 μm. It was observed that unlike the higher RE-containing Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy, the NZ30K alloy exhibited asymmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of R_ε = −1. This was mainly due to the presence of relatively stronger crystallographic texture, PFZ, and the resultant twinning–detwinning activities during cyclic deformation. While this alloy exhibited cyclic softening at lower strain amplitudes and cyclic hardening at higher strain amplitudes, it had an equivalent fatigue life to that of other extruded Mg alloys. Fatigue crack was observed to initiate from the specimen surface with some isolated facets of the cleavage-like planes near the initiation site. Crack propagation was basically characterized by serrated fatigue striations.     

Effect of process parameters on structure and macrosegregation upon direct chill casting of Mg–Nd–Zn–Zr alloy

Abstract

Direct chill (DC) semicontinuous casting process has been successfully used to produce sound Mg–3·0Nd–0·4Zn–0·4Zr (NZ30K) billets. The influence of process parameters such as casting speed, casting temperature on the microstructure and macrosegregation was studied. The results show that the casting speed affects the macrosegregation greatly while it has a slight influence on the grain size of the billet; the casting temperature has a slight influence on macrosegregation of the billet while the grain size of the billet increases as the casting temperature increases. The optimal process parameters have been experimentally determined as follows: casting temperature 700°C and casting speed 90 mm min^?1. The ultimate tensile strength, yield strength and elongation of billets cast at the optimal casting parameters are 196 MPa, 125 MPa and 16·5% respectively.     

Effect of aging treatment on mechanical properties and fracture behavior of friction stir processed Mg–Y–Nd alloy

In this study, precipitation behavior of Mg–Y–Nd cast alloy during friction stir processing (FSP), and the effect of subsequent artificial aging on mechanical properties and fracture behavior of the FSP alloy were investigated. It is found that the coarse α-Mg grains and large second phases are greatly refined after FSP. Moreover, due to the heat input during processing and the natural cooling, β′ and β₁ precipitates are also observed in the FSP alloy. The FSP specimens were subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 150 °C for 54 h and 180 °C for 30 h. Strengths of the peak–aged specimens are further increased, which is attributed to the large quantity of β″ and β₁ precipitates, respectively. Meanwhile, elongations of the peak-aged specimens are both decreased. Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of FSP and peak-aged specimens are all mixed ductile–brittle fracture mode. However, compared to the FSP specimens, different fracture paths are exhibited in peak–aged specimens.     

Influence of aging temperature on corrosion behavior of Al–Zn–Mg–Sc–Zr alloy

Al–Zn–Mg–Sc–Zr alloy sheets were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The effect of aging temperature on the corrosion characteristics of Al–Zn–Mg–Sc–Zr alloy was investigated by means of exfoliation corrosion testing, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) combined with transmission electron microscopy, scanning electron microscopy and optical microscopy observation. It is found that with increasing the aging temperature the susceptibility to exfoliation corrosion decreases. Electrochemical measurements reveal that at early stage of immersion in testing solution, EIS plots of the samples are composed of a capacitive arc and an inductive loop. Inductive loop disappears with the increasing of immersion time and two time constants in impedance diagrams appear. Moreover, the trends of corrosion resistance are further confirmed by polarization curve and EIS test. In addition, transmission electron microscopy observations show that the improved corrosion resistance from increasing aging temperature is duo to the coarsening of matrix and separated precipitates at the grain boundary, and the increased spacing of grain boundary precipitates.     

Effect of precipitate free zones on low cycle fatigue life of Al–Zn–Mg alloy

Abstract

The effect of precipitate free zones (PFZs) on the low cycle fatigue (LCF) life of an Al–4·70 wt-%Zn-2·80 wt-%Mg alloy at room temprature was investigated. The alloy used in the present study was uniquely prepared to have different widths of PFZ, whlle keeping the distribution and size of predominant precipitates in the matrix or on grain boundaries almost constant. As the width of the PFZ increased from 0·073 to 0·3 μm,the LCF life was observed to increase to some extent. However, the tensile properties such as yield strength, tensile strength, and elongation remained approximately unchanged. Studies using transmission electron mlcroscopy show that the extent of planar slip decreases with increasing PFZ width throughout cyclic deformation. Thls result suggests that the increase in LCF life is probably due to relaxation of the local stress in the soft PFZ regions.

