4Cr 4Ti Alloy Research Articles

A low cost and rapid analytical technique for direct spectrophotometric determination of chromium in V–Cr–Ti alloys without a chromogenic agent

A simple, rapid and accurate spectrophotometric method was developed for the determination of chromium in V–Cr–Ti ternary alloy without any prior separation and chromogenic agent.

Mechanical properties and precipitation evolution behavior of V–5Cr–5Ti alloy after cold rolling

The mechanical properties and precipitation behavior of V–5Cr–5Ti alloy were investigated after 80% cold rolling and annealing by optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), and tensile testing. The TEM analysis revealed that the cold-rolled V–5Cr–5Ti alloy had two kinds of precipitates with an hexagonal close-packed(HCP) structure. The rectangular precipitate had the following lattice parameters: a=0.3041–0.3048nm, c=0.5153–0.5158nm, and c/a=1.69. The elliptically shaped precipitate had the following lattice parameters: a=0.3080–0.3082nm, c=0.5003–0.5009nm, and c/a=1.62. The other precipitates had a face-centered cubic(FCC) structure. The precipitates were broken into small spherical particles, which were arranged in the vicinity of the band structure. After annealing (1273 K×1h), the stable precipitates were formed by increasing the components of Cr and N. The yield strength of cold-rolled V–5Cr–5Ti alloy after annealing was 21.5% higher than that before cold rolling, and the yield ratio increased from 0.69 to 0.79.

Enhancement of Mechanical Biocompatibility of Titanium Alloys by Deformation-Induced Transformation

Metastable β-type titanium alloys are highly suitable for use as structural biomaterials applied to hard tissue, i.e., as cortical bone (hereafter, bone) replacing implants. However, their mechanical biocompatibitities, such as the Young’s modulus, strength and ductility balance, fatigue strength, resistance against fatigue crack propagation and fracture toughness, require improvenent for increased compatibility with bone. Through deformation, the metastable β-phase in a metastable β-type titanium alloy is transformed into various phases, such as α’ martensite, α” martensite, and ω-phases with exact phase depending by metastable β-phase stability. In addition, twinning is also induced by deformation. Deformation twinning effectively enhances the work hardening in the metastable β-type titanium alloy, leading to increased strength and ductility. This improvement is accompanied by with other deformation-induced transformations including the appearance of deformation-induced martensite and ω-phase transformation. The enhancement of the mechanical biocompatibility of various materials using the abovementioned deformation-induced transformation is described in this paper, for both newly developed metastable β-type Ti-Mo and Ti-Cr alloys for biomedical applications.

Cold brittleness and fracture of metals with various crystal lattices: Dislocation mechanisms

The dislocation mechanisms of formation of the ductile–brittle transition temperature and the low-temperature brittle fracture of metals (single crystals, polycrystals) with various crystal lattices (bcc, fcc, hcp) are considered. The conditions of appearance of cold shortness and intracrystalline crack propagation (brittle fracture) are determined. These conditions can be met in bcc and some hcp metals and cannot be met in fcc and many hcp metals. The nondestructive internal friction (at 100 kHz) method is used to determine the temperature ranges of cold shortness (ductile–brittle transition temperatures) in bcc metals (ferritic–martensitic EK-181 steel, V–4Ti–4Cr alloy), which depend on their structure–phase state and strength (yield strength).

Effect of post-weld heat treatment on microstructure, hardness and low-temperature impact toughness of electron beam welds of NIFS-HEAT-2 and CEA-J57 heats of V–4Ti–4Cr alloy

Bead-on-plate electron beam welding in high vacuum atmosphere was applied to the plates of NIFS-HEAT-2 and CEA-J57 heats of V–4Ti–4Cr alloy. Effect of post-weld heat treatment (PWHT) in the temperature range 673–1273K on the hardness, impact toughness at 77K and microstructure of weld metal was investigated. After PWHT at 773K, hardness of weld metal slightly decreases from 180HV100 (as-welded state) to ∼170HV100 while absorbed energy increases up to ∼10J showing ductile fracture mode. PWHT at 973K results in re-hardening of weld metal up to ∼180HV100 caused by re-precipitation of Ti–C,O,N precipitates and corresponding decreasing absorbed energy to ∼2J with brittle fracture mode. PWHT in-between 1073–1273K results in gradual recovery of hardness towards values comparable with those of base metal. Impact toughness (77 K) of weld metal after PWHT at 1073K is not recovered nether to the value in as-welded state nor to that one of base metal.

