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.     

Microstructure of Rapidly Solidified Alloys of the Sn–Zn–Bi–In System

The results of a study of the phase composition and microstructure of foils of Sn–8.0 Zn–3.0 Bi–X In (X = 1.5, 2.5, 4.5, 9.0) (wt %) alloys formed by rapidly quenching from the melt at a cooling rate of up to 5 × 10⁵ K/s have been presented. The dependence of the phase composition of the rapidly quenched foils on the concentration of In has been determined. It has been shown that, in rapidly quenched foils, crystallization occurs with the formation of supersaturated solid solutions based on β-Sn and γ phase (Sn₄In). The mechanisms and rates of decomposition of the supersaturated solid solutions at room temperature have been established. The specific features of the formation of the microstructure of the foils have been discussed. The grain structure has been studied by the electron back-scatter diffraction (EBSD) method; the formation of an elongated shape of grains and the high specific surface area of small-angle boundaries has been explained.     

Non-equilibrium Phases Formed in Cu–In–Se–Te System Synthesized by Melt-Quench Method

Non-equilibrium phases formed in melt-quenched Cu In(SexTe1-x)2system, where x = 0.1, 0.2, 0.4, 0.5, 0.6,0.8 and 0.9, have been studied using Rietveld refinement of the crystal structure and Raman spectroscopy. Results of structure refinement have showed that all the samples, except the Cu In(Se0.1Te0.9)2, are heterogeneous. All the observed non-equilibrium phases are quaternary system and are found to have chalcopyrite structure(I"42d), in accordance with the Cu In Te2–Cu In Se2 phase diagram. The lattice constants deduced from the refinement have showed linear variation with Se content. A detailed analysis of the characteristic A1 modes present in the Raman spectrum of individual sample has corroborated the results obtained from the structure analysis. The position of A1 mode of individual phase is found to vary linearly with Se content, which suggests that Cu In(SexTe1-x)2system exhibits single-mode behaviour.     

Microstructure and melting properties of Ag–Cu–In intermediate-temperature brazing alloys

The microstructure and melting properties of ternary Ag–Cu–In intermediate-temperature alloys(400–600 °C) prepared by electric arc melting were investigated in this work. The melting properties, phase compositions, microstructure and hardness were characterized by differential scanning calorimetry(DSC), X-ray diffraction(XRD), scanning electron microscopy(SEM)and micro-hardness tester, respectively. The results show that the melting properties, phase compositions, microstructure and hardness of Ag–Cu–In brazing alloys are substantially different when adding different levels of indium. Indium element could effectively reduce the melting temperatures of(Ag–Cu28)–x In alloys, and the melting temperatures of(Ag–Cu28)–25In alloy are located at 497.86 and 617.48 °C. When the indium content varies from 5 wt% and 10 wt%, the dominant phases in the alloys are Ag-rich and Cu-rich phases, and their granular crystals are smaller than 0.5 lm. When the indium content is higher than 15 wt%, the phase compositions of the alloy are Ag4 In and Cu11In9, and the microstructure exhibits dendritic crystals with a uniform distribution. The hardness of(Ag–Cu28)–x In alloy decreases first and then increases with the content of indium increasing, and the highest hardness of(Ag–Cu28)–25In alloy is HV 266.0.     

Microstructure and shear strength of a Au–In microjoint

Two types of Au–In microjoints, i.e. Au/In/Au in which In foil was used and Au/In, were prepared by either solid state interdiffusion (SSID) or solid–liquid interdiffusion (SLID) bonding for single lap tensile test. Deposition of the Au and In thin films was carried out by thermal evaporation on a polyethylene terephthalate (PET) substrate. It is found that the shear strength of the Au/In microjoints is higher than that of Au/In/Au using In foil. It is also observed that the fracture mode of Au–In microjoints depends on the types of In used. Failure of the Au/In microjoints appeared to be along the joint–substrate interface, whereas it occurred within the In foil for the other type of specimens. Examination of the Au/In microjoints by glancing angle X-ray diffraction reveals the presence of the two major constituent phases, Au₇In₃ and Au, as well as other intermetallics AuIn₂, Au₁₀In₃, and Au₉In₄ in small amount. On the other hand, only the intermetallic AuIn₂ and pure In were observed in the Au/In/Au microjoints, where the total thickness of In is much higher than that of Au.     

