Effect of powder size on the consolidation of gas atomized Cu54Ni6Zr22Ti18 amorphous powders

Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass powders with spherical morphology and clean surfaces were synthesized using high-pressure gas atomization. In order to investigate the effect of powder size distribution on consolidation, which was performed using warm extrusion, the powders were divided into four groups. Little difference in the thermal behaviors of powders was found with the powder size distribution. Extruded bars are amorphous structure when consolidated at a suitable, temperature in their supercooled liquid state. As the powder size decreased, the compressive strength of the extrudated samples decreased. Fracture occurred along the maximum shear plane declined ∼45° to the loading direction.     

Mechanical behavior of a bulk Cu–Ti–Zr–Ni–Si–Sn metallic glass forming nano-crystal aggregate bands during deformation in the supercooled liquid region

The deformation behavior of a bulk Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ metallic glass, fabricated by injection casting, has been characterized in the supercooled liquid region. The alloy deforms homogeneously and exhibits large elongation above the glass transition temperature at constant true strain rate below 1×10⁻²s⁻¹, but it shows a variation of the flow stress during deformation. The flow stress reaches a peak just after yielding and then decreases significantly with increasing strain. After the plateau level of remarkably low flow stress, it rises again and then the alloy finally fails in a brittle manner. DSC data and TEM observations for the tested alloy reveal that the alloy evolves to being crystallized during deformation. Nano-crystals are aggregated and the aggregates are aligned along the load direction. When the volume fraction of the crystalline phase is in the range up to 0.5, the nano-crystal aggregates effectively slide over each other, lowering the apparent stress level. However, as the amount of the crystalline phase further increases, the flow stress continuously increases. This behavior can be explained based on the volume-fraction rule between the crystalline phase and the amorphous phase.     

Difference in compressive and tensile fracture mechanisms of Zr59Cu20Al10Ni8Ti3 bulk metallic glass

The compressive and tensile deformation, as well as the fracture behavior of a Zr₅₉Cu₂₀Al₁₀Ni₈Ti₃ bulk metallic glass were investigated. It was found that under compressive loading, the metallic glass displays some plasticity before fracture. The fracture is mainly localized on one major shear band and the compressive fracture angle, θ_C, between the stress axis and the fracture plane is 43°. Under tensile loading, the material always displays brittle fracture without yielding. The tensile fracture stress, σ_F^T, is about 1.58 GPa, which is lower than the compressive fracture stress, σ_F^C(=1.69 GPa). The tensile fracture angle, θ_T, between the stress axis and the fracture plane is equal to 54°. Therefore, both θ_C and θ_T deviate from the maximum shear stress plane (45°), indicating that the fracture behavior of the metallic glass under compressive and tensile load does not follow the von Mises criterion. Scanning electron microscope observations reveal that the compressive fracture surfaces of the metallic glass mainly consist of a vein-like structure. A combined feature of veins and some radiate cores was observed on the tensile fracture surfaces. Based on these results, the fracture mechanisms of metallic glass are discussed by taking the effect of normal stress on the fracture process into account. It is proposed that tensile fracture first originates from the radiate cores induced by the normal stress, then propagates mainly driven by shear stress, leading to the formation of the combined fracture feature. In contrast, the compressive fracture of metallic glass is mainly controlled by the shear stress. It is suggested that the deviation of θ_C and θ_T from 45° can be attributed to a combined effect of the normal and shear stresses on the fracture plane.     

双孔洞排布位置对C轴压缩镁单晶的影响研究

为探究双孔洞位置排布对于镁及镁合金塑性变形的影响,应用分子动力学方法模拟在300k下含不同排布位置的双孔洞镁单晶c轴压缩模型,结合三种模型的应力-应变曲线、势能曲线、径向分布函数和位错密度曲线,分析不同排布位置双孔洞镁单晶的压缩力学性能和结构演化过程。结果表明：双孔洞镁单晶在与加载方向平行时可承受的压应力峰值和势能峰值以及对应的应变程度最大;与加载方向垂直时次之,当与加载方向呈45°排布时最小,且与c轴呈90°排布的双孔洞镁单晶模型孔洞闭合速率最快。     

