Anisotropy of the fatigue behavior of extruded and rolled magnesium alloys

Fatigue tests were conducted using both extruded and rolled magnesium alloys AZ31 to study anisotropy of the fatigue behavior of the alloys. For the above purpose, two types of specimens whose longitudinal axes are parallel (E specimen) and perpendicular (T specimen) to the extrusion direction were prepared for the extruded Mg alloy. For the rolled Mg alloy, three types of specimens, whose longitudinal axes are parallel (R specimen) and perpendicular (T and S specimens) to the rolling direction, were prepared. S–N curves and crack propagation characteristics for both the extruded and rolled specimens with different longitudinal directions were studied to investigate the effects of the texture, microstructures and residual stresses on the fatigue behavior of the magnesium alloy AZ31. Anisotropy of the fatigue behavior was observed for both the extruded and the rolled magnesium alloys. In the extruded Mg alloy, differences in both fatigue lives and fatigue limit exist between E and T specimens. The fatigue resistance for the former is superior to that of the latter. In the rolled Mg alloy, lesser differences were found between R and T specimens. However, fatigue lives of the S specimens were clearly shorter than those of the former at the high stress amplitudes above the fatigue limit. In the extruded Mg alloy, the rod-like microstructure observed is an important factor contributing to the anisotropy of the fatigue behavior of the alloy. While in the rolled Mg alloy, the texture induced during processing is considered to play a dominant role in the anisotropy of the fatigue behavior of the alloy.     

Fracture mechanism of AZ31 magnesium alloy processed by equal channel angular pressing comparing three point bending test and tensile test

A commercial magnesium alloy, AZ31 in hot-rolled condition, has been processed by equal channel angular pressing (ECAP) to get microstructure modified. Uniaxial tensile tests were conducted along the rolling/extrusion direction for as-received AZ31 alloy and ECAPed AZ31 alloy. Then, three point bending fracture tests were conducted for specimens with a pre-crack perpendicular to the extruded direction. Digital image correlation (DIC) technique was adopted to determine the deformation field around the crack tip. The fracture surfaces of the failed specimens after tensile tests and fracture tests were observed by Scanning Electron Microscope (SEM). To explore the deformation mechanism, the microstructure and texture of different regions on the deformed specimens were examined through electron backscatter diffraction (EBSD). The results show ECAP process improves both the tensile elongation and fracture toughness of AZ31 alloy. Different from the slip dominated deformation mechanism in the tensile test, deformation twinning presents in the deformation zone adjacent to the crack tip in the three point bending fracture tests. The fracture surface is characterized by co-occurrence of dimple and cleavage features.     

AZ31与AZ61异种镁合金的TIG焊研究

采用TIG焊对AZ31与AZ61镁合金薄板进行连接,分别采用了AZ31和AZ61焊丝,比较焊接效果的影响.通过显微镜、扫描电镜、X-ray物相检测等实验方法,分析了两种焊丝焊接接头的外观形貌、显微组织、焊缝析出相及力学性能等差异.研究结果表明:采用AZ31和AZ61两种焊丝都能完成AZ31与AZ61镁合金薄板的对焊,获...     

Zwillingsbildung bei der thermischen Ermüdung der Mg‐Basislegierung AZ31

Twinning at thermal fatigue of magnesium alloy AZ31 In this paper results of thermal fatigue tests of the magnesium base alloy AZ31 carried out in a temperature range between ‐50 °C and +290 °C are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stresses. The resulting materials behaviour is described by stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that AZ31 shows different stress‐strain behaviour during tensile and compressive loading resp. at lower temperatures due to the fact that mechanical twinning depends on the loading direction. However untwinning processes may occur during unloading and reloading in the opposite direction. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins are observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.     

17-4PH末级长叶片叶根的疲劳强度与缺口效应

采用成组法和升降法开展了室温某17-4PH末级长叶片叶根纵、横方向光滑与缺口试样的疲劳试验,获得了两个方向上材料的S-N曲线,并对典型疲劳断裂试样进行了断口宏微观形貌观察。结果表明:叶片叶根纵、横方向的疲劳强度基本一致,在长寿命区,纵向疲劳强度略高于横向;光滑试样的疲劳断裂主要呈现单裂纹源特征,而缺口试样呈现多裂纹源汇聚的断裂形貌。结合试样缺口根部的弹塑性应变分析,进一步讨论了叶片叶根纵、横方向的疲劳缺口效应。     

Anisotropy of mechanical properties in quaternary Al-Li-Cu-Mg alloys

Considerable anisotropy in the mechanical properties of quaternary Al-Li-Cu-Mg alloys was observed in both plate and sheet products. These alloys showed more than 100% increase in tensile ductility in the test direction oriented at 45–60° to the rolling direction as compared to that in the rolling direction (longitudinal, L). A concomitant decrease in strength was also found. These alloys exhibit superior low cycle fatigue resistance in the long-transverse (LT) direction as compared to the longitudinal (L) direction. Another observation is the occurrence of strength differential (S-D), which is seen to be directional. The trends in S-D are similar under monotonic as well as cyclic loading conditions. The fracture resistance is also highly anisotropic. An attempt is made here to correlate the observed anisotropy in the mechanical behaviour of these alloys with the microstructure and crystallographic texture.     