Ta/Zr-Alloyed V–Cr–Ti Alloys via a Cluster-Plus-Glue-Atom Model for BCC Solid Solutions

A cluster formula of [M − V14]M1 was formed for vanadium alloys based on a cluster-plus-glue-atom model for BCC solid solutions, where the clusters [M − V14] were centered by one solute M and surrounded by fourteen solvent atoms V, and M could also be served as a glue atom to link clusters. The [M − V14]M1 formula with M = V1/3Cr1/3Ti1/3, an equal-molar combination of V, Cr and Ti, corresponded to the typical V–4Cr–4Ti alloy (wt.%). Based on this formula, a series of new alloys with Ta and Zr substitution for V and Ti respectively in M, were designed and molded into ϕ3 mm rods by copper-mold suction-cast method. These alloys were solid-solutioned at 1273 K for 2 h followed by water-quenching. For Zr-added alloys, the second phase V2Zr was prone to be precipitated, that made alloys much brittle and worse corrosion-resistant in Cl− solution. While Ta-alloyed alloys exhibited a single BCC structure, the Vickers hardness HV of alloys were enhanced obviously. Among them, the Ta-added alloy with M = Ta1/3Cr1/3Ti1/3 (V79.21Ta13.4Cr3.85Ti3.54 wt.%) displayed both higher microhardness and better corrosion-resistance in Cl− solution.

The Oxidation Behaviour of Alloys Based on the Ni–Co–Al–Ti–Cr System

The isothermal oxidation behaviour of a series of quinary Ni–Co–Al–Ti–Cr alloys were studied at 800 °C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-h exposure, as did the alloys richest in Ni and Al for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni and Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni and Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the elemental partitioning behaviour and the increased mole fraction of γ′ precipitates forming in the alloy.

The research on the constitutive modeling and hot working characteristics of as-cast V–5Cr–5Ti alloy during hot deformation

Isothermal compression tests of as-cast V–5Cr–5Ti vanadium alloy are conducted in the deformation temperature ranging from 1150 to 1400 °C with an interval of 50 °C, strain rate ranging from 0.001 to 1 s−1 and height reductions of 55% on a computer controlled thermal simulation machine. The hot deformation behavior of as-cast is characterized based on the analysis of the stress–strain behaviors, the constitutive equations and process map for obtaining optimum processing parameters and achieving desired microstructure during hot working. A constitutive equation by which the flow stress was expressed as a function of strain rate and deformation temperature was established and the apparent activation energy of deformation was calculated to be 428.480 kJ/mol. The processing maps are constructed at the true strain of 0.5 and 0.7 based on dynamic materials model (DMM) to delineate the safe and unsafe regions in the axes of temperature and strain rate. The area of unsafe region increases with the increasing of strain. These instability domains exhibited localized flow and cracking along grain boundaries which should be taken care of during hot processing. The recommended domain is in the condition of the temperature range of 1250–1400 °C and strain rate range of 0.001–0.02 s−1. And in this state the main soften mechanism is dynamic recovery. It is worth noting that when as-cast V–5Cr–5Ti alloys deformed in the recommended deformation zone, the continuous dynamic recrystallization occurred, which display the feature of serrate grain boundaries and the equiaxed grains in the grain boundaries. Microstructure observation of deformed specimens validated the applicability of the processing map at obtaining the optimum processing parameters of as-cast V–5Cr–5Ti alloy.

