Fabrication of Mg alloy tubes for biodegradable stent application

Though Mg alloys are promising candidates for biodegradable stents, it is very difficult to fabricate stent tubes with high dimensional accuracy using Mg alloys because of their low deformability. This study aimed to develop thin-walled, high-quality Mg alloy tubes with good performance in stent applications. Cold drawing with a fixed mandrel was carried out for extruded Mg-0.8%Ca and AZ61 alloy tubes using optimized drawing parameters and lubrication, and stent tubes with 1.5–1.8 mm outer diameter and 150 μm thickness were fabricated. A dimensional evaluation showed that the tube dimensional errors were within 0.02–2.5%. Also, an immersion test of pure Mg with different crystal orientations showed that the crystal orientation affected the corrosion properties, results that are the same with other Mg alloys. The crystal orientation of the stent tube could be controlled by changing the deformation amount and direction in the drawing, showing that it is possible to further improve the biodegradability of stents by approaching their fabrication from a processing aspect.     

Effect of strain ratio on cyclic deformation and fatigue of extruded AZ61A magnesium alloy

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (R_ɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.     

EBSD analysis of {10–12} twinning activity in Mg–3Al–1Zn alloy during compression

The {10–12} twinning activity of Mg–3Al–1Zn magnesium alloy during uniaxial compression at room temperature has been investigated by electron backscatter diffraction. The results indicated that the twinning activity was closely related with two angles: one was the angle between the c-axis and the compression direction and the other was the angle between the a-axis and the titling direction in the basal plane for a given relation between the c-axis and the compression direction. These two parameters can be used to explain which twinning variant will operate under the given strain path. For the grains containing a single {10–12} twinning variant, the {10–12} twinning variant occurred in a wide range of Schmid factor values (0 < Schmid factor < 0.5) and the Schmid factor rank of 1 or 2 was the most commonly observed. By contrast, for the grains containing two {10–12} twinning variants, the {10–12} twinning activity exhibited a stronger orientation dependence and the combinations of Schmid factor ranks 1–3 and 1–2 were the most commonly observed.     

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

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

Numerical formability assessment in single crystals of magnesium

In this paper a rate-sensitive elastic–viscoplastic crystal plasticity constitutive model (CPCM) together with the Marciniak–Kuczynski (M–K) approach have been used to assess the formability of a magnesium single crystal sheet by simulating the forming limit diagrams (FLDs). Sheet necking is initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field is assumed inside and outside the band, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs have been simulated under two conditions: (a) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, and (b) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, as well as extension and contraction twinning systems. The FLDs have been generated for two grain orientations. In the first orientation pyramidal 〈c + a〉 and extension twinning systems, and in the second orientation basal and pyramidal 〈c + a〉 slip systems, as well as contraction twinning systems have favourable orientation for activation. The effects of shear strains outside the necking band, strain rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations have been individually explored.     

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.     

Microstructure and mechanical properties of LA51 and LA51–0.5Y alloys with different accumulated strains and rolling temperatures

Mg–5Li–1Al (LA51) and Mg–5Li–1Al–0.5Y (LA51–0.5Y) alloys were smelted and rolled with different accumulated strains (36% and 68%) and rolling temperatures (373 K and 573 K). The microstructure, mechanical properties, fracture morphology and texture of the specimens were investigated. Results show that, due to the PSN (particle stimulate nucleation) mechanism, the addition of 0.5 wt.% Y improves the deformation resistance and weakens the basal texture of LA51 alloy. The effect of Y on UTS (ultimate tensile strength) of as-rolled alloys is more obvious than that of the as-cast alloy. Accumulated strain and rolling temperature could influence the twinning number, slip systems and DRX (dynamic recrystallization), thus affecting the microstructure and mechanical properties of the alloy. Under the proper combination of the above factors, the as-rolled LA51–0.5Y alloy with an accumulated strain of 68% at 573 K possesses the best comprehensive mechanical properties.     

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.     

Influence of texture on corrosion rate of AZ31 Mg alloy in 3.5 wt.% NaCl

In this study, AZ31 Mg alloys with different grain orientation were prepared to investigate the influence of texture on corrosion rate in 3.5 wt.% NaCl. Experimental results showed that texture had significant influence on the corrosion rate of AZ31 in 3.5% NaCl. The corrosion rate of AZ31 dramatically increased with the (0 0 0 1) texture intensity decreased and the (1 0 ?1 0)/(1 1 ?2 0) texture intensity increased. The corrosion rate reached a maximum at about 3 h immersion and subsequently decreased slightly due to the formation of a Mg(OH)₂ corrosion product layer. This study indicates that the corrosion rate of AZ31 Mg alloy in NaCl solution can be modified to some extend by controlling texture.     

Fracture mechanism of Mg–3Al–1Zn sheet at the biaxial state with respect to forming temperatures

Magnesium (Mg) sheet has been of great interest in automobile industries to make a light-weight design although it has low formability at the room temperature compared with steel sheets. It is required to elevate forming temperature to enhance the formability of Mg sheet, which enables increase of active slip systems in Hexagonal close packed (HCP) crystal structure. This paper demonstrates the effect of forming temperature on the formability of Mg–3Al–1Zn sheet, which is evaluated by the Limit Dome Height (LDH) test at temperature of 423 K, 523 K, and room temperature. The variation of dome heights depending on the forming temperature has been investigated to stand for its formability, and punch stroke and loads have been compared with each other. It has been tried to correlate the fracture mechanism with formability of AZ31 sheet with respect to the forming temperature by investigating the fracture surfaces with optical microscopy (OM) and orientation imaging microscopy (OIM) analyses.     

