Comparison of mechanical properties for equiaxed fine-grained and dendritic high-palladium alloys

Two Pd-Cu-Ga alloys and a Pd-Ga alloy were selected for study. Bars of each alloy were tested in tension for the as-cast and simulated porcelain-firing conditions, and values of mechanical properties were measured. Fracture surfaces and microstructures of axially sectioned fracture specimens were observed with the SEM. The two Pd-Cu-Ga alloys exhibited similar mechanical properties. The Pd-Ga alloy had lower strength and higher percentage elongation. Heat treatment simulating porcelain firing cycles decreased the strength of both Pd-Cu-Ga alloys and increased their ductility. However, this heat treatment did not significantly affect the mechanical properties of the Pd-Ga alloy. All three high-palladium alloys had the same modulus of elasticity. The amount of overall porosity was relatively minimal (< 1%) and not significantly different among the three alloys. However, porosity was a significant factor for UTS of one Pd-Cu-Ga alloy and the Pd-Ga alloy.     

石墨烯增强镁合金塑性加工的组织和力学性能

为获得优异力学性能的复合材料,选用石墨烯作为增强体.本文采用粉末冶金方法,经高能球磨法、冷压、烧结、热压和热挤压制备了AZ31镁合金及石墨烯(GNPs)增强AZ31镁基复合材料棒状试样,通过光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射(XRD)和室温拉伸、压缩表征了该材料的组织和力学性能.结果表明:制备的复合材料及基体中生成了Mg_(17)Al_(12)和MgO,加入GNPs后复合材料的屈服强度与维氏硬度都优于基体材料;加入GNPs质量分数为0.5%和1.0%的GNPs复合材料分别比基体屈服强度增加13.2%和14.2%(258和259 MPa),显微维氏硬度分别增加11.4%和14.3%(78和80 HV),主要的强化机制为载荷转移强化、奥罗万强化、热错配强化,但材料的拉伸延伸率分别降低到3.9%和4.3%,比基体分别降低了38%和32%,材料的致密度分别为99.6%、98.5%、97.8%,随着GNPs的增加,致密度降低;GNPs的加入未改变材料的断裂方式,材料的断裂方式均主要为脆性断裂;GNPs的添加使复合材料的基面{0002}织构弱化,从而降低材料的屈服不对称性.     

Improvement of formability and mechanical properties of magnesium alloys via pre-twinning: A review

Twinning can generate the change of texture and a large of twin boundaries, which can greatly influence the mechanical properties of magnesium alloys. Thus, pre-twinning can be considered to be a simple and feasible method to improve the mechanical properties of magnesium alloys. Recently, some studies have confirmed that pre-twinning can be an effective way to enhance the strength, formability and mechanical anisotropy of magnesium alloys. Based on these results, some aspects of the present research on the improvement of mechanical properties via pre-twinning are reviewed. The relevant mechanisms have been summarized. Finally, for this research field, a few critical scientific problems are also proposed.     

Superplasticity of fine-grained magnesium alloys for biomedical applications: A comprehensive review

The superplastic behavior of medical magnesium alloys is reviewed in this overview article. Firstly, the basics of superplasticity and superplastic forming via grain boundary sliding (GBS) as the main deformation mechanism are discussed. Subsequently, the biomedical Mg alloys and their properties are tabulated. Afterwards, the superplasticity of biocompatible Mg-Al, Mg-Zn, Mg-Li, and Mg-RE (rare earth) alloys is critically discussed, where the influence of grain size, hot deformation temperature, and strain rate on the tensile ductility (elongation to failure) is assessed. Moreover, the thermomechanical processing routes (e.g. by dynamic recrystallization (DRX)) and severe plastic deformation (SPD) methods for grain refinement and superplasticity in each alloying system are introduced. The importance of thermal stability (thermostability) of the microstructure against the grain coarsening (grain growth) is emphasized, where the addition of alloying elements for the formation of thermally stable pinning particles and segregation of solutes at grain boundaries are found to be major controlling factors. It is revealed that superplasticity at very high temperatures can be achieved in the presence of stable rare-earth intermetallics. On the other hand, the high-strain-rate superplasticity and low-temperature superplasticity in Mg alloys with great potential for industrial applications are summarized. In this regard, it is shown that the ultrafine-grained (UFG) duplex Mg-Li alloys might show remarkable superplasticity at low temperatures. Finally, the future prospects and distinct research suggestions are summarized. Accordingly, this paper presents the opportunities that superplastic Mg alloys can offer for the biomedical industries.     