Microstructure and mechanical properties of V–4Ti–4Cr alloy as a function of the chemical heat treatment regimes

The regularities of the formation of a heterophase structure and mechanical properties of V–4Ti–4Cr alloy as a function of thermomechanical and chemical heat treatments are studied. The regimes of thermomechanical treatment which provide the formation of a heterophase structure with a homogeneous volume distribution of oxycarbonitride nanoparticles with a size of about 10 nm and an increase in the volume content and thermal stability of this phase and which provide an increase in the temperature of alloy recrystallization are developed. The formation of the heterophase structure results in a substantial (up to 70%) increase in the short-term high-temperature strength of the alloy at T = 800°C. The increase in the strength is achieved while keeping a rather high level of plasticity.

Formation and evolution of platelet-like Ti-rich precipitates in the V–4Cr–4Ti alloy

The goal of the present investigation is to explain the obviously different appearances of Ti-rich precipitates in vanadium alloy and in steels. To achieve the goal, the formation and evolution of the precipitates in the as-cast and the heat treated V–4Cr–4Ti samples were investigated using optical and electron microscopies. The precipitates were found to be rare in the as-cast samples, and a high density of the precipitates occur in the samples subjected to isothermal holding at 600–1300°C. The precipitates preferentially distribute within the grains rather than at the grain boundaries. All of the precipitates are platelet-like, with NaCl structure, in three-dimensional space. The further observation using high-resolution electron microscopy (HREM) reveals that a high density of twins occurs in the growth front of the precipitates, whereas the middle of the precipitates is twin-free. Meanwhile, enrichment of titanium atoms was observed in the middle of the precipitates. These results indicate that the precipitates form by a displacive transformation, followed by a diffusional process to enrich titanium further and eliminate the twined structure.

Abnormal hardening effect induced by the lath-like precipitates in the V–4Cr–4Ti alloy

The present work focuses the effect of the precipitate's shape on the hardness of V–4Cr–4Ti alloy. It is found that there are numerous lath-like Ti-rich precipitates occurring in the samples subjected to different heat treatment. The habit plane of these precipitates is the {200} plane of the matrix. The distribution density of the precipitates is closely related to heating temperatures and times. The hardness valley of the samples occurs at the heating temperatures corresponding to the high distribution density of precipitates. The abnormal hardening effect can be interpreted by a model, in term of which lath-like precipitates with high distribution density result in single gliding of dislocations and reduced work-hardening.

Thermal activation parameters of V–5Cr–5Ti alloy under hot compression

In order to well understand the elementary mechanisms that govern the hot working process of a V–5Cr–5Ti alloy (mass fraction, %), thermal activation parameters under compression were measured in a temperature range of 1373–1673 K by a Gleeble–3800 system. The results show that the stress exponent n is 4.87 and the activation energy Q is 375.89 kJ/mol for the power law equation. The activation energy is determined as 288.34 kJ/mol, which is close to the self-diffusion energy of alloy (270–300 kJ/mol) by introducing a threshold stress(σ0) variable. The typical values of physical activation volume (Vp) and strain rate sensitivity (m) are measured as (120–700) b3 and 0.075–0.122, respectively, by the repeated stress relaxation tests. These activation parameters indicate that the rate controlling mechanism for V–5Cr–5Ti alloy compressed in ranges of 1373–1673 K and 0.001–1.0 s−1 is the dislocation climb by overcoming of forest dislocations.

Microstructure and wear resistance of c-BN/Ni–Cr–Ti composites prepared by spark plasma sintering

c-BN/Ni–Cr–Ti composites were fabricated by spark plasma sintering. The microstructure and the interfacial structure were analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis. The results show that the Ni–Cr–Ti alloy exhibits a good wettability to cover the c-BN particles. The interfacial bonding is strong and an interfacial reaction layer was found at the interface. The microscopic analysis revealed that Ti is the active element in the Ni–Cr–Ti alloy which is enriched in the c-BN particles and interacts with N and B, resulting in the formation of TiB2, TiB and TiN and promoting the chemical metallurgical bonding between the Ni–Cr–Ti alloy and c-BN. The non-smooth surface formed by the relative protrusion of the c-BN particles with their high hardness can resist the wear of the friction pairs during the wear experiments. The failure mode of the c-BN/Ni–Cr–Ti composites is fatigue shedding of the reinforcing hard phases of the c-BN particles as well as the micro-cutting and transfer of the Ni-based alloy surface.