Forming process of cross-connected finned micro-grooves in copper strips

Using ploughing-extrusion method, a cross-connected finned micro-grooves structure was formed on the surface of copper strips with thickness of 0.4 mm. The structure was fabricated by making ‘V’-grooves in copper strips and perpendicular ‘V’-grooves on the opposite side that intersect the first set of grooves. Micro pores appear at the intersection of these cross-connected grooves, and micro fins appear on the groove fringes. So it can be defined as ‘pore-groove-fin’ structure. The preferable ‘pore-groove-fin’ structure can be obtained under the condition that the tool edge inclination angle (χγ) is 45°, both the major extrusion angle (γo) and the minor extrusion angle (γ 0′ ) are 30°, both the major formation angle (β) and the minor formation angle (β′) are 10°, the ploughing-extrusion depth (fd) is 0.32 mm and the groove pitch is 0.4 mm on surfaces A and B. The formed included angle of groove A is 70°, and the groove depth is 0.3 mm, while the included angle of opposite perpendicular groove B is 20° with the groove depth of 0.35 mm. The obtained fin height is 0.15 mm, the elliptical pore length is 0.2 mm and the width is 0.05 mm. Experiments show that fd has the greatest influence on the formation of micro pores. Bulges appear on the opposite surface B when the ploughing-extrusion depth on surface A (fdA) reaches a critical value. The ploughing-extrusion depth on surface B (fdB) has great influence on the re-growth of fin structure.     

基于VPSC仿真的ZK60镁合金拉伸变形行为

通过实验和粘塑性自洽（VPSC）模型,研究了在室温下挤压态ZK60镁合金沿不同方向拉伸时的变形机制开动情况,及其与流动曲线、织构演变和显微组织的对应关系。通过调节VPSC模型的参数,建立了滑移和孪生耦合的晶体塑性力学模型。比较了不同方向拉伸过程中织构演变的差异,分析了变形机制对屈服不对称性的影响。实验和模拟结果表明：当沿垂直于挤压方向（PED）拉伸时,由于｛102｝孪晶开动,大部分晶粒发生大角度旋转（约90°）。柱面<a>滑移是导致ZK60合金沿不同方向拉伸时出现明显屈服不对称的主要变形机理。当ZK60合金沿挤压方向（ED）拉伸时,由于晶粒的择优取向分布,｛101｝孪晶难以开动,导致ZK60挤压态镁合金拉伸屈服强度较高。ZK60镁合金沿着与ED成45°的方向拉伸时,屈服应力高于沿PED拉伸,但随着拉应力逐渐增大,由于沿PED拉伸时柱面<a>滑移逐渐开动,沿PED应变后期的应力曲线逐渐高于沿与ED成45°方向应变的应力曲线。     

Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO₂ at 300 °C, while a mixture of monoclinic-ZrO₂ (m-ZrO₂) and tetragonal-ZrO₂ (t-ZrO₂) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al₂O₃, some tetragonal-ZrO₂, and a slight amount of monoclinic-ZrO₂ at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO₂, some CuO and Cu₂O, and limited tetragonal-ZrO₂. It is suggested that the formation of Al₂O₃ (at 300 °C) and CuO/Cu₂O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.     

Plasma-electrolytic formation of Ta-containing oxide coatings on titanium. Their composition and properties

Coatings with a thickness of 2–18 μm that contain up to 20 at % tantalum and Ta₂O₅ oxide phase are formed in an aqueous NH₄[TaF₆]-containing electrolyte for 2 min. The coatings are pierced with pores with a size of from 0.5 to 2 μm. The number of pores in the coatings can be decreased by additionally applying a Ta-containing paste to the surface. The contact angle of the coatings in distilled water is 68°–85°. This approach is promising for application of Ta-containing coatings on titanium implants and stents for increasing their corrosion resistance, chemical inertness, and biocompatibility.     

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

Microstructures and properties of porous TiAl-based intermetallics prepared by freeze-casting

Preparation of porous TiAl-based intermetallics with aligned and elongated pores by freeze-casting was investigated. Engineering Ti−43Al−9V−1Y powder (D₅₀=50 μm), carboxymethyl cellulose, and guar gum were used to prepare the aqueous-based slurries for freeze-casting. Results showed that the porous TiAl was obtained by using a freezing temperature of −5 °C and the pore structure was tailored by varying the particle content of slurry. The total porosity reduced from 81% to 62% and the aligned pore width dropped from approximately 500 to around 270 μm, with increasing the particle content from 10 to 30 vol.%. Furthermore, the compressive strength along the aligned pores increased from 16 to 120 MPa with the reduction of porosity. The effective thermal conductivities of porous TiAl were lower than 1.81 W/(m·K) and showed anisotropic property with respect to the pore orientation.     