Structure and mechanical properties of rapidly solidified magnesium based Mg-Al-Zn-RE alloys consolidated by extrusion

Magnesium based Mg-9Al-lZn-5RE (RE = La or Nd) alloys were rapidly solidified by chill block melt spinning. The resulting ribbons were cold packed into an aluminium alloy can and extruded at temperatures of 230 and 340°C and ratios of 20:1 or 25:1. Tensile and hardness tests of the extruded and heat treated materials were carried out. The tensile strength and elongation to fracture of the as extruded material were 478 MN m^?2 and 6·5% respectively and those of the material heat treated for 2 h at 350° C were 420 MN m^?2 and 20% respectively. The microstructure of these specimens was studied by X-ray diffraction and transmission electron microscopy. Intermetallics of Al₁₁ La₃ or Al₂Nd were found at grain boundaries and in the matrix which had a grain size of between 0–26 and 0–8 μm, while Mg₁₇Al₁₂ precipitates were present in the specimens extruded at a lower temperature (230° C). Yield strengths were consistent with the Hall-Petch relationship with grain size established earlier for this class of material.

MST/3495     

Texture and mechanical anisotropy in three extruded magnesium alloys

Abstract

One ZM61 alloy (6·2%Zn, 1·2%Mn) and two magnesium alloys containing nominally 3% of neodymium and yttrium respectively have been prepared in the form of hot extruded flat strips. Their textures and microstructures have been quantified and series of mechanical tests were carried out to determine plane stress yield loci in both the solution treated and aged conditions. The ZM61 alloy had a sharp texture and marked anisotropy of strength that could be rationalised in terms of deformation by basal <a> slip and {1012}<1011> twinning. This material was much weaker in compression than in tension. Precipitation hardening on aging caused a greater impedance to twinning than to slip with the result that the anisotropy was somewhat reduced. The Mg–3Nd alloy had a very weak and different texture but nevertheless demonstrated a pronounced anisotropy of strength. Aging increased the yield stress by about 80% and also inhibited twinning to some extent although the degree of anisotropy remained almost unaffected. The Mg–3Y alloy which showed a texture of intermediate strength was different in type from either of the others. Its strength behaviour was close to isotropic; in particular, no difference existed between tensile and compressive loading, and aging produced only a marginal increase in strength. Twins were relatively infrequent in the deformed Mg–3Y alloy but its mechanical behaviour could not be rationalised according to simple models.     

Review of the atmospheric corrosion of magnesium alloys

Mg atmospheric corrosion is induced by a thin surface aqueous layer. Controlling factors are microgalvanic acceleration between different phases, protection by a continuous second phase distribution, protection by corrosion products, and degradation of protective layers by aggressive species such as chloride ions. The Mg atmospheric corrosion rate increases with relative humidity (RH) and concentrations of aggressive species. Temperature increases the corrosion rate unless a protective film causes a decrease. O₂, SO₂ and NO₂ accelerate the atmospheric corrosion rate, whereas the corrosion rate is decreased by CO₂. The traditional gravimetric method can evaluate effectively the corrosion behavior of Mg alloys.     

