Some useful approximations for wrought aluminum alloys based on monotonic tensile properties and hardness

The objective of the present paper is to derive some useful approximations for estimating the strain‐controlled fatigue properties and cyclic deformation of wrought aluminum alloys from hardness and monotonic tensile properties. A variety of relationships and correlations among monotonic tensile properties, Brinell hardness, cyclic deformation and strain‐controlled fatigue properties are developed for wrought aluminum alloys. A simple method is proposed for prediction of the strain‐life curve requiring only ultimate tensile strength and modulus of elasticity. Prediction capability of the proposed method is evaluated for 25 kinds of wrought aluminum alloys with ultimate tensile strength between 120 MPa and 650 MPa. The proposed method provides good approximations of the strain‐life curve.     

Crack initiation and propagation characteristics of a dual-phase Mg–Li alloy under high-cycle and very-high-cycle fatigue regimes

Ultralight Mg–Li alloys are promising aerospace materials as they are the lightest structural alloys at present; however, their fatigue behaviors remain to be explored. This work focuses on the fatigue strength and crack initiation behavior of an extruded dual-phase Mg–Li alloy (LZ91) under high-cycle and very-high-cycle fatigue regimes. The fatigue limit of LZ91 alloy at 10⁹ cycles was determined to be 78 MPa, and the fatigue ratio is 0.46. Microstructural characterization demonstrates that fatigue cracks tend to initiate at β-Li phase-enriched regions. The α-Mg phase presents a < > fiber texture with a basal plane that has low deformation in the extrusion direction and acts as an enhanced phase in relation to the β-Li phase. Deformation discrepancies cause localized cyclic plasticity at the Li phase that leads to fatigue crack initiation.     

Evaluation of low cycle fatigue life in AZ31 magnesium alloy

Magnesium alloys are attracting engineers for their practical application to structural components. Here fatigue properties, which is essential for structural use, have been examined on extruded AZ31 bar under uniaxial cyclic loading by both strain and stress controlled conditions. Adding fatigue tests with mean stresses under stress controlling conditions, fatigue life evaluation method has been discussed along with the analysis of cyclic stress–strain behavior. The specimen is easy to yield in compression by twinning. This leads to the asymmetric hysteresis curves. It also tends to deform quasi-elastically during unloading from compression; this makes the plastic strain amplitude smaller to the maximum one in the hysteresis curve. These asymmetric features fairly disappear at half-life in the stress controlled tests. The fatigue lives and deformation characteristics can be expressed nicely by Manson–Coffin type equation. On the contrary, the strain controlled tests retain the asymmetry till the end and produce tensile mean stresses. The fatigue lives are unsuccessfully evaluated by the above equation. Various mean stress correction models for cubic metals are not operative in magnesium alloys. A new model has been devised adding a correction term of −σ_m/2E to the above mentioned Manson–Coffin type equation. Strain controlled test, as it retains pyriform shape till the end, could be evaluated more accurately with the maximum plastic strain amplitude in the hysteresis curve.     

The effect of PVD coatings on the tensile strength and low‐cycle fatigue resistance of stainless steel and titanium alloys

PVD coatings applied to components form hard, stronger layers and generate high residual compressive stresses that limit the plastic deformation in surface layers of the base metal thus increasing its tensile strength and resistance to fatigue loading. The purpose of this paper is to experimentally determine the influence of the deposition of 2 to 16.5‐μm‐thick PVD coatings of TiN, Cr, (Cr+TiN), (TiC)N, (TiAl)N onto specimens of stainless steel 321 and titanium alloys of types MILT‐81556A and (10‐2‐3; 4966) on their tensile strength and low‐cycle fatigue resistance when the development of large elastic–plastic strains takes place. The tensile and low‐cycle fatigue tests were conducted under conditions of axial zero‐to‐tension cycle of the stress‐controlled loading on flat 1‐ to 1.5‐mm‐thick specimens in the initial state (uncoated specimens) and after application of a PVD coating, including those after pretensioning or after cyclic prestraining in the low‐cycle fatigue range. The deposition of PVD coatings is found to enhance the characteristics of tensile strength and low‐cycle fatigue resistance in the quasi‐static fracture range. The deposition of PVD coatings on specimens cyclically prestrained to the values of 53–86% of the number of cycles to fracture, changes the cyclic properties of the material and predetermines the fatigue fracture mode only.     