In situ TEM study of deformation twinning in Ni–Mn–Ga non-modulated martensite

The straining of non-modulated (NM) Ni–Mn–Ga martensite was studied by in situ transmission electron microscopy (TEM). Initially, the self-accommodated NM martensitic structure consists of internally twinned domains. During straining, the detwinning process starts within these domains. The internal twin variant more favorably oriented to the stress grows at the expense of the other one. In the detwinned, single-variant domain, a new twin variant can form, gradually replacing the existing variant via the twinning process. Both processes—detwinning and new twinning—proceed by the same mechanism, namely by the movement of twinning dislocations along the twin boundary. Lattice dislocations are also created in the detwinning process. While the boundaries between the internal twins are coherent and mobile, the boundaries between the internally twinned domains are incoherent, strained and not mobile. The planes of the coherent twin boundary are {2 0 2) planes and the Burgers vectors of the twinning dislocations are parallel to the 〈1 0 1] direction. The magnitude of the Burgers vector determined from the TEM observations disagrees with the calculation from the lattice constant measurement by X-ray diffraction. Possible reasons for this discrepancy are discussed.     

Direct measurement of large reversible magnetic-field-induced strain in Ni–Co–Mn–In metamagnetic shape memory alloys

Direct measurements of reversible magnetic-field-induced strain (MFIS) on a single crystalline Ni₄₅Co₅Mn_36.5In_13.5 metamagnetic shape memory alloy were attained via magnetic-field-induced martensitic transformation under different stress levels and at various temperatures. This was achieved using a custom-designed micro-magneto-thermo-mechanical testing system capable of applying constant stress while measuring strain and magnetization simultaneously on the samples, which can fit into conventional superconducting magnets. MFIS levels are reported as a function of temperature, magnetic field and external bias stress. It was necessary to apply an external bias stress in these materials to detect a notable MFIS because a magnetic field does not favor a specific martensite variant resulting in no shape change even though magnetic field leads to reversible martensitic transformation. Fully recoverable transformation strains up to 3.10% were detected under repeated field applications in the presence of different compressive stress levels up to 125 MPa. The bias stress opposes the field-induced martensite-to-austenite phase transformation and causes the critical field for the transformation to increase at a given temperature in accordance with the Clausius Clapeyron relationship. The effect of the bias stress on the kinetic arrest of austenite is also explored.     

Surfaces,interfaces and phase transitions in Al–In monotectic alloys

Surface and bulk liquid phase transitions are measured by a unique method currently used to determine surface and interfacial tension of liquid alloys. Focusing on the Al–In system, the location of the liquid miscibility gap was determined from the critical to the monotectic temperatures. The surface tensions of nine liquid alloys, the interfacial tension between coexisting liquids and their densities were measured as a function of temperature. Implementing the bulk data extracted from the asymmetric miscibility gap into a sub-regular model reproduced the experimental surface and interfacial tensions. The wetting temperature was estimated to lie well below the monotectic temperature. The micrometer thickness of the In-rich films which wet the surface of the Al-rich liquid phase after solidification is suggested to be due to the growth of the equilibrium wetting film by diffusion from the Al-rich phase during cooling.     

In situ assessment of lattice strain in an Al–Li alloy

The lattice strains of individual grains are measured in an Al–Li alloy, AA 2195, using high-energy X-ray diffraction at a synchrotron source. The diffraction of individual grains in this highly textured production alloy was isolated through use of a depth-defining aperture. It is shown that hydrostatic stress, and in turn the stress triaxiality, can vary significantly from grain to grain.     