HIGH TEMPERATURE CYCLIC DEFORMATION OF A DIRECTIONALLY SOLIDIFIED Ni-BASE SUPERALLOY

Abstract— The high temperature low cycle fatigue behaviour of a directionally solidified Ni-base superalloy hardened by about 65% volume fraction of γ'-precipitates was investigated in order to determine the fatigue life parameters for longitudinal (L) and longitudinal transverse (LT) grain orientations. The fatigue resistance was compared with that of two oxide dispersion strengthened (ODS) Ni-base superalloys with a similar elongated grain structure.
The fatigue life of the alloy can be adequately predicted by Basquin and Coffin-Manson empirical relationships and the fatigue ductility parameters in these relationships show a similar trend with the tensile ductility properties.
The studied alloy exhibits a fairly stable cyclic stress response, with only a slight stress softening. Fatigue crack initiation occurs mainly at shrinkage pores on the surface or sub-surface of the specimens. The crack growth direction is predominantly perpendicular to the applied load. The fracture mode in the LT-direction is transgranular and fatigue life is shorter by a factor of about six compared to the L-direction. The fatigue life of the alloy is longer than that of the ODS Ni-base superalloys with which it is compared.     

Symmetric and asymmetric dynamic characterization of mode‐III fracture in epoxy/unidirectional carbon‐fibre composites

This paper experimentally analyses the delamination behaviour of a composite material under mode III fracture as a function of the loading mode applied during the fatigue tests was carried out in two different ways. First, the test specimens were loaded cyclically in a single direction of delamination until reaching a certain fracture energy and subsequently returning to the starting point. Second, cyclic loading was applied both in one direction of delamination and in the opposite direction until reaching the same fracture energies, though in two opposite directions. The experimental results revealed clear differences in the fatigue behaviour of the material when the way of applying the load varied. In both cases, a statistical analysis of the results was carried out in order to evaluate the existing differences more suitably. Furthermore, a fractographic analysis was conducted for both types of loading in order to distinguish possible differences at the microstructural level.     

Fatigue behaviour of cast magnesium alloy AZ91 microstructurally modified by friction stir processing

Friction stir processing (FSP) was applied to cast magnesium alloy AZ91-F to modify the as-cast microstructure, and the effect of FSP on fatigue behaviour was discussed based on microstructural consideration, crack initiation, crack growth behaviour, and fracture surface analysis. Fully reversed axial fatigue tests have been performed using as-cast, T5-aged and their FSPed specimens (as-cast/FSP and T5/FSP). It was found that both FSPed specimens exhibited significantly higher fatigue strength than the as-cast and T5-aged specimens. FSP resulted in the break-up of coarse as-cast microstructure, grain refinement of the matrix, finely dispersed precipitates and increase of hardness, thereby both the crack initiation resistance and the crack growth resistance were considerably enhanced compared with the as-cast and T5-aged specimens, resulting in the improved fatigue strengths of the FSPed specimens.     

Observations and modeling of the small fatigue crack behavior of an extruded AZ61 magnesium alloy

The objective of this paper is to quantify the microstructurally small fatigue crack growth of an extruded AZ61 magnesium alloy. Fully reversed and interrupted load-controlled tests were conducted on notched specimens that were taken from the material in the longitudinal and transverse orientations with respect to the extrusion direction. In order to measure crack growth, replicas of the notch surface were made using a dual-step silicon-rubber compound at periodic cyclic intervals. By using microscopic analysis of the replica surfaces, crack initiation sites from numerous locations and crack growth rates were determined. A marked acceleration/deceleration was observed to occur in cracks of smaller length scales due to local microheterogeneities consistent with prior observations of small fatigue crack interaction with the native microstructure and texture. Finally, a microstructure-sensitive multistage fatigue model was employed to estimate the observed crack growth behavior and fatigue life with respect to the microstructure with the most notable item being the grain orientation. The crack growth rate and fatigue life estimates are shown to compare well to published findings for pure magnesium single crystal atomistic simulations.     