Effect of Zn content on the microstructures and mechanical properties of laser beam-welded ZK series magnesium alloys

The effect of Zn content on the microstructures and mechanical properties of laser beam welded ZK series magnesium alloys (ZK21, ZK40, and ZK60) has been studied. Owing to the lower heat input, laser beam welding can successfully be employed to weld ZK series magnesium alloys having Zn content up to 4 wt%, which are difficult to weld by means of conventional arc welding. However, ZK60 is susceptible to solidification cracking and presents a poor weldability, which may originate from the net-like distribution of more Mg₅₁Zn₂₀ precipitates along grain boundaries (GBs) in the fusion zone (FZ). With increasing Zn content, the amount and size of precipitates along GBs in the FZ increase, and the morphology of grains in the FZ adjacent to fusion boundary changes from cellular to equiaxed dendritic. The grains in the FZ of ZK40 alloy are the finest among the three alloys, whose size is only about 4.8 μm, and the ZK40-welded joint achieves the highest ultimate tensile strength of 312 MPa, which is up to 90.4% of the base metal.     

Gas tungsten arc welding of fine-grained AZ31B magnesium alloys made by powder metallurgy

AZ31B magnesium alloys with various grain sizes and oxygen contents were prepared by powder metallurgy (P/M) combined with hot extrusion, and the P/M magnesium alloys were subjected to gas tungsten arc welding (GTAW). Porosities are observed in weld joints of the P/M AZ31B alloys with high oxygen contents. Gas composition analysis of porosity shows that the porosity originates mainly from the decomposition of magnesium hydroxide formed during the fabrication of magnesium alloys. The porosity can be decreased or prevented by reducing the amount of magnesium hydroxide (which is expressed as oxygen content in the present study) in the base materials, through controlling the P/M processing time. Use of a filler rod and/or an insert sheet containing rare earth element La tends to decrease the porosity in weld joints. When oxygen content in P/M AZ31B alloys is reduced to 440 ppm or less, sound weld joints without porosity are obtained. Mechanical test demonstrates that tensile strength of the sound weld joints of P/M AZ31B alloys is at the same level as that of weld joint of a commonly hot-extruded AZ31B alloy.     

Injectability and mechanical properties of magnesium phosphate cements

Up to now magnesium phosphate cements are mainly being utilized in wastewater treatment due to their adsorptive properties. Recently they also have been shown to have a high potential as degradable biocements for application as replacement materials for bone defects. In comparison to degradable calcium phosphate cements they have the advantage of setting at neutral pH, which is favorable in biological environment. In this study two parameters of the cement composition, namely powder-to-liquid ratio (PLR) and citrate content, were varied in order to optimize the injectability properties of the cement paste and the mechanical properties of the reaction product. These properties were determined by means of testing setting time and temperature, paste viscosity, and injectability as well as phase composition and compressive strength of the set cements. Best results were obtained, when the cements were prepared with a PLR of 2.5 and a binder liquid consisting of an aqueous solution of 3 mol/l diammonium hydrogen phosphate and 0.5 mol/l diammonium citrate.     

Microstructure and mechanical properties of FexCoCrNiMn high-entropy alloys

The microstructure and tensile properties of Fe_xCoCrNiMn high-entropy alloys (HEAs) were investigated. It was found that the Fe_xCoCrNiMn HEA has a single face-centered cubic (fcc) structure in a wide range of Fe content. Further increasing the Fe content endowed the Fe_xCoCrNiMn alloys with an fcc/body-centered cubic (bcc) dual-phase structure. The yield strength of the Fe_xCoCrNiMn HEAs slightly decreased with the increase of Fe content. An excellent combination of strength and ductility was achieved in the Fe_xCoCrNiMn HEA with higher Fe content, which can be attributed to the outstanding deformation coordination capability of the fcc/bcc dual phase structure.     

