Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf)2Ni precipitates

The martensite structure in a Ti₃₆Ni₄₄Hf₁₅Cu₅ ribbon annealed at different temperatures is investigated. When the annealing temperature is <873 K, spherical (Ti,Hf)₂Ni particles 20–40 nm in diameter precipitate in the grain interior. Transmission electron microscopy analysis shows that (0 0 1) compound twins are dominant in the ribbon containing homogeneously distributed (Ti,Hf)₂Ni precipitates. When the annealing temperature is 773 K, the boundaries between the martensite domains with the (0 0 1) twins are blurry and vague. When the annealing temperature is 873 K, four types of boundaries among the martensite domains are found: {1 1 1}, (0 0 1)//{1 1 1}, {1 1 3} and (1 1 0)//{1 1 3} types. When the annealing temperature is 973 K, the (0 1 1) twins become dominant, and the martensite variants show mainly spear-like and mosaic-like morphologies. However, martensite domains with (0 0 1) twins also exist around the coarse (Ti,Hf)₂Ni precipitates. Fine (Ti,Hf)₂Ni precipitates should be responsible for the improvement in shape memory effect and the superelasticity of Ti–Ni–Hf–Cu ribbons.     

Ti–Cu–Ni shape memory bulk metallic glass composites

New Ti–Cu–Ni shape memory bulk metallic glass composites were obtained by carefully controlling the cooling rate upon quenching. This allows for the formation of a metastable microstructure consisting mainly of ductile, spherical martensitic Ti(Ni,Cu) precipitates embedded in an amorphous matrix also containing a small volume fraction of TiCu and Ti₂Cu precipitates. These composites exhibit large ductility and high strength combined with a strong work-hardening behaviour. A deformation mechanism is proposed with the help of experimental observations and finite-element simulation. The simulation results demonstrate that stress concentrations occur around the precipitates, which promotes a heterogeneous stress distribution and the formation of multiple shear bands. Additionally, different transformation temperatures were observed for martensitic precipitates depending on whether they are completely or partially embedded in the amorphous matrix.     

Thermal Electrochemical, and Plasma Oxidation of Ti–50Zr, Cu–50Zr, Cu–50Ti, and Cu–33Ti–33Zr Alloys

The three main methods for oxidation of metallic substrates, thermal, anodic and plasma have been applied to a copper, titanium, and zirconium alloy and its corresponding binaries (Cu–33Ti–33Zr, Cu–50Ti, Cu–50Zr and Ti–50Zr). Polished polycrystalline samples of these alloys were examined before treatment, after vacuum thermal annealing at 100°C and heating in 20 mTorr oxygen at 100, 200, and 300°C. ISS depth profiles were taken of selected samples. The least-noble component oxidizes first, but at high temperatures and with plasma oxidation the noble component segregates to the surface. A comparison of the resulting structures on the ternary and binary alloys with different oxidation methods is used to explore the physico-chemical processes during oxidation. Results from these three methods are discussed in terms of physical/chemical parameters that influence the chemical nature and structure of the resulting oxides. The electrochemical processes that occur during the materials reaction with a chosen environment are used to discuss the physical and chemical mechanisms involved. Intrinsic (thermal and plasma oxidation) and extrinsic (electrochemical oxidation) electric fields are shown to influence the chemical and structural nature of the resulting oxide structures. The influence of transport phenomena is discussed.     

Machining of a Zr–Ti–Al–Cu–Ni metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass machining chips were characterized using SEM, X-ray diffraction and nano-indentation. Above a threshold cutting speed, oxidation of the Zr produces high flash temperatures and causes crystallization. The chip morphology was unique and showed the presence of shear bands, void formation and viscous flow.     

Fatigue behavior of Zr–Ti–Ni–Cu–Be bulk-metallic glasses

The high-cycle fatigue (HCF) behavior of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (in at.%) bulk-metallic glass (BMG) was studied. Two batches of samples that are from different lots (Batches 59 and 94) are employed in present experiments. The HCF experiments were conducted, using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 in air at room temperature and under tension-tension loading, where R=σ_min./σ_max.. (σ_min. and σ_max. are the applied minimum and maximum stresses, respectively). A high-speed and high-sensitivity thermographic-infrared (IR) imaging system was employed for the nondestructive evaluation of temperature evolutions during fatigue testing. No distinct sparking phenomenon was observed at the final fracture moment for this alloy. The fatigue lifetime of Batch 59 is longer than that of Batch 94 at high stress levels (maximum stresses >864 MPa). Moreover, the fatigue-endurance limit of Batch 59 (703 MPa) is somewhat greater than that of Batch 94 (615 MPa). The vein pattern and liquid droplets were observed in the apparent-melting region along the edge of the fractured surfaces. The fracture morphology suggests that fatigue cracks initiated from casting defects, such as porosities and inclusions, which have an important effect on the fatigue behavior of BMGs.     