Strength differential in Al---Li alloy 8090

Cyclic strength differential (SD) data have been derived from the variation of the tensile and the compressive stress amplitude with elapsed cycles during low cycle fatigue (LCF) for the quaternary Al---Li alloy 8090 in the T8E51 condition. LCF test specimens were machined out of the rolled plate such that the direction of stressing was along the longitudinal (L), L+45° and long transverse (LT) directions. The analysis of cyclic stress amplitude data corresponding to the half-life revealed an SD effect at all strain levels in the L+45° and LT directions. In the case of the L direction, at strain levels below 8.5 × 10⁻³, the SD is more than compensated by the Bauschinger effect resulting from the prestretch. The alloy exhibits a similar SD behaviour under monotonic loading conditions. The magnitude of cyclic SD at intermediate strain amplitudes in all the three test directions decreases upon cycling, which behaviour can be attributed to the relaxation of prestretch-related residual stresses leading to a decrease in σ_T only in the L direction and σ_C only in the L+45° and LT directions. As microstructural features vary in the differently heat-treated conditions, a comparison has been made of the monotonic SD data in the stretched and aged (T8E51) condition with those in the solution-treated, in the peak-aged (T6) and in the overaged (T7) conditions.     

Adding carbon nanotubes and integrating with AA5052 aluminium alloy core to simultaneously enhance stiffness, strength and failure strain of AZ31 magnesium alloy

New bimetal AZ31-CNT/AA5052 macrocomposite comprising: (a) carbon nanotube (CNT) reinforced magnesium alloy AZ31 shell and (b) aluminium alloy AA5052 millimeter-scale core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterisation revealed more rounded intermetallic particle of decreased size, reasonable CNT distribution, and dominant (1 0 −1 1) texture in the longitudinal and transverse directions in the AZ31-CNT nanocomposite shell. Interdiffusion of Mg and Al across the core-shell macrointerface into each other was also significant. Compared to monolithic AZ31, the AZ31-CNT shell had significantly higher hardness (+30%). In tension, the presence of CNT (in the AZ31 shell) and AA5052 core significantly increased stiffness (+39%), ultimate strength (+13%), failure strain (+17%) and work of fracture (+27%) of AZ31, while yield strength (−2%) was marginally decreased. In compression, the presence of CNT (in the AZ31 shell) and AA5052 core significantly increased yield strength (+35%), failure strain (+42%) and work of fracture (+70%) of AZ31, while ultimate strength (+1%) was marginally increased. The effect of joint presence of: (a) CNT (in the AZ31 shell) and (b) AA5052 millimeter-scale core on tensile and compressive properties of AZ31 is investigated in this paper.     

Experimental investigation of pulse current on mechanical behaviour of AZ31 alloy

The effect of pulse current on the mechanical properties of AZ31 alloy is investigated through uniaxial tensile test at different temperatures. The electroplastic effect is evaluated by the change of ultimate strength. During tensile test, both microstructure evolution and fracture behaviour are sensitive to the applied deformation conditions. In this work, the influence of pulse current is discussed from the point of view of microstructure evolution and fracture characteristics. The experimental results show that the dynamic recrystallisation temperature of AZ31 is reduced by the pulse current and continuous dynamic recrystallisation is found at 100°C for tensile test with current. The data also indicate that the pulse current accelerates the precipitation and dissolved of the second phase particles in AZ31 alloy.     

Cavitation Behavior during Superplastic Deformation of AZ31 Magnesium Alloy

Superplasticity of AZ31 Mg alloy at the temperature range of 250～450℃ and stain rate range of 0.7x10-3～ 1.4x 10-1 s-1 was examined through uniaxial tensile test. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of cavities and surface relief near fracture surface, respectively.It is shown that AZ31 Mg alloy starts to exhibit superplasticity from 300℃. The maximum elongation of 362.5%was obtained at 400℃ and strain rate of 0.7×10-3 s-1. There exist many O-shaped cavities and filaments at the boundaries near fracture surface. The fracture of filaments results in intergranular cavity and the model for the formation of intergranular cavities is proposed. The growth of cavities is plasticity-controlled and the serrated boundaries of intergranular cavities agree with the results of surface relieves.     

Effects of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy

Load‐controlled fatigue tests were performed at 20 and 50 °C using two relative humidity levels of 55 and 80% to characterize the influence of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy. Fatigue tests were also conducted at 150 °C. No significant variation in fatigue properties was noticed with respect to temperature over the range from 20 to 50 °C for both the humidity levels. Fatigue limits in the range 140–150 MPa were observed for relative humidity of 55%. Fatigue strength decreased significantly with increase in temperature to 150 °C. Further, a significant reduction in fatigue strength with a fatigue limit of ～110 MPa was observed with increase in relative humidity to 80% at 20 and 50 °C. The crack initiation and propagation remained transgranular under all test conditions. The fatigue fracture at low stress amplitudes and high relative humidity of 80% results from the formation of corrosion pits at the surface and their growth to a critical size for fatigue‐crack initiation and propagation. The observed reduction in fatigue strength at high humidity is ascribed to the effects associated with fatigue–environment interaction.     