钛氮合金的铸态组织与性能

采用熔铸工艺法制备了含氮量为0.045%～0.27%的原位自生氮化物增强钛基复合材料,分析并测试了合金的铸态组织和力学性能.研究结果表明:在Ti-N合金中,随着氮含量的增加,合金中氮化物的形态和相组成发生了明显的改变;当氮含量在0.045%～0.18%时,合金的基体为α-Ti,增强相为TiN0.3;氮含量增加到0.225%时,增强相转变为块状Ti2N;复合材料的硬度、抗压强度和弹性模量均高于纯钛基体且随着氮含量的增加而增加;当增强相由TiN0.3转变为Ti2N时,抗压强度显著增加;由压缩断口分析可知,基体为韧性断裂,随着氮含量增加合金由韧窝 解理断口向具有解理特征的脆性断裂转变.     

Microstructural evolution and tensile mechanical properties of thixoforged ZK60-Y magnesium alloys produced by two different routes

Semi-solid processing of magnesium alloys is generally based on conventional magnesium-based casting alloys such as Mg–Al series. However, these casting alloys do not give such high mechanical properties as the alloys that are conventionally wrought such as Mg–Zn series. In this paper, a ZK60 magnesium alloy with the addition of Y was thixoforged. The semi-solid thermal transformation (SSTT) route and the recrystallisation and partial melting (RAP) route were used to obtain the semi-solid feedstocks for thixoforging. Microstructural evolution during partial remelting was studied at temperatures for times. Tensile mechanical properties of thixoforged components at room temperature were examined. Results show that a fine spheroidal microstructure can be obtained by the RAP route. Compared to the RAP route, the SSTT alloy shows coarsened solid grains with a relatively high proportion of intragranular liquid droplets. With prolonged holding time, the solid grain sizes of the SSTT alloy and the RAP alloy increased. Coalescence was dominant in the SSTT alloy and Ostwald ripening was dominant in the RAP alloy. Thixoforging for the SSTT alloy and the RAP alloy resulted in successful filling of the die. The tensile properties of the thixoforged RAP alloy were satisfactory and exceeded those of the thixoforged SSTT alloy. However, the mechanical properties of both the thixoforged SSTT alloy and the thixoforged RAP alloy decreased with prolonged holding time.     

The effects of calcium and yttrium additions on the microstructure,mechanical properties and biocompatibility of biodegradable magnesium alloys

In this study, the effects of calcium (Ca) and yttrium (Y) on the microstructure, mechanical properties, corrosion behaviour and biocompatibility of magnesium (Mg) alloys, i.e. Mg–xCa (x = 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 and 20.0%, wt%, hereafter) and Mg–1Ca–1Y, were investigated. Optical microscopy, X-ray diffractometry (XRD), compressive and Vickers hardness testing were used for the characterisation and evaluation of the microstructure and mechanical properties. The in vitro cytotoxicity of the alloys was assessed using osteoblast-like SaOS2 cells. The corrosion behaviour of these alloys was evaluated by soaking the alloys in simulated body fluid (SBF) and modified minimum essential medium (MMEM) at 37 °C in a humidified atmosphere with 5% CO₂. Results indicated that the increase of the Ca content enhances the compressive strength, elastic modulus and hardness of the Mg–Ca alloys, but deteriorates the ductility, corrosion resistance and biocompatibility of the Mg–Ca alloys. The Y addition leads to an increase in the ductility; but a decrease in the compressive strength, hardness, corrosion resistance and biocompatibility of the Mg–1Ca–1Y alloy when compared to the Mg–1Ca alloy. Solutions of SBF and MMEM with the immersion of Mg–xCa and Mg–1Ca–1Y alloys show strong alkalisation. Our research results indicate that Mg–xCa alloys with Ca additions less than 1.0 wt% exhibited good biocompatibility, low corrosion rate as well as appropriate elastic modulus and strength; whilst the Y is not a proper element for Mg alloys for biomedical application due to its negative effects to the corrosion resistance and biocompatibility.     