The axial compressive failure of titanium reinforced with silicon carbide monofilaments

Monofilament-reinforced titanium has been subjected to compressive loading, with a range of angles between the fibre axis and the loading direction. Under axial loading, the failure stress is about 4 GPa, which is well below levels predicted for kink band formation. It is proposed that compressive failure occurs under these circumstances by the crushing of individual fibres. A model is proposed for prediction of the composite strength as controlled by this mechanism. Observed strengths are consistent with monofilament crushing stresses of about 8–10 GPa. Composites were also studied after a post-consolidation heat treatment and with weak fibre–matrix interfacial bonding. In both cases, slightly higher compressive strengths were recorded than for the standard material. These increases are attributed to an enhanced matrix yield stress and to a higher monofilament compressive strength, respectively. Under off-axis loading, strengths fell from about 4 GPa at low misalignment angles to just above 1 GPa at an angle of 16°. A transition occurs between fibre crushing at low angles and kink band formation at higher angles. The transition range is around 3–4°, which is consistent with model predictions. Microstructural studies confirmed that the expected failure modes were operative in these two regimes.     

Effect of Compressive Mode I on the Mixed Mode I/II Fatigue Crack Growth Rate of 42CrMo4

Subsurface cracks in mechanical contact loading components are subjected to mixed mode I/II, so it is necessary to evaluate the fatigue behavior of materials under mixed mode loading. For this purpose, fatigue crack propagation tests are performed with compact tension shear specimens for several stress intensity factor (SIF) ratios of mode I and mode II. The effect of compressive mode I loading on mixed mode I/II crack growth rate and fracture surface is investigated. Tests are carried out for the pure mode I, pure mode II, and two different mixed mode loading angles. On the basis of the experimental results, mixed mode crack growth rate parameters are proposed according to Tanaka and Richard with Paris’ law. Results show neither Richard’s nor Tanaka’s equivalent SIFs are very useful because these SIFs depend strongly on the loading angle, but Richard’s equivalent SIF formula is more suitable than Tanaka’s formula. The compressive mode I causes the crack closure, and the friction force between the crack surfaces resists against the crack growth. In compressive loading with 45° angle, da/dN increases as K _eq decreases.     

The impact of different volume fractions of crystalline structures on the electrochemical behaviour of Mg67Zn29Ca4 alloys for biomedical applications

ABSTRACT

In this work, the influence of various volume fractions of the crystalline phase in an amorphous matrix of Mg₆₇Zn₂₉Ca₄ alloys was investigated for its corrosion resistance for biodegradable applications. An amorphous Mg₆₇Zn₂₉Ca₄ alloy was successfully fabricated using melt casting into a copper mould. Then, to obtain different ratios of the crystalline phase in an amorphous matrix, the obtained amorphous rods with 3?mm diameters were annealed at 190, 230, 250, and 400°C. The volume fraction of the crystalline phase was measured by X-ray diffraction, and the microstructures of the obtained alloys were determined based on scanning electron microscopy images. Electrochemical testing was conducted in simulated body fluid at 37°C. This report shows that the ratio of the volume fractions of amorphous and crystalline phases in alloy microstructures strongly influences their corrosion behaviours. The alloy with a fully amorphous structure was the most resistive in the analysed media.     

压痕应变法中压痕周围的应力应变分布规律

利用有限元数值模拟方法,对压痕应变法测量残余应力中球形压痕周围不同受力方向的应力应变分布规律进行了分析.结果表明,在单向应力场中,与主方向成62°的分界线两侧,应变增量与外加应变之间的关系曲线特征不同,并随夹角而变化.在特定范围内,应变增量与拉应力间的关系夹角小于45°时近似为线性关系,夹角大于45°为二次曲线关系.而应变增量与压应力间的关系在分界线两侧可用线性函数和二次函数分别描述.在双向应力场中,如果把乘以泊松比的垂直方向应力值叠加到主应力上,则得到的规律与单向应力场的相同.     

Deformation behavior of Zr-based bulk metallic glass in an undercooled liquid state under compressive loading

High temperature deformation behavior of a Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk amorphous alloy has been studied in a temperature range between 355 and 460°C under compressive loading after rapid heating. A transition of flow behavior, viz. from, a Newtonian to a non-Newtonian flow, has been reported by many researchers as the temperature is decreased at a given strain rate. In the present study, two different theoretical relations based on a viscous flow model and a transition state theory have been applied to analyze the transition behavior of deformation in terms of viscosity and flow stress. An experimental deformation map was then constructed to specify the boundaries between Newtonian and non-Newtonian flow, based on the relationship between the flow stress and strain rate in an undercooled liquid state. It has further been confirmed that the stress overshoot phenomena can be observed mostly in a non-Newtonian flow regime appearing in an intermediate temperature and strain rate region in this deformation map.     

