Cr Ta Alloys Research Articles

Phase constituent and microstructure manipulations via annealing for enhancements of mechanical property and functionalities of Ti–Au–Cr–Ta biomedical shape memory alloys

Shape memory alloys (SMAs) are considered as one of the most important smart materials for the future biomedical applications in consideration of the rapid world population aging. The promising Ti–Au–Cr–Ta alloys, which are good in biocompatibility, excellent in X–ray contrast, and high in corrosion resistance, were synthesized and evaluated in this study for realizing new biomedical materials. The specimens with the specific chemical composition of Ti–4Au–5Cr–5Ta (at.%) were chosen as the target material based on our preliminary findings. The Ti–4Au–5Cr–5Ta alloys were thereafter subjected to annealing–treatments at various annealing time periods for the manipulations of the phase constituents and microstructures. The fundamental mechanical properties and functionalities, such as shape deformation/recovery behaviors in the loading/unloading processes, were investigated. The two optimized specimens, which were subjected to a 1073 K annealing–treatment for 1.8 ks and 3.6 ks, performed good mechanical properties (i.e., strength and elongation) and excellent shape recovery rate of 96 % and 97 %, respectively. Via the systematic fine–tuning strategies, the Ti–4Au–5Cr–5Ta alloy with appropriate heat–treatment surpassed all other Ti–Au–Cr–Ta alloys, which were study by our research group.

Combinatorial Materials Design Approach to Investigate Adhesion Layer Chemistry for Optimal Interfacial Adhesion Strength

A combinatorial material adhesion study was used to optimize the composition of an adhesion promoting layer for a nanocrystalline diamond (NCD) coating on silicon. Three different adhesion promoting metals, namely W, Cr, and Ta, were selected to fabricate arrays of co-sputtered binary alloy films, with patches of seven different, distinct alloy compositions for each combination, and single element reference films on a single Si wafer (three wafers in total; W–Cr, Cr–Ta, Ta–W). Scratch testing was used to determine the critical failure load and practical work of adhesion for the NCD coatings as a function of adhesion layer chemistry. All tested samples eventually exhibit delamination of the NCD coating, with buckles radiating perpendicularly away from the scratch track. Application of any of the presented adhesion layers yields an increase of the critical failure load for delamination as compared to NCD on Si. While the influence of adhesion layers on the maximum buckle length is less pronounced, shorter buckles are obtained with pure W and Cr–Ta alloy layers. As a general rule, the addition of an adhesion layer showed a 75% improvement in the measured adhesion energies of the NCD films compared to the NCD coating without an adhesion layer, with specific alloys and compositions showing up to 125% increase in calculated practical work of adhesion.

The creep deformation behavior of a single-crystal Co–Al–W-base superalloy at 900 °C

The creep deformation behavior of a single-crystal Co–Al–W–Ni–Cr–Ta alloy with low tungsten content has been studied at stresses between 275 and 310MPa at 900°C. The alloy exhibits comparable creep strength with that of Co–Al–W-base alloys containing more tungsten. The creep deformation consists of three stages, the primary stage, the steady-state stage and the tertiary stage, when described by the creep strain rate versus time curve. At 900°C, γ′ precipitates tend to raft along the direction of applied tensile stress in the steady-state creep stage and a topologically inverted and rafting γ/γ′ microstructure is formed in the tertiary stage. The main deformation mechanism in the primary creep stage is dislocation shearing of γ′ precipitates, and in the following creep stages, the dominant deformation mechanism is dislocations bypassing γ′ precipitates.

Temperature dependence of deformation behavior in a Co–Al–W-base single crystal superalloy

Abstract Tensile properties of a single-crystal Co–Al–W–Ni–Cr–Ta alloy with low tungsten content have been studied within the temperatures ranging from 20 to 1000 °C at a constant strain rate of 1.0×10 −4 s −1 . The alloy exhibits comparable yield strength with that of Co–Al–W-base alloys containing more tungsten. From 600 °C to 800 °C, a yield strength anomaly is observed, probably due to the cross-slip of superdislocations from the octahedral plane to the cube plane. TEM analysis demonstrates that stacking faults (SFs) appear both in γ channels and γ′ precipitates in a wide temperature range. These SFs are responsible for the obvious strain hardening observed in stress–strain curves. From room temperature to 900 °C, the deformation is dominated by dislocations shearing γ′ particles. At 1000 °C, the main deformation mechanism is dislocations bypassing γ′ particles.