Microstructure and mechanical properties of Fe–Si–Ti–(Cu,Al) heterostructured ultrafine composites

The influence of partial substitution of Fe by Cu or Al in Fe_75?xSi₁₅Ti₁₀(Cu, Al)_x (x = 0 and 4) ultrafine composites on the microstructure and mechanical properties has been investigated. The Fe₇₁Si₁₅Ti₁₀Cu₄ ultrafine composite exhibits a favorable microstructural evolution and improved mechanical properties, i.e., large plastic strain of ～5% and pronounced work hardening characteristics. The mechanical properties of the ultrafine eutectic composite are strongly linked to the length scale heterogeneity and the distribution of the constituent phases.     

Comparison of high temperature shape memory behaviour for ZrCu-based,Ti–Ni–Zr and Ti–Ni–Hf alloys

New ZrCu-based high temperature shape memory alloys with M_s close to 500 K are under development. The shape memory behaviour of this material is compared to those of Ti–Ni–Zr and Ti–Ni–Hf alloys. The optimal compositions show a shape recovery of not less than 3% at temperatures above 470 K.     

In situ formed Ti–Cu–Ni–Sn–Ta nanostructure-dendrite composite with large plasticity

A group of Ti–Cu–Ni–Sn–Ta multicomponent alloys is prepared by copper mold casting and arc melting, respectively, in which nanostructured (or ultrafine-grained) matrix-dendrite composites can be obtained. With increasing Ti and Ta contents, the volume fraction of the dendritic phase increases. The grain size of the matrix phase depends on the preparation method, and is 30–70 nm for as-cast 2–3 mm diameter cylinders and about 100–200 nm for the as-arc melted samples. Compression test results indicate that fully nanostructured samples exhibit very high yield strength of 1800 MPa with a limited plastic strain of 1.4%. The nanostructure-dendrite composites exhibit high yield strengths of 1525–1755 MPa together with large plastic strains of 4.7–6.0%. The as-arc melted samples exhibit relatively lower yield strengths of 1037–1073 MPa with very large plastic strains of 16.5–17.9% because of the coarser grain size of the matrix. The large plasticity of the composites is attributed to the retardation of localized shear banding and the excessive deformation in the nanostructured matrix due to the in situ formed dendrites. The deformation and the fracture mechanisms of the nanostructure-dendrite composites are discussed based on fractography observations.     

(Ti,Zr,Hf)-(Cu,Ni,Ag)-Al多组元合金体系的机械驱动非晶化

研究了由前过渡族金属和后过渡族金属按照等原子比替代的(Ti0.33Zr0.33Hf0.33)50(Ni0.33Cu0.33Ag0.33)40Al10多组元合金在机械研磨作用下的非晶化转变,并与由熔体急冷制备的同一成分玻璃态合金进行了对比.两种途径获得的玻璃态相似.在机械研磨的稳态产物中,仍残留有少量尺寸小于30 nm的晶体相颗粒,即非晶化转变未能完全实现.机械研磨形成的非晶相表现有明确的玻璃转变和约80 K宽的过冷液态温度区间,晶化过程分两步进行,第一步晶化转变完成后,剩余的非晶相亦具有玻璃态行为,呈现另一玻璃转变及约100 K宽的过冷液态温度区间.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Deformation and fracture toughness of a bulk amorphous Zr–Ti–Ni–Cu–Be alloy

The flow behavior and fracture toughness of two different plate thicknesses (i.e. 4 and 7 mm) of a bulk amorphous Zr–Ti–Ni–Cu–Be alloy was investigated. It is shown that the flow/fracture stress was independent of superimposed hydrostatic pressure over the range 50–575 MPa, suggesting that the flow behavior follows the von Mises criterion. However, the macroscopic orientation of the fracture plane relative to the stress axis was strongly affected by changes in stress state, suggesting some normal stress dependence to the flow/fracture behavior. The fracture behavior was also studied on both notched and precracked bend bars for both plate thicknesses. The average fracture toughness obtained from seven fatigue precracked specimens taken for both plate thicknesses was 17.9±1.8 MPa√m, while the notched toughness obtained on specimens with notch root radii ranging from 65 to 250 μm taken from both plate thicknesses were in the range of 9l–131 MPa√m.     