In situ visualization of Ni–Nb bulk metallic glasses phase transition

We report the results of in situ investigation of the structural evolution and crystallization behavior of Ni-based bulk metallic glass. The X-ray diffraction, transmission electron microscopy, nanobeam diffraction, differential scanning calorimetry, radial distribution function and scanning tunneling microscopy (STM)/spectroscopy techniques were applied to analyze the structure and electronic properties of Ni_63.5Nb_36.5 glasses before and after crystallization. According to our STM measurements, the primary crystallization originally starts with the Ni₃Nb phase formation as a leading eutectic phase. It was shown that surface crystallization differs drastically from bulk crystallization due to the possible surface reconstruction. The mechanism of Ni_63.5Nb_36.5 glass alloy two-dimensional crystallization was suggested, which corresponds to the local metastable (3 × 3) ?  Ni(1 1 1) surface phase formation. The possibility of different surface nanostructures developing by annealment of the originally glassy alloy in an ultrahigh vacuum at a temperature lower than the bulk crystallization temperature was shown. The increase of the mean square surface roughness parameter R_q while transforming from a glassy to a fully crystallized state can be caused by concurrent growth of Ni₃Nb and Ni₆Nb₇ bulk phases. The simple empirical model for the estimation of Ni_63.5Nb_36.5 cluster size was suggested, and the value obtained (about 8 Å) is in good agreement with the corresponding STM measurements (8–10 Å).     

Characterizations on Mechanical Properties and In Vitro Bioactivity of Biomedical Ti–Nb–Zr–CPP Composites Fabricated by Spark Plasma Sintering

To alleviate the bio-inert of Ti alloys as hard tissue implants, Ti–35Nb–7Zr–xCPP(calcium pyrophosphate,x = 5, 10, 15, 20 wt%) composites were prepared by mechanical alloying(MA) and following spark plasma sintering(SPS). Mechanical behaviours and in vitro bioactivity of these composites were investigated systematically. Results showed that the composites consisted of β-Ti matrix, α-Ti, and metal–ceramic phases such as CaO, CaTiO_3, CaZrO_3, and Ti_xP_y. With increasing CPP content, the composites had higher strength(over 1500 MPa) and higher elastic modulus, but suffered almost zero plastic deformation together with lower relative density. When the CPP contents were 5 and 10 wt%,the compressive elastic moduli were 44 and 48 GPa, respectively, which were close to those of natural bones. However, the compressive elastic modulus of the composites increased significantly when CPP contents exceed 10 wt%, thus deteriorating the mechanical compatibility of the composites owing to more α-Ti and metal–ceramic phases. Besides, the surface of Ti–35Nb–7Zr–10CPP composite was deposited as a homogeneous apatite layer during soaking in simulated body fluid(SBF). It indicates a good bioactivity between the implant materials and living bones.     

In situ observation of solidification behavior in undercooled Pd–Cu–Ni–P alloy by using a confocal scanning laser microscope

In the undercooled melt of Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy, the solidification behavior including the nucleation and growth of crystals at the micrometer level has been observed in situ by use of a confocal scanning laser microscope combined with an infrared image furnace. The Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy specimens were cooled from the liquid state to various undercooled states under a helium gas flow. Images of solidification progress were obtained by the charge-coupled device image sensor of the confocal scanning laser microscope. Depending on the degree of undercooling, the morphology of the solidification front changed among various types: faceted front, columnar dendritic front, cellular grain and equiaxed grain, etc. The velocities of the solid–liquid interface were measured to be 10⁻⁵–10−7 m/s, which are at least two orders of magnitude higher than the theoretical crystal growth rates. Combining the morphologies observed in the three undercooling regimes and their solidification behaviors, we conclude that phase separation takes place in the undercooled molten Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy. The continuous-cooling–transformation (CCT) diagram was derived from the experimental time–temperature-transformation diagram constructed from solidification onset times under various isothermal annealing conditions. The CCT diagram suggests that the critical cooling rate for glassy solidification is about 1.8 K/s, which is in agreement with previous calorimetric findings.     

In situ observation of nucleation,fragmentation and microstructure evolution in Sn–Bi and Al–Cu alloys

Abstract

This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn–13 at.-%Bi alloys and the Al–15 mass%Cu alloys. In the Al–15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.     