Structural durability criteria for commercial vehicle components from the self strengthening cast ausferrite nodular iron EN‐GJS‐800–8 (ADI) in comparison to the ferritic EN‐GJS‐400–15

The structural durability of safety components in the chassis comprises not only the fatigue behaviour under cyclic variable amplitude service loading, but also its interaction with prestrains caused by special events and the rupture behaviour under impact loading due to misuse . From this background, the structural durability behaviour of Panhard rods made from ferritic cast nodular iron EN‐GJS‐400–15 was compared with the behaviour of rods made from the austempered EN‐GJS‐800–8. The components investigated, Panhard rods and cast plugs, made from the austempered material revealed a higher impact resistance than the components made from the ferritic cast nodular iron. Due to their ausferrite microstructure, Panhard rods made from EN‐GJS‐800–8 display a significantly superior fatigue strength behaviour, especially under spectrum loading, and offer a potential for lightweight design. Prestrains do not affect the fatigue behaviour under variable amplitude loading and the plastic deformation of the component under impact loading can be increased by appropriate design reducing the stiffness in the shaft area and achieving a weight reduction by 15 %.     

Effect of dynamic strain aging on the appearance of the rare earth texture component in magnesium alloys

Binary Mg alloys were prepared containing Zn, Ce and Gd. These were extruded and the resulting mechanical properties were determined. The intensities of the rare earth (RE) texture components were measured and linked to the extrusion conditions. Tension and compression testing was carried out on samples taken from extruded bars and a Mg-0.5Ce cast alloy. Over particular temperature and strain rate ranges, dynamic strain aging (DSA) was observed. The ranges over which DSA occurred during testing are compared with the conditions under which the RE texture components were produced during extrusion. It is concluded that formation of the RE texture components can be enhanced by extruding when the rate sensitivity is negative, i.e. under conditions where DSA is taking place.     

Effect of heat treatment on strain–controlled fatigue behavior of cast Mg–Nd–Zn–Zr alloy

The influence of heat treatment on the strain-controlled fatigue behavior of cast NZ30?K alloy was investigated. Compared with the as-cast and solutionized (T4) alloys, the peak-aged (T6) and over-aged (T7) counterparts have a higher cyclic stress and a lower plastic strain value due to the precipitation strengthening. The as-cast and T4-treated alloys have a higher fatigue strength/yield strength ratio than the aged alloys, which is mainly attributed to their higher cyclic hardening. Under stress-controlled loading, the aged alloys show lower hysteresis energies than the as-cast and T4-treated counterparts, leading to longer fatigue lifetimes. For the T4-treated alloy, the cyclic hardening and fatigue failure are controlled by the dislocations-slip and twinning, while for both the as-cast and T6-treated counterparts, they are controlled by the dislocation-slip. For the T7-treated alloy, cyclic deformation and failure behavior are mainly dependent on dislocations-slip and grain boundary sliding.     

Texture and microstructure development during hot deformation of ME20 magnesium alloy: Experiments and simulations

The influence of deformation conditions and starting texture on the microstructure and texture evolution during hot deformation of a commercial rare earth (RE)-containing magnesium alloy sheet ME20 was investigated and compared with a conventional Mg sheet alloy AZ31. For all the investigated conditions, the two alloys revealed obvious distinctions in the flow behavior and the development of texture and microstructure, which was primarily attributed to the different chemistry of the two alloys. The presence of precipitates in the fine microstructure of the ME20 sheet considerably increased the recrystallization temperature and suppressed tensile twinning. This gave rise to an uncommon Mg texture development during deformation. Texture simulation using an advanced cluster-type Taylor approach with consideration of grain interaction was employed to correlate the unique texture development in the ME20 alloy with the activation scenarios of different deformation modes.     

Fatigue of Magnesium Alloys

Magnesium alloys show a high specific strength and are therefore increasingly used for light‐weight constructions in transportation industry.^[1,2] To predict the behaviour of the material under the influence of cyclic loading it is vital to understand the fatigue behaviour of magnesium alloys. Only when understood properly, it is possible to fully apply the potential weight reduction by using magnesium alloys. A very important aspect in fatigue of magnesium alloys is the influence of a corrosive media and elevated temperatures, of which both are relevant in automotive applications. These two factors tend to have deleterious effects on magnesium alloys and therefore also have to be considered in investigations on the fatigue behaviour of magnesium alloys.     