High-strength Cu-based bulk glassy alloys in Cu–Zr–Ti and Cu–Hf–Ti ternary systems

New Cu-based bulk glassy alloys were formed in Cu–Zr–Ti and Cu–Hf–Ti systems by the copper mold casting method. The critical diameter is 4 mm for the Cu₆₀Zr₃₀Ti₁₀ and Cu₆₀Hf₂₅Ti₁₅ alloys which are larger than 1 mm for the Cu₆₀Zr₄₀ and Cu₆₀Hf₄₀ glassy alloys. The substitution of Zr or Hf for Ti causes an increase in the glass-forming ability (GFA). As the Ti content increases, the glass transition temperature (T_g), crystallization temperature (T_x), and the supercooled liquid region ΔT_x(=T_x−T_g) decrease for both Cu₆₀Zr_40−xTi_x and Cu₆₀Hf_40−xTi_x alloys. In contrast, the liquid temperature (T_l) has a minimum value of 1127 K for the Cu₆₀Zr₂₀Ti₂₀ alloy and 1175 K for the Cu₆₀Hf₂₀Ti₂₀ alloy, resulting in a maximum T_g/T_l of 0.63 and 0.62, respectively. The alloys with the highest T_g/T_l value showed the highest GFA for these Cu-based alloys. The bulk glassy alloys exhibit high tensile fracture strength of 2000–2160 MPa, compressive fracture strength of 2060–2150 MPa and compressive plastic elongations of 0.8–1.7%. The finding of the new Cu-based bulk glassy alloys with high GFA, high fracture strength above 2000 MPa and distinct plastic elongation is encouraging for the future development of a new type of bulk glassy alloy which can be used for structural materials.     

Ti(Cu,Pt)_2和Ti(Cu,Pt)_3相的晶体结构和弹性性能（英文）

利用X射线粉末衍射数据结合Rietveld结构精修方法研究并确定两个新三元相Ti(Cu,Pt)_2和Ti(Cu,Pt)_3的晶体结构。使用电子探针(EPMA)检测样品的成分,同时结合纳米压痕技术和第一性原理计算对其弹性性能进行研究。研究发现,Ti(Cu,Pt)_2的空间群为Amm2(No.38),与VAu_2有着相同的结构类型。Ti(Cu,Pt)_3的结构为四方晶系的AlPt_3结构类型,属于P4/mmm空间群(No.123)。纳米压痕测量和第一性原理计算表明,Ti(Cu,Pt)_2的弹性模量随Pt含量的增加先增大,然后减小;而Ti(Cu,Pt)_3的弹性模量随Pt含量的增加几乎呈线性增加。     

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.     

Structure of martensite in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures

After annealing at different temperatures, there are different types of precipitates in Ti-rich Ti–Ni–Cu thin films: plate-like Guinier–Preston (GP) zones, Ti₂Cu precipitates and spherical Ti₂Ni precipitates. The (0 1 1) compound twins and (1 1 1) type I twins are dominant in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures, which suggests that the precipitates do not change the twinning modes of the B19 martensite. However, the amount of the (0 1 1) compound twin increases with increasing annealing temperature due to its small twinning shear. In thin films with GP zones or Ti₂Ni precipitates, the amount of martensite with a single-pair morphology is less than that in thin films without precipitates. And in thin film with Ti₂Cu + Ti₂Ni precipitates, hardly any martensite with a single-pair morphology is observed. For the different types of precipitates at the different annealing temperatures, the obstacle of the precipitates to the growth of the B19 martensite plate also varies. The GP zones slightly hinder the growth in the width of martensite, resulting in wavy twin boundaries at the martensite variant tip. The Ti₂Cu precipitates can change both the width and the direction of the martensite plates. Ti₂Ni precipitates also significantly disturb or impede the growth of the martensite variants. These effects lead to a decrease in the maximum shape recoverable strain with increasing annealing temperature.     

Transient liquid phase (TLP) bonding of TiAl/Ti6242 with Ti–Cu foil

Abstract

Defect-free joints of TiAl/Ti6242 have been produced by TLP bonding using Ti, Cu foils as insert metals. The influence of the bonding parameters such as bonding pressure, bonding temperature, dwell time and the stacking sequence of the parent materials on the microstructure of the joined zone has been investigated. The results showed that bonding pressure played an important role even though there was a liquid phase during bonding. The stacking sequence of the parent materials also had a major effect on joint formation. Elimination of defects at the interface of the TiAl/ interlayer in order to obtain sound joints was a major problem. Similar interface structures were observed in all cases except that the thickness of the joined zone was different. The results of the study revealed that the surface oxide layer on TiAl had a major effect on the interface diffusion process. Based on the interface diffusion and interface structure, the joint formation process has been investigated and a mechanism for the effect of experimental parameters on joint formation has been proposed.     

