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.     

Fatigue behavior and plane-strain fracture toughness of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

In this study, the tensile properties, high cycle fatigue behavior and plane-strain fracture toughness of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy were investigated, comparison to that of sand-cast plus T6 heat treated magnesium alloy which named after sand-cast-T6. The results showed that the tensile properties of the sand-cast alloy are greatly improved after T6 heat treatment, and the fatigue strength (at 10⁷ cycles) of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy increases from 95 to 120 MPa after T6 heat treatment, i.e. the improvement of 26% in fatigue strength has been achieved. The plane-strain fracture toughnesses K_IC of the sand-cast and sand-cast-T6 alloys are about 12.1 and 16.3 MPa m^1/2, respectively. In addition, crack initiation, crack propagation and fracture behavior of the studied alloys after tensile test, high cycle fatigue test and plane-strain fracture toughness test were also investigated systematically.     

Correlation between ultimate tensile strength and solidification microstructure for the sand cast A357 aluminium alloy

Many studies have demonstrated a relationship between secondary dendrite arm spacing (SDAS) and the mechanical behaviour of cast aluminium–silicon alloys, both for tensile and fatigue strength. SDAS is related to the solidification time and can be predicted, with a good approximation, by finite-element simulation. However, other microstructural features can affect the tensile behaviour of cast aluminium alloys such as size and morphology of the eutectic Si particles, grain size, composition and morphology of the intermetallic compounds. The present investigation was aimed at finding valuable relationships between ultimate tensile strength and the previously mentioned microstructural parameters for the sand cast A357 aluminium alloy. The microstructural characterization was carried out by optical microscopy and image analysis on more than about 2500 micrographs. Starting from the microstructural parameters and taking into account the material hardness, a relationship able to predict the ultimate tensile strength of the alloy, with an error less than 5%, was found. This relationship can be used to evaluate the local values of the UTS in complex cast components knowing only the hardness and the microstructural parameters, even in positions where the extraction of tensile specimens is not possible.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

Tensile and fatigue evaluation of Ti–15Al–33Nb (at.%) and Ti–21Al–29Nb (at.%) alloys for biomedical applications

In this work the fatigue and tensile behavior of Ti–15Al–33Nb (at.%) and Ti–21Al–29Nb (at.%) was evaluated and compared to that for other titanium-based biomedical implant alloys, in particular Ti–6Al–4V (wt.%). The mechanical properties of interest were fatigue strength, tensile strength, elastic modulus, and elongation-to-failure. Fatigue stress versus life curves were obtained for tests performed at room temperature in air as well as in Ringer's solution at R = 0.1 for maximum stresses between 35% and 90% of the ultimate tensile strength. The results indicated that the fatigue strength and lives and elastic modulus of these alloys is comparable to that for Ti–6Al–4V (wt.%). Considering the data scatter and deformation behavior, the Ringer's solution did not significantly affect the fatigue behavior. Heat treatment reduced the tensile strength and this corresponded to a reduction in the fatigue strength. The tensile strength of the as-processed Ti-Al-Nb alloys was slightly lower than that for Ti–6Al–4V (wt.%), and the Ti–15Al–33Nb (at.%) exhibited lower strengths and higher elongations than Ti–21Al–29Nb. Based on the current results, it is proposed that titanium–aluminum–niobium alloys will be of considerable future interest for biomedical applications.     

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.     

A high strength and ductility Mg–Zn–Al–Cu–Mn magnesium alloy

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn (wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228 MPa, 328 MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.     

Hot tensile and fatigue behaviour of zinc–aluminum alloys produced by gravity and squeeze casting

The tensile and fatigue properties of zinc–aluminum alloys (ZA-8, ZA-12 and ZA-27) in squeeze and gravity cast forms have been investigated. Tensile tests were conducted at ambient and elevated temperatures up to 150 °C. At low temperatures, the ultimate tensile strength and yielding strength of the squeeze cast alloys have been found to be superior those of the gravity-cast alloys, as the temperature increased they decreased. In the same way, Brinell hardness of the squeeze cast alloys were obtained at higher values than gravity castings. The fatigue tests were performed at a constant speed of 400 rev/min and under a number of stress levels ranging from 100 to 150 MPa. The fatigue behaviour results of the ZA alloys were similar to obtained from the tensile testing. The squeeze cast alloys exhibited good fatigue resistance in proportion to the gravity castings. Metallography examinations showed that the microstructure of the castings differed according to the method of casting used. It was considered that the mechanical properties of the alloys were affected from these micro-structural changes.     

Fatigue and wear evaluation of Ti-Al-Nb alloys for biomedical applications

In this work the fatigue and wear behavior of Ti–15Al–33Nb(at.%) and Ti–21Al–29Nb(at.%) was evaluated and compared to that for other titanium-based biomedical implant alloys, in particular Ti–6Al–4V(wt.%). Fatigue stress versus life curves were obtained for tests performed at room temperature in air at a stress ratio of R = 0.1 for maximum stresses between 75%–90% of the ultimate tensile strength. The results indicated that the fatigue strength and lives of the as-processed alloys are comparable to that for Ti–6Al–4V(wt.%). Heat treatment significantly increased the orthorhombic-phase volume fractions in the alloys and resulted in reduced fatigue strength. The wear resistance for the alloys was significantly greater than that for Ti–6Al–4V(wt.%). Based on the current results, it is proposed that titanium–aluminum–niobium alloys will be of considerable future interest for biomedical applications.     