MST/2093     

Effect of two step strain rate on superplastic behaviour of Al–Zn–Mg–Cu–Zr alloy 7010 containing Sc

Abstract

A suitable thermomechanical process, different from the Rockwell route, before the superplastic deformation together with the utilisation of a ‘two step strain rate’ during the superplastic deformation has been employed to evaluate the total percentage elongation in a high strength Al–Zn–Mg–Cu–Zr alloy containing Sc. It is shown that the utilisation of a two step strain rate under appropriate combinations of temperature and strain rate results in superior values of elongations.     

Effect of strain on cavity development during Al–Zn–Mg–Cu alloy superplastic flow

Cavitation behaviour has been investigated in an Al–Zn–Mg–Cu alloy with an average grain size of 10?µm during superplastic deformation. The superplastic tensile tests were interrupted at different true strains at 530°C and 3?×?10^?4?s^?1. The results showed that cavity nucleation occurred above a critical strain in the optimum loading condition. It was easy for cavities to form at the triple junction due to the stress concentration caused by cooperative grain boundary sliding. Since the tensile stress was higher in the middle of the sample, the cavities were arranged in a straight line parallel to the tensile axis in the centre of the sample. A more appropriate cavity growth equation considering the critical strain was proposed to describe the cavitation behaviour.     

Effects of the enhanced heat treatment on the mechanical properties and stress corrosion behavior of an Al–Zn–Mg alloy

In this article, the effects of the enhanced solution treatment (EST) and high-temperature pre-precipitation (HTPP) on the microstructures, mechanical properties, and stress corrosion cracking resistance of an Al–Zn–Mg alloy have been investigated. The results indicated that EST and HTPP can substantially affect the microstructures of the alloy. The width of the continuously distributed grain boundary precipitates decreases after the EST, while the continuous grain boundary precipitation changes to a discontinuous precipitation structure after both the EST and HTPP. The yield strength, tensile strength, elongation, and fracture toughness of the specimens after the EST are much higher than those of the specimens only after traditional solution treatment, since the EST substantially decreases the size and volume fraction of the constituents. The stress corrosion cracking resistance of the specimens after both the EST and HTPP has been greatly improved due to the discontinuous distribution, and high Cu and low Mg concentrations of the grain boundary precipitates.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Effect of Y on the bio-corrosion behavior of extruded Mg–Zn–Mn alloy in Hank's solution

The bio-corrosion properties of Mg–Zn–Mn alloys with and without Y in Hank's solution at 37 °C were investigated by using electrochemical test and electrochemical impedance spectra (EIS). The results of open circuit potential (OCP) and polarization tests indicated that Y could reduce the cathodic current density. A passivative stage appeared in the Tafel curve of the Y containing magnesium alloy, indicating that a passivative film was formed on the surface of the Y containing magnesium alloy. EIS results showed that the Y containing alloy had higher charge transfer resistance and film resistance, but lower double layer capacity than the alloy without the Y element. The surface reaction product identification by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) showed that the surface corrosion products were hydroxide and phosphate (Mg₃Ca₃(PO₄)₄) for Mg–Zn–Mn alloy and phosphate (MgNaPO₄) for the Y containing Mg–Zn–Mn alloys. The XPS results also showed that a Y₂O₃ protective film was formed on the surface of the Y containing magnesium alloy which contributed mainly to the low cathodic current density and the high resistance.     

Effect of grain size on strain rate sensitivity of cryomilled Al–Mg alloy

Al–Mg alloy powder was cryomilled to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic forging. The consolidation resulted in a bulk material with ultrafine grains of about 250 nm, and the material exhibited enhanced strength compared to conventionally processed Al–Mg alloy. The hardness of as-cryomilled powder, the forged ultrafine-grained (UFG) material, and the conventional coarse-grained (CG) alloy were measured by nanoindentation using various loading rates, and the results were compared with strain rate sensitivity (SRS) from uniaxial compression tests. Negative SRS was observed in the cryomilled NC powder and the forged UFG material, while the conventional alloy was relatively insensitive to strain rate. The dependence on loading rate was stronger in the NC powders than in the UFG material.