Effects of specimen aspect ratio on the compressive properties of Mg alloy foam

Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. The effects of specimen aspect ratio (the thickness/width ratio, AR) on the compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that the length of stress–strain plateau stage extended and ideality energy absorption efficiency improved with the specimen AR increasing and the yield strength decreased. Specimens with the AR = 1.00 possess good combination of yield strength, plateau stage length and compressive stability when compressed under the experiment conditions.     

Localized microtwin formation and failure during out-of-phase thermomechanical fatigue of a single crystal nickel-based superalloy

The deformation and damage mechanisms of a single crystal nickel-based superalloy CMSX-4 have been investigated under out-of-phase thermomechanical fatigue (OP TMF) condition. The deformation was highly localized to the area near the crack tip, where multiple groups of parallel twin plates on {1 1 1} planes formed during the high temperature-compressive half cycle. The atomistic a/6 〈1 1 2〉 twinning shear-based approach is presented which explains the origin of twinning. The localized twins provided a preferential path for crack propagation. OP TMF deformation was dominated by partial dislocation movement with {1 1 1}〈1 1 2〉 slip system, resulting in the formation and propagation of deformation twins.     

The effect of twinning and detwinning on the mechanical property of AZ31 extruded magnesium alloy during strain-path changes

In order to investigate the effect of twinning–detwinning on the mechanical properties of AZ31 extruded magnesium alloy pre-compression and pre-stretch deformation were conducted along extrusion direction (ED) at 1%, 3%, 5% strain levels. After pre-strain, the strain-path was inverted by performing tensile or compressive tests at room temperature. Results showed that the detwinning behavior occurred during the inverse tension after the pre-compression. Although due to the aforementioned effect the tensile yield strength decreased, by increasing the pre-compressive levels both fracture elongation and peak strength improved. In the inverse compressive tests after pre-stretch the {1 0 −1 2} twinning was restrained and the volume fraction of twins decreased, leading to the improvement of yield strength by increasing in pre-stretching levels.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Effect of strain amplitude on the low-cycle fatigue behavior of a new Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy

The low-cycle fatigue (LCF) properties and post-fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si austenitic alloy were investigated under an axial strain control mode with total strain amplitudes, Δε_t/2, ranging from 2.5 × 10⁻³ to 2 × 10⁻². The fatigue resistance of the alloy was described by Coffin–Manson’s and Basquin’s relationships, and the corresponding fatigue parameters were evaluated. In addition, the Masing behavior, which is associated with a constant deformation mode during fatigue, was revealed at the examined strain amplitudes. Microstructural observations of the fatigue fractured samples showed that the strain induced ε-martensitic transformation accompanied by a planar slip of the Shockley partial dislocations in the austenite is the main deformation mode controlling the fatigue behavior of the studied alloy at Δε_t/2 < 2 × 10⁻². However, at Δε_t/2 = 2 × 10⁻², the formation of a cell structure was found in the austenite in addition to ε-martensitic transformation. The LCF resistance of the alloy was compared with conventional Cr–Ni austenitic stainless steels, ferrous base TRIP and TWIP steels and low yield point damping steels. It was found that at the studied strain amplitudes the alloy possessed a higher LCF resistance compared to conventional Fe-base alloys and steels. Remarkably, the fatigue ductility coefficient, ε_f′, of the studied alloy is 1.3–6 times higher than that of the stainless steels because of a cyclic deformation-induced ε-martensitic transformation. The results showed that the ε-martensitic transformation that occurred in the studied alloy during LCF is the main reason for the improved LCF resistance.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Ultrafine grain structure features in spray-formed AZ31 magnesium alloy

The ultrafine grain structure was developed in spray-formed AZ31 magnesium alloy by optimizing delivery tube orifice diameter. A significant refinement of grain size ∼1 μm in ultrafine level was achieved by using 2.25 mm delivery tube orifice diameter. The tensile strength value was increased from 145 MPa of as-cast alloy to 250 MPa of spray-formed alloy, registering an enhancement of ∼72%. On the other hand, elongation was increased from 6% to 13% using 2.25 mm orifice, registering more than onefold increase in elongation. Also, hardness enhancement of ∼49% was observed in spray-formed AZ31 alloy compared to as-cast alloy. The fracture surface of spray-formed AZ31 Mg alloy evidences the mixed type of ductile and brittle fracture.     

Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000 °C

An ultrasonic fatigue testing system capable of operating at temperatures up to 1000 °C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000 °C and loading frequency of approximately 20 kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000 °C for PWA 1484 occurred in a crystallographic manner on {1 1 1} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850 °C) with conventional servo-hydraulic loading frequencies (<100 Hz).     

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.     

Tensile properties of hot rolled Mg–3Sn–1Ca alloy sheets at elevated temperatures

Mechanical behavior of hot rolled Mg–3Sn–1Ca (TX31) magnesium alloy sheets were studied in the temperature range 25–350 °C. The microstructure of the alloy consisted of the eutectic structure of α-Mg + Mg₂Sn and a dispersion of needle-like CaMgSn. The highest room-temperature ductility of 18% was obtained by hot rolling of the cast slabs at 440 °C, followed by annealing at 420 °C. The high temperature tensile deformation of the material was characterized by a decrease in work hardening exponent (n) and an increase in strain rate sensitivity index (m). These variations resulted in respective drops of proof stress and tensile strength from 126.5 MPa and 220 MPa at room temperature to 23.5 MPa and 29 MPa at 350 °C. This was in contrast to the ductility of the alloy which increased from 18% at room temperature to 56% at 350 °C. The observed variations in strength and ductility were ascribed to the activity of non-basal slip systems and dynamic recovery at high temperatures. The TX31 alloy showed lower strength than AZ31 magnesium alloy at low temperatures, while it exhibited superior strength at temperatures higher than 200 °C, mainly due to the presence of thermally stable CaMgSn particles.