Relations between fatigue strength and other mechanical properties of metallic materials

The relations between fatigue strength and other mechanical properties especially the tensile strength of metallic materials are reviewed. After analyzing the numerous fatigue data available, the qualitative or quantitative relations between fatigue strength and hardness, strength (tensile strength and yield strength) and toughness (static toughness and impact toughness) are established. Among these relations, the general relation between fatigue strength σ_w and tensile strength σ_b, σ_w = σ_b(C ? P ? σ_b), where C and P are parameters, (hereafter, the general fatigue formula) can well predict the fatigue strength with increasing the tensile strength in a wide range for many materials such as conventional metallic materials, newly developed materials and engineering components. On the basis of the experimental results of many materials, the fatigue damage mechanism, especially for high‐strength steels, is proposed. It is suggested that the general fatigue formula can provide a new clue to predict the fatigue strength and design the materials by adjusting material parameters P and C adequately.     

On the role of matrix grain size and particulate reinforcement on the hardness of powder metallurgy Al–Mg–Si/MoSi2 composites

Six Al–Mg–Si composites reinforced with 15 vol.% of MoSi₂ intermetallic particles, together with three unreinforced monolith Al–Mg–Si (AA6061) alloys have been processed by powder metallurgy to quantify the roles of alloy matrix grain size and reinforcement particle on their solutionized hardness and ageing response. In the range studied, hardness of solutionized composites follows a Hall–Petch mechanism. Moreover, it can be rationalised as the sum of the hardness of the alloy matrix with the same matrix grain size (d) and a term H_R, that accounts for 17–27% of total hardness, is roughly constant and independent of reinforcing size and distribution. Matrix grain size is responsible for 50–65% of hardness, whereas the contributions of solid solution and Orowan strengthenings account for 17–26%. Upon heat treatment at 170 °C, hardening ability decreases linearly with d^?1/2, fitting all data points to a single equation independently of whether they correspond to the composites or to the monolith alloys.     

Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

In this study, Cu was added into the high-pressure die-casting Al-5.5Mg-0.7Mn (wt%) alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5Mg-0.7Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al₂CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al₂CuMg precipitates with a size of 15–20 nm × 2–4 nm were generated in the T5-treated Al-5.5Mg-0.7Mn-xCu (x = 0.5, 0.8, 1.5 wt%) alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al₂CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5Mg-0.7Mn-0.8Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5Mg-0.7Mn-0.8Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.     

Fatigue assessment of high strength leaf springs based on the FKM guideline

Analytical fatigue strength calculations based on the FKM guideline have been performed for hot tapered and stress‐shot‐peened high‐strength leaf spring specimens subjected to three‐point fatigue bending. The ultimate tensile strength of the decarburized specimens' surface has been approached by means of Rockwell‐C hardness measurements, and used as input for the approximation of its fatigue limit and mean stress sensitivity. Surface roughness and residual stress measurements were performed to take account for the technological life influencing factors. Fatigue tests at a constant mean stress and various stress amplitude levels were performed to determine the specimens' S–N curve and validate the calculation's accuracy. Comparison of calculated with experimentally determined fatigue lives, though satisfactorily, pinpoints the necessity for more accurate implementation of the stress‐shot‐peening process within the FKM guideline.     

The influence of post‐heat treatments on the tensile strength and surface hardness of selectively laser‐melted AlSi10Mg

To improve the mechanical properties of cast aluminium alloys several post‐heat treatments are known. However, these treatments cannot directly be transposed to additively via selective laser melting manufactured aluminium alloys, e. g., aluminium‐silicon‐magnesium (AlSi10Mg). Therefore, this study aims to determine suitable post‐heat treatments to optimise the mechanical properties of SLM‐built AlSi10Mg specimen. The influence of various post‐heat treatment conditions on the material characteristics was examined through hardness and tensile tests. The findings indicate that the Vickers hardness and ultimate tensile strength could not be improved via secondary precipitation hardening, whereas the fracture elongation shows a value which is distinctly higher than the values of a comparable cast alloy. Solution annealing at 525 °C reduces the hardness and the ultimate tensile strength by about 40 % and increases the fracture elongation three times. A subsequent precipitation hardening allows recovery of 80 % of the as‐built hardness, and 90 % of the previous ultimate tensile strength combined with maintaining an improved fracture elongation of about 35 % compared to the respective as‐fabricated condition.     