Thermodynamical and mechanical properties of Pd–Ag alloys

The mechanical and thermodynamical properties of Pd, Ag pure metals and especially Pd_xAg_1−x alloys are studied by using the molecular dynamics simulation. The effects of temperature and concentration on the physical properties of Pd_xAg_1−x are analyzed. Sutton–Chen (SC) and quantum Sutton–Chen (Q-SC) many-body potentials are used as interatomic interactions which enable one to investigate the thermodynamic, static, and dynamical properties of transition metals. The calculated results such as cohesive energy, density, elastic constants, bulk modulus, pair distribution functions, melting points and phonon dispersion curves of Pd, Ag and Pd_xAg_1−x are in good agreement with the available experimental data at the various temperatures. The predicted melting points of Pd, Ag and their binary alloys from Q-SC potential parameters are closer to experimental values than the ones predicted from SC potential parameters.     

Thermal conductivity and mechanical properties of Sm-containing Mg-Zn-Zr alloys

This study investigated the effect of Sm additions on the microstructure, thermal conductivity, and mechanical properties of Mg-Zn-Zr alloy. The results indicate that the addition of Sm led to the formation of a rare-earth phase at the grain boundaries, and the grain size was significantly refined in the extruded state. The thermal conductivity of Mg alloy increased with the increase in Sm content because of the formation of a rare-earth phase that helps to dissolve the Zn atoms in the α-Mg matrix. Moreover, the as-extruded Mg alloy exhibited a higher thermal conductivity (up to124?W?(m?K)^?1) than its as-cast counterparts. The Sm-containing as-extruded Mg alloy showed excellent yield strength of up to 254?MPa, and also a good plastic deformation ability.     

Microstructure and mechanical properties of hot rolled TiNbSn alloys

Titanium alloys with lower elastic modulus and free from toxic elements such as Al and V have been studied for biomedical matters. Ti–Nb–Sn alloys showed up as presenting great potential for the aforementioned purpose. The current study got Ti–35Nb-XSn alloys (x = 2.5; 5.0; 7.5) by applying the following techniques: arc melting, homogenizing and cooling in furnace, homogenizing and water quenched, hot rolling and water quenched. According to each step of the study, the microstructures were featured by means of optical microscopy, by applying a scanning electron microscopy (SEM) analysis as well as X-ray diffraction. The mechanical properties were gotten by means of: Vickers microhardness, tensile and ultrasonic tests. Their ratio between tensile strength and elastic modulus as well as the ductility were compared to other biomedical alloys already available in the literature. The mechanical behavior of the Ti–Nb alloys directly depends on the Sn rates that constitutes the phases as well as on the thermomechanical background to which the alloy was submitted to. The hot rolled Ti–35Nb–2.5Sn alloy showed high ratio between strength and elastic modulus as well as high ductility, just as high as those of some cold rolled Ti alloys.     

Long-range ordering behaviour and mechanical properties of Ni-Mo-based alloys

Thermal ageing for up to 1000 h at 600–800 °C was used to study the long-range ordering behaviour to Ni₄Mo and related ordered phases in selected Ni-Mo based alloys with varying Fe and Cr contents, and the corresponding effects on mechanical properties. Analytical electron microscopy and X-ray diffraction were used to characterize the microstructure. Mechanical properties were determined from microhardness and tensile tests. During the early stages of ordering, the crystallographically-related Pt₂Mo-type, DO₂₂ and D1_a superlattices coexisted in all alloys studied. However, depending upon the exact chemical composition, particularly the Mo, Cr and Fe contents, some of these superlattices became unstable as the ordering reaction progressed. Chromium was found to act as a stabilizer of either a Pt₂Mo-type superlattice or Ni₃Mo depending upon the Mo content; however Fe acted as a stabilizer only of Ni₃Mo. For both Cr and Fe, the tendency to stabilize Ni₃Mo was realized at relatively higher Mo contents. Ordering behaviour of commercial alloys containing minor concentrations of Cr and Fe was found to significantly vary from one heat to another depending upon the exact Mo content. Although ordering to Ni₄Mo and Ni₃Mo could lead to severe embrittlement, ordering to a Pt₂Mo-type superlattice was found to have beneficial effects on mechanical strength.