Transient liquid phase (TLP) brazing of Mg–AZ31 and Ti–6Al–4V using Ni and Cu sandwich foils

Transient liquid phase (TLP) brazing of Mg–AZ31 alloy and Ti–6Al–4V alloy was performed using double Ni and Cu sandwich foils. Two configurations were tested; first, Mg–AZ31/Cu–Ni/Ti–6Al–4V and second, Mg–AZ31/Ni–Cu/Ti–6Al–4V. The effect of set-up configuration of the foils on microstructural developments, mechanical properties and mechanism of joint formation was examined. The results showed that different reaction layers formed inside the joint region depending on the configuration chosen. The formation of ? phase (Mg), ρ (CuMg₂), δ (Mg₂Ni) and Mg₃AlNi₂ was observed in both configurations. Maximum shear strength obtained was 57 MPa for Mg–AZ31/Ni–Cu/Ti–6Al–4V configuration and in both configurations, the increase in bonding time resulted in a decrease in joint strength to 13 MPa. The mechanism of joint formation includes three stages; solid state diffusion, dissolution and widening of the joint, and isothermal solidification.     

Strain glass in Fe-doped Ti–Ni

We report the existence of strain glass in Ti₅₀Ni_50?xFe_x, beyond a critical Fe doping level x > x_c(5 < x_c < 6). The strain glass state is confirmed by the appearance of a frequency-dependent anomaly in the AC mechanical modulus/loss at a freezing temperature T₀ and by the breaking of ergodicity shown in a zero-field-cooling/field-cooling experiment. Based on the experimental results, a phase diagram is established in which both the normal martensitic transformations (for x < x_c) and the strain glass transition (for x > x_c) are indicated. The new phase diagram allows for explanations of two long-standing puzzles in Ti₅₀Ni_50?xFe_x and Ti–Ni alloys: (i) the origin of nano-domains present prior to the martensitic transformation (for x < x_c) and (ii) the negative temperature dependence of electrical resistivity in abnormal non-transforming compositions (for x > x_c).     

Infrared joining strength and interfacial microstructures of Ti–48Al–2Nb–2Cr intermetallics using Ti–15Cu–15Ni foil

Infrared joining of Ti–48Al–2Nb–2Cr using Ti–15Cu–15Ni (wt%) foil as brazing filler metal was investigated at the temperature range of 1100∼1200°C for 30∼60 s in a flowing argon environment. The compressive tests show three types of fracture morphologies in which type I fails at the joint interface, but types II and III are fractured in the base-metal with the crack direction parallel to and perpendicular to the loading axis, respectively. Most of joined specimens were fractured through the base metal indicating that the infrared joined interface has relatively good joint strength. The compressive strength of type I specimen is about 319–322 MPa. Experimental results show that the shorter the real holding time or the higher the joining temperature, the larger the strength variation will be. The observed interfacial microstructures of Ti–48Al–2Nb–2Cr joint interfaces indicate that seven characteristic zones can be distinguished in the joint interfaces and each characteristic structure corresponds to one or more stable phases at T_w temperature. The observed microstructures and their evolutions of each zone are explained in detail in this study. The major difference between joint interfaces of Ti–48Al–2Nb–2Cr and Ti₅₀Al₅₀ alloys takes place on the base-metal interface zone and the columnar two-phase zone. The existence of Nb and Cr atoms in Ti–48Al–2Nb–2Cr alloy also has some influences on the microstructural evolution of the columnar two-phase zone and the continuous α₂-layer.     

Effect of ultra-fine powders on the microstructure of Ti(CN)–xWC–Ni cermets

Microstructural and compositional analyses of Ti(CN)–xWC–20Ni cermets were done in order to better understand the dissolution behavior of ultra-fine sized Ti(CN) and WC in comparison with a micro-sized system. The WC content was varied from 5 to 75 wt%. A discrete composition (∼50 wt%WC addition) at which WC does not dissolve any longer in the ultra-fine system was found. At this composition, the W concentrations, not only in the rim, (Ti,W)(CN), but in the binder as well were at a maximum. Depending on the powder size and morphology, these values are strongly affected due to the surface areas of Ti(CN) and WC for dissolution and precipitation sites. Furthermore, a significant improvement in the mechanical properties of simple ultra-fine systems was found, compared to that of corresponding micron-sized systems (Hv 14–15 GPa, K_IC 8–10 MPa m^1/2), which are comparable to commercial cermets. The refined microstructure and high fraction of rim phase in the ultra fine systems are likely responsible for this difference.