Thermoelectric properties for P-type (Bi2Te3)0.2(Sb2Te3)0.8 alloys fabricated by shear extrusion

P-type (Bi₂Te₃)_0.2(Sb₂Te₃)_0.8 compounds were synthesized via bulk mechanical alloying (BMA), and subsequently prepared by a shear extrusion process in order to create the developed texture. The shear extrusion process improved the preferred orientation factor of the anisotropic crystallographic structure. It was found by an electron backscattered diffraction pattern (EBSP) analysis that approximately 90% of the crystallographic orientations for shear-extruded sample are aligned in the range deviated from 60° to 90° from the c-axis. The electric resistivity is well controlled at 1.008×10⁻⁵Ωm, which is nearly equal to that of the unidirectionally grown sample. The maximum figure of merit for the (Bi₂Te₃)_0.2(Sb₂Te₃)_0.8 alloy was found to be z=3.03×10⁻³K⁻¹. The bending strength of the material produced is also improved to 120 MPa, six times larger than that of the unidirectionally grown sample.     

Preparation of self-organized porous anodic niobium oxide microcones and their surface wettability

Porous anodic niobium oxide with a pore size of ～10 nm was formed at 10 V in glycerol electrolyte containing 0.6 mol dm^?3 K₂HPO₄ and 0.2 mol dm^?3 K₃PO₄ at 433 K. After prolonged anodizing for 5.4 ks, niobium oxide microcones develop on the surface. X-ray diffraction patterns of the anodized specimens revealed that the initially formed anodic oxide is amorphous, but an amorphous-to-crystalline transition occurs during anodizing. As a consequence of the preferential chemical dissolution of the initially formed amorphous oxide, due to different solubility of the amorphous and crystalline oxides, crystalline oxide microcones appear on the film surface after prolonged anodizing. The surface is superhydrophilic. After coating with fluorinated alkylsilane, the surface becomes superhydrophobic with a contact angle of 158° for water. The surface is also oil repellent, with a contact angle as high as 140° for salad oil.     

Numerical Modeling of Frictional Stress in the Contact Zone of Direct Extrusion of Aluminum Alloys under Starved Lubrication

The objective of this article is to investigate numerically frictional stress in the contact zone at the die/billet interface in the direct extrusion of aluminum alloys considering starved lubricated conditions. In the modeling, both the inlet and work zones have been investigated by coupled solution of the governing equations. The influences of the billet material’s strain hardening and its heating due to the plastic deformation are accounted for in the numerical computation. The frictional shear stress at the die/billet interface is computed using three different lubricating oils. Numerical results have been presented herein for the various operating parameters viz. starvation factor (ψ = 0.2–0.6), lubricants’ viscosities (η ₀ = 0.05 Pa s–0.2 Pa s), semi die angle (β = 10°–20°), and material parameter (G = 0.56–2.25). It has been observed that the frictional stress increases with an increase in the severity of the lubricant’s starvation for the given values of semi-die angle, extrusion speed, and material parameter.     

Effect of superimposed uniaxial stress on rumpling of platinum-modified nickel aluminide coatings

Thermal cycling of a platinum modified, nickel aluminide (Ni,Pt)Al coated single crystal superalloy, between 1000 and 1150 °C with 10 min holds at each temperature, and subject to a compressive uniaxial stress is reported. There are two major effects of the superimposed compressive stress not observed in the absence of the stress. One is that the rumpling pattern exhibits an asymmetry with an increase of the bond coat surface roughness perpendicular to the applied loading axis. The other is the formation of cracks in the thermally grown oxide aligned parallel to the stress axis.     

Effects of Crystallographic Orientation on Corrosion Behavior of Magnesium Single Crystals

The corrosion behavior of magnesium single crystals with various crystallographic orientations was examined in this study. To identify the effects of surface orientation on the corrosion behavior in a systematic manner, single-crystal specimens with ten different rotation angles of the plane normal from the [0001] direction to the $ [ 10\overline{1} 0] $ direction at intervals of 10° were prepared and subjected to potentiodynamic polarization and potentiostatic tests as well as electrochemical impedance spectroscopy (EIS) measurements in 3.5?wt.% NaCl solution. Potentiodynamic polarization results showed that the pitting potential (E _pit) first decreased from ?1.57?V _SCE to ?1.64?V _SCE with an increase in the rotation angle from 0° to 40°, and then increased to ?1.60?V _SCE with a further increase in the rotation angle to 90°. The results obtained from potentiostatic tests are also in agreement with the trend in potentiodynamic polarization tests as a function of rotation angle. A similar trend was also observed for the depressed semicircle and the total resistances in the EIS measurements due to the facile formation of MgO and Mg(OH)₂ passive films on the magnesium surface. In addition, the amount of chloride in the passive film was found first to increase with an increase in rotation angle from 0° to 40°, then decrease with a further increase in rotation angle, indicating that the tendency to form a more protective passive film increased for rotation angle near 0° [the (0001) plane] or 90° [the $ ( 10\overline{1} 0) $ plane].