Fatigue strength of die-cast magnesium components

There is a commercial interest to extend the use of die‐cast magnesium from low stress applications, such as interior components of motor vehicles, to components carrying significant loads. In high stress applications it is the strength and fatigue properties of die‐cast magnesium alloys that limit their use. Manufacturing defects, such as microscopic shrink holes, pores and oxide inclusions, impair the strength of components under fatigue loads, but are unavoidable with present‐day magnesium casting technology. In the present study, the effects of different rib thicknesses and notch radii on the fatigue strength were investigated on realistic cast specimens with unmachined surfaces. The tests were performed on ribbed specimens of magnesium alloys AZ91 and AM60 under pulsating bending stress with a constant amplitude at a stress ratio R = 0. As indicated by the results of the investigation, the real material must be considered together with its defects in designing die‐cast magnesium components. For this purpose, the influence of defects must therefore be given a higher priority than the local stresses at the surface.     

锆合金的低周疲劳特性

锆合金被普遍用做核反应堆中的燃料包壳和结构材料。在反应堆运行时，堆功率的波动和水冷却介质的流动．使燃料组件及其它构件发生循环变形，在极端情况下出现破损。本文概述了堆内包壳循环变形的特点，并分析了锆合金的循环变形行为，疲劳裂纹的形核与扩展，疲劳寿命及影响疲劳寿命的因素。     

Corrosion fatigue behavior of extruded AZ80, AZ61, and AM60 magnesium alloys in distilled water

Rotary bending fatigue tests were conducted in laboratory air and distilled water using three extruded magnesium (Mg) alloys AZ80, AZ61, and AM60 with different chemical compositions. In laboratory air, the fatigue strengths at high stress levels were similar in all alloys because cracks initiated at Al-Mg intermetallic compounds, whereas AZ80 with the largest Al content exhibited the highest fatigue strength at low stress levels, which was attributed to the crack initiation due to cyclic slip deformation in the matrix microstructure. In distilled water, fatigue strengths were considerably decreased due to the formation of corrosion pits in all alloys, and the difference of fatigue strength at low stress levels among the alloys disappeared, indicating that the addition of Al that improved the fatigue strength in laboratory air was detrimental to corrosion fatigue. __________ Translated from Problemy Prochnosti, No. 1, pp. 141–145, January–February, 2008.     

Numerical Investigation of the Effect of Loading Multiaxiality and Deformation Process Parameters on the Fatigue Life of Metals

Known experimental data on the effect of loading multiaxiality and the type of deformation path on the fatigue damage accumulation process have been analyzed. In terms of the mechanics of a damaged medium, a variant of the defining relations has been developed that describes the processes of elastoplastic deformation and damage accumulation in structural materials (metals and their alloys) in the case of multiaxial nonproportional low-cycle loading paths. The damaged-medium model consists of three interrelated components: relations defining elastoplastic behavior of the material with allowance for the influence of the fracture process, equations describing the damage accumulation kinetics, and a strength criterion for damaged material. To qualitatively assess the defining relations, a numerical experiment to construct equal-damage surfaces has been carried out. The calculated data are compared with the experimental ones. The effect of the stress-state multiaxiality and the type of the deformation path on the low-cycle fatigue life of metals has been investigated. It has been shown that the developed variant of defining relations for a damaged medium reflects correctly the main effects of cyclic elastoplastic deformation and damage accumulation at a multiaxial stress-state and arbitrary material deformation paths.     

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

The 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 → slip and detwinning exhaustion → 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 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.     

Low‐cycle fatigue behavior of a newly developed cast aluminum alloy for automotive applications

The drive for increasing fuel efficiency and decreasing anthropogenic greenhouse effect via lightweighting leads to the development of several new Al alloys. The effect of Mn and Fe addition on the microstructure of Al‐Mg‐Si alloy in as‐cast condition was investigated. The mechanical properties including strain‐controlled low‐cycle fatigue characteristics were evaluated. The microstructure of the as‐cast alloy consisted of globular primary α‐Al phase and characteristic Mg₂Si‐containing eutectic structure, along with Al₈(Fe,Mn)₂Si particles randomly distributed in the matrix. Relative to several commercial alloys including A319 cast alloy, the present alloy exhibited superior tensile properties without trade‐off in elongation and improved fatigue life due to the unique microstructure with fine grains and random textures. The as‐cast alloy possessed yield stress, ultimate tensile strength, and elongation of about 185 MPa, 304 MPa, and 6.3%, respectively. The stress‐strain hysteresis loops were symmetrical and approximately followed Masing behavior. The fatigue life of the as‐cast alloy was attained to be higher than that of several commercial cast and wrought Al alloys. Cyclic hardening occurred at higher strain amplitudes from 0.3% to 0.8%, while cyclic stabilization sustained at lower strain amplitudes of ≤0.2%. Examination of fractured surfaces revealed that fatigue crack initiated from the specimen surface/near‐surface, and crack propagation occurred mainly in the formation of fatigue striations.     