Alloy design and microstructural evolution in V–Ti–Cr alloys

V-alloys in combination with Ti and Cr have been identified as potential candidate materials for various energy technologies as an alternative to fossil fuel based technologies. Phase formation and microstructural evolution in ternary V–Ti–Cr alloys along with the constituent binaries have been studied by Miedema approach and the selected binary and ternary alloy compositions have been studied experimentally in order to validate the results obtained from Miedema's model. According to Miedema's model V–Ti–Cr ternary alloys in the V-rich side of the phase diagram should form a solid solution phase and the ternary alloy composition in the vicinity of binary TiCr2 Laves phase should form amorphous or intermetallic phase. The expected solid solution phase in binary V–Ti and V–Ti–20Cr alloy system may undergo phase separation. Four alloy compositions, as identified by the Miedema approach e.g., V–4Ti–4Cr, V–39Ti–54Cr, V–50Ti and V–40Ti–20Cr, have been studied experimentally. V–4Ti–4Cr alloy forms BCC solid solution phase upon solidification. In V–39Ti–54Cr alloy, V-substituted cubic TiCr2 Laves phase is seen upon solidification. The BCC solid solution phase in binary V–50Ti and ternary V–40Ti–20Cr alloys undergoes phase separation giving rise to V-rich and V-poor domains. In the ternary V–40Ti–20Cr alloy, the phase separation is essentially binary as Cr is distributed homogeneously in the phase separated domains. Interesting possibilities exist for V–Ti–Cr alloys so far as phase transformation, microstructural evolution and tailoring of properties are concerned. Moreover, Miedema's approach could be used to design alloys in this system.

Influence of grain boundaries and pre-precipitates on ion implantation-induced precipitation in the V–4Cr–4Ti alloy

The V–4Cr–4Ti alloy which contains the pre-precipitates has been subjected to implantation of high energy ions at room temperature. It was found that the Ti-rich precipitates occur at grain boundaries while the far finer precipitates form within the grains. The ion implantation also results in variation of appearance of the pre-precipitates in the samples. These results indicate that diffusion in the samples is obviously enhanced by ion implantation-induced vacancies. The vacancies will disappear at the grain boundaries and the precipitates/matrix interfaces, which will transfer vanadium atoms prior out of the boundaries or the interfaces due to Kirkendall effect. In this way, the Ti-rich precipitates will form at the boundaries. Meanwhile, the pre-precipitates will change their shape.

Arrhenius-type constitutive model and dynamic recrystallization behavior of V–5Cr–5Ti alloy during hot compression

To clarify the high temperature flow stress behavior and microstructures evolution of a V–5Cr–5Ti (mass fraction, %) alloy, the isothermal hot compression tests were conducted in the temperature range of 1423–1573 K with strain rates of 0.01, 0.1, and 1 s−1. The results show that the measured flow stress should be revised by friction and the calculated values of friction coefficient m are in the range of 0.45–0.56. Arrhenius-type constitutive equation was developed by regression analysis. The comparison between the experimental and predicted flow stress shows that the R2 and the average absolute relative error (AARE) are 0.948 and 5.44%, respectively. The measured apparent activation energy Qa is in the range of 540–890 kJ/mol. Both dis-continuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) mechanisms are observed in the deformed alloy, but dynamic recovery (DRV) is the dominant softening mechanism up to a true strain of 1.5.

Pulse current electrodeposition of tungsten coatings on V–4Cr–4Ti alloy

Tungsten coatings with high (220)-orientation were formed on V alloy substrate by pulse current electrodeposition in air atmosphere. The coatings’ microstructure, crystal structure and adhesive strength between coatings and substrates were investigated. It could be observed the tungsten coatings consisted of two sub-layers with the inner tooth-like layer, and the outer columnar layer. The tungsten coatings deposited at lower duty cycle have a better surface quality with a little change in the adhesive strength. The tungsten coating was exposed to electron beam with power density of 200MW/m2 in the thermal shock test, the tungsten crystal grain surface melt, the microcracks are found among the crystal grains. Exfoliation, flaking and dense needle-like holes were observed on the tungsten coating after irradiation with helium ions at an energy of 65keV and an implanted dose of 22.67×1018cm−2.