Banded-like morphology and martensitic transformation of dual-phase Ni–Mn–In magnetic shape memory alloy with enhanced ductility

Two of the current challenges facing producers of Ni–Mn–In alloys are the achievement of small hysteresis and good ductility. Here, we present a dual-phase (β-Ni_51.8Mn_31.4In_16.8 and γ-Ni_62.4Mn_32.5In_5.1) Ni₅₂Mn₃₂In₁₆ alloy prepared by the zone melting liquid metal cooling directional solidification method, which simultaneously shows small hysteresis (ΔT < 10 K) and good ductility (6.6%). In addition, and more importantly, an inter-martensitic transition with a large magnetization jump occurs in this alloy. This is expected to further broaden the working temperature range of actuators and sensors that use this magnetic shape memory alloy. The sequence of the martensitic transformation can be shown by in situ X-ray diffraction to be austenite → 10M → 14M. Additionally, the second (γ) phase dramatically enhances the entropy change of these structural transformations and shifts them to higher temperatures. During the directional solidification, a novel banded-like microstructure, consisting of two layers, one of the β single phase and the other of the two phases coupled, forms at the low growth rate. A qualitative model is presented to explain the experimental observation, taking into account both the competitive nucleation and the growth of the phases. Experimental and theoretical analysis in the present work shows a linear relationship between the maximum spacing of the β single phase layer and the growth rate.     

Influence of microstructural characteristics on corrosion behavior of Mg–5Sn–3In alloy in Hank's solution

The microstructural observation, the mass loss test, potentiodynamic polarization measurements and corrosion morphology examinations were conducted to study the influence of microstructural characteristics on corrosion behavior of Mg–5Sn–3In alloys in Hank's solution after extrusion. The results show that the corrosion rate of the as-cast alloy is similar to that of as-extruded alloy; however, the local corrosion susceptibility is greatly weakened in the as-extruded alloy, especially in the extrusion direction. The relatively uniform corrosion morphology of the as-extruded alloy is attributed to refined Mg₂Sn particles, uniform distribution of Mg₂Sn particles and favorable crystal orientation. Meanwhile, the cytotoxicity tests confirm that the Mg–5Sn–3In alloy exhibits cytotoxicity of Grade 0–1 for NIH3T3 cells, suggesting an acceptable cytotoxicity of this alloy in the vitro assay.     

In Situ Elimination of Pores During Laser Powder Bed Fusion of Ti–6.5Al–3.5Mo–l.5Zr–0.3Si Titanium Alloy

The printing quality of components manufactured using laser powder bed fusion(LPBF) generally depends on the presence of various defects such as massive porosity. Thus, the efficient elimination of pores is an important factor in the production of a sound LPBF product. In this study, the efficacy of two in situ laser remelting approaches to eliminating pores during the LPBF of a titanium alloy Ti–6.5 Al–3.5 Mo–l.5 Zr–0.3 Si(TC11) was assessed both experimentally and computationally. These two re...     

Observation of dendritic growth and fragmentation in Ga–In alloys by X-ray radioscopy

Abstract

Capabilities of the X-ray attenuation contrast radioscopy were utilised to provide a real time diagnostic technique for observations of dendritic growth and fragmentation during solidification of a Ga–30In (wt-%) alloy. The solidification process was visualised by means of a microfocus X-ray tube providing shadow radiographs at spatial resolutions of about 10 μm. Experiments have been carried out to solidify the Ga–In alloy unidirectionally either starting from the bottom or the top of the specimen. The first case is significantly affected by solutal convection, which governs a redistribution of solute concentration. A detachment of dendrite side arms, which is unambiguously caused by melt flow, was not observed. Dendritic fragmentation occurs during the solidification in the reverse top down direction. Variations of the applied cooling rate excited a transition from a columnar to an equiaxed dendritic growth (CET).     

Influence of Cryorolling on the Precipitation of Cu–Ni–Si Alloys: An In Situ X-ray Diffraction Study

The effect of cryorolling on the precipitation process of deformed Cu–Ni–Si alloys was investigated through in situ synchrotron X-ray diffraction technique. The results demonstrate that the precipitation process is significantly accelerated by cryorolling. Cryorolling produces higher dislocation density, which provides more heterogeneous nucleation sites for Ni2 Si precipitates, hence promotes precipitation. In the early stage of aging, the enhanced nucleation of precipitates accelerates the depletion of supersaturation, and finer precipitates are obtained. In addition, recrystallization is promoted as a result of high stored energy in the cryorolled Cu–Ni–Si alloys, which facilitates the formation of discontinuous precipitation in the late stage of aging.