Microstructural evolution during hot shear deformation of an extruded fine-grained Mg–Gd–Y–Zr alloy

Mg–Gd–Y–Zr alloys are among recently developed Mg alloys having superior mechanical properties at elevated temperatures. Dynamic recrystallization (DRX) and rare earth-rich particles play important roles in enhancing the high-temperature strength of these alloys. Accordingly, the microstructural evolution of a fine-grained extruded Mg–5Gd–4Y–0.4Zr alloy was investigated after hot shear deformation in the temperature range of 350–450 °C using the shear punch testing (SPT) method. The results reveal the occurrence of partial dynamic recrystallization at the grain boundaries at 350 °C while the fraction of DRX grains increases with increasing deformation temperature. A fully recrystallized microstructure was achieved after SPT at 450 °C. The Gd-rich and Y-rich cuboid particles, having typical sizes in the range of ~50 nm to ~3 μm, show excellent stability and compatibility after hot shear deformation, and these particles enhance the high-temperature strength during hot deformation at elevated temperatures. The textural evolution, examined using electron backscattered diffraction, revealed a non-fibrous basal DRX texture after SPT which is different from the conventional deformation texture.     

Fatigue of metals under nonuniform stressed state. Part 1. Stressed state assessment methods and results of investigation

We discuss the methods of assessment of stress-strain state and stress gradient in nonuniformly stressed structural components under elastic and elastic-plastic cyclic deformation. We systematize the research findings on fatigue of metals and alloys, including the stress concentration case, as well as the methods that describe the dependence of fatigue characteristics on the stress gradient.     

镁合金化阻燃的研究进展

介绍了添加各种合金元素(包括Ca、Be、Zn、Ba、RE以及复合添加)对镁合金的阻燃和抗氧化性能的影响规律,展望了阻燃镁合金的发展趋势和潜在应用领域,即通过多元微合金化设计提高镁合金的阻燃性能,并兼顾其塑性变形能力,发展高速交通工具用阻燃变形镬合金挤压型材.     

Monotonic and Cyclic Deformation Behavior of MIG‐CMT Welded and Heat‐Treated Joints of Aluminum Cast and Wrought Alloys

While the fatigue behavior of die cast aluminum as well as welded aluminum wrought alloys have been subject of several studies, no systematic work has been carried out on hybrid structures made as a combination of welded sand castings and wrought alloys. Aim of the present study is to correlate the monotonic and cyclic deformation behavior of thin sheet welded joints with the microstructure in the heat affected zone of the material combination sand cast EN AC‐Al Si7Mg0.3 and wrought alloy EN AW‐Al Si1MgMn (EN AW‐6082). The metal sheets were welded using a metal inert gas cold metal transfer process under variation of the welding gap, the heat treatment parameters, as well as the surface finishes. It was demonstrated by Wöhler diagrams based on bending fatigue tests that the fatigue life could be increased for the welded and heat treated specimens as compared to the as‐received cast specimens. By means of optical microscopy this effect was attributed to microstructural changes due to the optimized welding and heat treatment process. A detailed analysis of the mechanical tests was possible by the application of an optical 3D strain analysis.     

Multiaxial fatigue of extruded ZK60 magnesium alloy

Multiaxial monotonic and cyclic behaviors of ZK60‐T5 magnesium extrusion are investigated. Strain‐controlled tests were performed at standard laboratory condition with fully reversed straining. Twinning‐detwinning deformation plays an important role in the cyclic axial behavior for tests that were performed under strain amplitudes higher than 0.4%. However, the hysteresis loop for the 0.4% was found symmetric and no sign of twinning‐detwinning deformation was observed. On the contrary, the cyclic shear behavior was found to be similar to conventional alloys and no significant asymmetric or twinning‐detwinning deformations were observed. The multiaxial fatigue tests suggest that multiaxiality and nonproportionality are not detrimental to fatigue life. Three multiaxial fatigue damage models were used: Smith‐Watson‐Topper, Fatemi‐Socie, and Jahed‐Varvani. While Fatemi‐Socie and Jahed‐Varvani models show comparable estimation, Smith‐Watson‐Topper overestimates shear and nonproportional lives.