Nanoquasicrystalline Al-based matrix/γ-Al2O3 nanocomposites

Quasicrystalline aluminium alloys have been studied in the past years achieving higher strength than commercial Al alloys and retaining high strength at high temperature. In this work a quasicrystalline Al alloy matrix nanocomposite containing nanoceramic particles has been manufactured using ball milling and hot extrusion. For that purpose a nanoquasicrystalline Al–Fe–Cr–Ti alloy was manufactured by powder atomisation. Nanocomposites consisting of a quasicrystalline Al–Fe–Cr–Ti alloy matrix and reinforcement of γ-Al2O3 nano particles were manufactured. The effect of ball milling time on the microstructure and microhardness of the nanocomposite powders was investigated. Bulk materials were produced by consolidation and hot extrusion. The microstructure and microhardness of the extruded materials were characterised.The milling regime behaviour is discussed, and shows three different steps that have a significant effect on the rate of change of uniformity of the reinforcement distribution, matrix microstructure, powder size distribution and its microhardness. No significant decomposition of the quasicrystalline phase occurred over 30h of milling. Strain increased and the crystallite size of the aluminium phase decreased with milling time, with the Al crystallite size reaching a steady state. Although the quasicrystalline phase decomposed during hot extrusion, the microhardness of the nanocomposite produced is significantly harder (227±3μHV500) than both the unreinforced quasicrystalline alloy (159±1μHV500) and crystalline aluminium nanocomposites reported in the literature [1]. Methods and analysis of material behaviour put forward in this work inform further understanding and optimisation of this and other nanocomposite systems containing a metastable microstructure matrix.

The ability of silicide coating to delay the catastrophic oxidation of vanadium under severe conditions

V–4Cr–4Ti vanadium alloy is a potential cladding material for sodium-cooled fast-neutron reactors (SFRs). However, its affinity for oxygen and the subsequent embrittlement that oxygen induces causes a need for an oxygen diffusion barrier, which can be obtained by manufacturing a multi-layered silicide coating. The present work aims to evaluate the effects of thermal cycling (using a cyclic oxidation device) and tensile and compressive stresses (using the three-point flexure test) on the coated alloy system. Tests were performed in air up to 1100°C, which is 200°C higher than the accidental temperature for SFR applications. The results showed that the VSi2 coating was able to protect the vanadium substrate from oxidation for more than 400 1-h cycles between 1100°C and room temperature. The severe bending applied to the coated alloy at 950°C using a load of 75MPa did not lead to specimen breakage. It can be suggested that the VSi2 coating has mechanical properties compatible with the V–4Cr–4Ti alloy for SFR applications.

Mechanical properties and cytocompatibility of oxygen-modified β-type Ti–Cr alloys for spinal fixation devices

In this study, various amounts of oxygen were added to Ti–10Cr (mass%) alloys. It is expected that a large changeable Young’s modulus, caused by a deformation-induced ω-phase transformation, can be achieved in Ti–10Cr–O alloys by the appropriate oxygen addition. This “changeable Young’s modulus” property can satisfy the otherwise conflicting requirements for use in spinal implant rods: high and low moduli are preferred by surgeons and patients, respectively. The influence of oxygen on the microstructures and mechanical properties of the alloys was examined, as well as the bending springback and cytocompatibility of the optimized alloy. Among the Ti–10Cr–O alloys, Ti–10Cr–0.2O (mass%) alloy shows the largest changeable Young’s modulus following cold rolling for a constant reduction ratio. This is the result of two competing factors: increased apparent β-lattice stability and decreased amounts of athermal ω phase, both of which are caused by oxygen addition. The most favorable balance of these factors for the deformation-induced ω-phase transformation occurred at an oxygen concentration of 0.2mass%. Ti–10Cr–0.2O alloy not only exhibits high tensile strength and acceptable elongation, but also possesses a good combination of high bending strength, acceptable bending springback and great cytocompatibility. Therefore, Ti–10Cr–0.2O alloy is a potential material for use in spinal fixture devices.