High cycle fatigue behavior of as-extruded ZK60 magnesium alloy

Tensile and high cycle fatigue properties of hot extruded ZK60 magnesium alloy have been investigated, in comparison to that of hot-extruded plus T5 heat-treated ZK60 magnesium alloy which was named as ZK60-T5. High cycle fatigue tests were carried out at a stress rate (R) of −1 and a frequency of 100 Hz using hour-glass-shaped round specimens with a gage diameter of 5.8 mm. The results show that tensile strength greatly improved and elongation is also slightly enhanced after T5 heat treatment, and the fatigue strength (at 10⁷ cycles) of ZK60 magnesium alloy increases from 140 to 150 MPa after T5 heat treatment, i.e., the improvement of 7% in fatigue strength has been achieved. Results of microstructure observation suggest that improvement of mechanical properties of ZK60 magnesium alloy is due to precipitation strengthening phase and texture strengthening by T5 heat treatment. In addition, fatigue crack initiations of ZK60 and ZK60-T5 magnesium alloys were observed to occur from the specimen surface and crack propagation was characterized by striation-like features coupled with secondary cracks.     

Influence of Mn content on mechanical properties and fatigue behavior of extruded Mg alloys

Effect of manganese addition, which has been known to improve formability during extrusion, on mechanical properties and fatigue strength was investigated to confirm the total performance of the Mn addition to the magnesium alloys. The grain size decreased with increasing Mn content and attained a constant grain size at the Mn contents higher than 0.4 wt%, where the Mn–Al–Mg intermetallic particles were precipitated. The tensile strength and hardness increased with increasing Mn content and attained a constant value at the Mn contents higher than 0.4 wt%, which was consistent with the grain size variation. The fatigue life increased with increasing Mn content and attained a constant value similar to the case of tensile strength. However, the fatigue life was significantly reduced at the Mn content of 0.79 wt%. It is speculated to result from a large number of precipitated intermetallic particles, which would degrade the fatigue crack growth resistance. The magnesium alloys with Mn contents between 0.4 and 0.6 wt% have a good balance of mechanical properties and fatigue strength.     

超高强韧Mg-Gd-Y-Zn-Zr变形镁合金研究进展

超高强韧镁合金的研发对推广镁合金在高技术领域的应用具有重要意义。镁与稀土均是我国的优势资源,因此在我国发展超高强韧稀土镁合金具有得天独厚的优势,其中Mg-Gd-Y-Zn-Zr系变形镁合金因其接近高强铝合金的超高强度和塑性,近年来受到研究者的广泛关注。综述了超高强韧Mg-Gd-Y-Zn-Zr系变形镁合金的合金成分、常规塑性变形工艺、新型剧烈塑性变形工艺和热处理工艺对该合金显微组织和力学性能的影响规律,以及该超高强韧变形镁合金的显微组织特征和强韧化机理。T5峰时效态超高强韧Mg-8.2Gd-3.8Y-1Zn-0.4Zr(质量分数)挤压合金具有双峰分布的晶粒尺寸“软-硬”复合层片微结构,以及由高密度的基面γ′纳米片状析出相和棱柱面β′纳米析出相形成的近连续网状结构,该挤压合金室温拉伸屈服强度、拉伸强度和断裂延伸率分别为466 MPa、514 MPa和14.5%。介绍了哈尔滨工业大学等单位在超高强韧Mg-Gd-Y-Zn-Zr系变形镁合金的规模化制备和应用方面的研究进展,并展望了Mg-Gd-Y-Zn-Zr系变形镁合金的发展趋势。     

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.     

Mechanical and microstructural characterization of 2124Al/25 vol.%SiCp joints obtained by linear friction welding (LFW)

The aim of this study is to evaluate the possibility of using the linear friction welding (LFW) technique to produce sound joints on a 2124Al/25 vol.%SiC_p composite. The MMC joints were subjected to microstructural and mechanical characterization, including hardness, tensile and fatigue tests, without any post-weld heat treatment. The microstructural analyses showed substantially defect-free joints, with a uniform particle distribution in the central zone and a relevant plastic flow of the aluminium matrix alloy. The hardness decrease in the welded zone was approximately 10% in respect to the base material. The joint efficiency was higher than 80%, both in respect to the ultimate tensile strength and fatigue strength at 10⁷ cycles. S–N probability curves were calculated using the maximum likelihood method. Generally, the fracture occurred in the Thermo-Mechanically Affected Zone (TMAZ), with a relevant reduction in the elongation to failure.     

Effect of shotpeening on sliding wear and tensile behavior of titanium implant alloys

Titanium has good biocompatibility and so its alloys are used as implant materials, but they suffer from having poor wear resistance. This research aims to improve the wear resistance and the tensile strength of titanium alloys potentially for implant applications. Titanium alloys Ti–6Al–4V and Ti–6Al–7Nb were subjected to shotpeening process to study the wear and tensile behavior. An improvement in the wear resistance has been achieved due to surface hardening of these alloys by the process of shotpeening. Surface microhardness of shotpeened Ti–6Al–4V and Ti–6Al–7Nb alloys has increased by 113 and 58 HV_(0.5), respectively. After shotpeening, ultimate tensile strength of Ti–6Al–4V increased from 1000 MPa to 1150 MPa, higher than improvement obtained for heat treated titanium specimens. The results confirm that shotpeening pre-treatment improved tensile and sliding wear behavior of Ti–6Al–4V and Ti–6Al–7Nb alloys. In addition, shotpeening increased surface roughness.