Zum Wechselverformungsverhalten der Magnesiumbasislegierung AZ31

The cyclic deformation behaviour of Mg – base alloy AZ31 The cyclic deformation behaviour of Mg – base alloy AZ31 was investigated in stress controlled tension-, compression-tests. Experiments with zero mean stress (R = -1) as well as with tensile or compression mean stress (R = 0, R = - ∞ resp.) were carried out. Cyclic strain hardening and a pronounced anisotropy of strength during the first loading cycles was observed with higher yield strength in tension compared with compression. Consequently, in tests with zero mean stress cyclic creep and compressive mean strains occured.     

An investigation on the high stress sensitivity of fatigue life of rolled AZ31 magnesium alloy under constant amplitude fatigue loading

In the present work, an investigation on the high stress sensitivity of the fatigue life of the AZ31 rolled magnesium alloy under constant amplitude fatigue loading has been carried out. Different damage parameters were involved to quantify fatigue damage accumulation at the various scales of material volume corresponding to the changing fatigue damage mechanisms that prevail at the various stages of the fatigue life. The experimental work included mainly nano‐indentation measurements to evaluate hardness evolution at the nano‐scale due to cyclic plasticity, results of micro‐crack monitoring by using the replication technique and fractographic analysis to obtain the fracture characteristics of the fatigue specimens after failure. The hexagonal close‐packed structure of the alloy and the resulting difficulty for the activation of five independent slip systems required for homogeneous plastic deformation were considered to determine the high stress sensitivity of the fatigue life observed for the rolled AZ31 alloy under the investigated loading conditions.     

The effect of cerium on high-cycle fatigue properties of die-cast magnesium alloy

The effect of cerium (Ce) on high‐cycle fatigue behaviour of die‐cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio, R= 0.1 on specimens of AZ91D alloys with different Ce additions. The microstructure and fatigue fracture surfaces of specimens were examined using a scanning electron microscope (SEM) to reveal the micromechanisms of fatigue crack initiation and propagation. The results show that the grain size of AZ91D is refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D evaluated by the up‐and‐down load method increases from 96.7 MPa to 116.3 MPa (1% Ce) and 105.5 MPa (2% Ce), respectively. The fatigue cracking of AZ91D alloy initiates at porosities and inclusions of the alloy's interior, and propagates along the grain boundaries. The fatigue fracture surface of test specimens shows the mixed fracture characteristics of quasi‐cleavage and dimple.     

Fatigue fracture behaviour and thermographic analysis of friction stir-welded AZ31

The fatigue fracture behaviour and thermographic analysis of friction stir-welded (FSW) AZ31 was studied. It showed that fatigue fracture at the advancing side (AS). Heat-affected zone (HAZ) contained a greater fraction of coarse grains and a small amount of twins. The grains in the thermo-mechanically affected zone were smaller, indicating that grains may originate from incompletely dynamic recrystallisation during the FSW. The nugget zone is composed of equiaxed grains caused by the dynamic recrystallisation. During cyclic deformation, hysteresis loops from the AS of the FSW joints with the stain amplitude were higher than the retreating side of the FSW joints, the temperature of HAZ at AS is higher than other regions.     

Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting

Cyclic deformation behavior and fatigue life of squeeze-cast AZ31 magnesium alloy was studied under stress amplitude-control at room temperature. Low and high cycle fatigue tests with engineering stress amplitudes in the range from 40 to 110 MPa were conducted. Analysis of hysteresis curves was performed. Tension–compression asymmetry of hysteresis loops was not observed; the alloy exhibited cyclic hardening in tension and compression. The fatigue life in the low cycle fatigue region was expressed by Wöhler and derived Manson–Coffin curves. Experimental data in both, the low and high cycle fatigue regions were fitted by means of regression functions. S–N curves exhibited a smooth transition from the low to the high cycle fatigue regions and significant scattering of experimental points was observed. Furthermore, metallographic and fractographic analyses were performed. Crack initiation occurred from the specimen surface or on clusters of secondary particles; the region of final fracture was characterized by a transgranular ductile fracture.It can be concluded that the fatigue properties of squeeze cast magnesium alloy AZ31 are significantly improved comparing to materials prepared by common methods of casting. Squeeze casting also enables the cost-effective fabrication of complicatedly shaped parts.