A study on the influence of Mo, Al and Si additions on the microstructure of annealed dual phase Cr–Ta alloys

The effects of additions of 5 at.% Mo, Al and Si on the long-term annealed microstructures of a two phase Cr–Cr2Ta alloy have been studied. Following 200 h at 1300 °C, the lamellar eutectic constituent of all the alloys disintegrated into discrete particles of the Laves phase embedded within a Cr-rich solid solution phase, along with the formation of fine Laves phase precipitates. One of the predominant differences between the three alloying additions was the extent of the C14 to C15 polytypic transformation of the Cr2Ta-based Laves phase. With Mo and Al additions, the Cr2Ta Laves phase transformed from C14 to either C15 or intermediate hexagonal polytypes following 200 h annealing at 1300 °C. In contrast, Si additions stabilised the C14 polytype, with no transformation to other polytypes observed after prolonged annealing at 1000, 1100 and 1300 °C.

Effect of silicon additions on the high temperature oxidation behaviour of Cr–Cr2Ta alloys

The effect of Si additions between 0 and 15 at.% on the microstructure and oxidation resistance of Cr–Ta alloys has been studied. After annealing at 1300 °C for 500 h, the microstructures of the alloys were observed to consist predominantly of a Cr-rich solid solution and a Cr2Ta Laves phase, with the Si partitioning preferentially to the Laves phase. Si was found to be beneficial to high temperature oxidation resistance, with the ternary alloys showing a lower weight gain than binary Cr–Ta alloys after isothermal exposure at 1100 °C for 100 h. However, no further improvement in oxidation behaviour under these conditions was achieved with increasing the Si-content beyond 7 at.%. The oxide scales formed on the alloys were found to comprise typically of Cr2O3 and mixed (Cr,Ta)-based oxides. The scales formed at 1100 °C and 1300 °C spalled readily, whilst those formed at 1000 °C showed better adherence to the substrate. Importantly, the presence of Si was also found to eliminate the internal nitridation otherwise encountered during the oxidation of binary Cr–Ta alloys. The formation of SiO2 in the alloys was observed only at 1300 °C, where the alloys displayed a massive weight gain with rapid oxidation of the Cr-rich solid solution.

Microstructural evolution and interfacial crystallography in Cr–Cr2Ta

The microstructural features of a Cr–Ta alloy containing 10at% Ta have been studied in the as-cast and stress-relieved condition, and after long-term annealing at 1300 °C and 1500 °C. In the as-cast and stress-relieved state, the microstructure consisted of dendrites of the Cr2Ta Laves phase and a lamellar eutectic mixture of Cr(Ta) solid solution and Cr2Ta. After protracted annealing at 1300 °C, the lamellar eutectic was observed to spherodise, producing a microstructure consisting of a Cr(Ta) matrix, within which fine particles of Cr2Ta were embedded. This was accompanied by solid state precipitation of fine Cr2Ta particles within the Cr(Ta) matrix. In addition, the Cr2Ta phase transformed progressively from the hexagonal C14 polytype, found in the as-cast and stress-relieved condition, to either the cubic C15 polytype or intermediate, faulted, hexagonal-based structures. The cubic Laves phase was seen to contain either coarse or fine twin bands. An orientation relationship between the cubic Laves phase and the Cr(Ta) matrix, clearly different from ones previously reported elsewhere was observed. A significant density of dislocations was also observed in the matrix, which persisted even after annealing.

Pseudoelasticity of thermo-mechanically treated Fe–Mn–Si–Cr–Ta alloys

Abstract The effects of slight Tantalum (Ta) addition on the microstructures, martensitic transformation and pseudoelasticity of Fe–28Mn–6Si–5Cr alloy were investigated. Experimental results show that the slight addition of 0.1 wt% Ta can effectively inhibit the slip deformation, increase the M d temperature and hence improve the pseudoelasticity of Fe–28Mn–6Si–5Cr alloy. Meanwhile, the cold rolling can also significantly increase the alloy's pseudoelasticity, especially when the specimens are tested at near M d temperature. The pseudoelasticity can reach 62% at the sixth loading–unloading cycle for the 30% cold-rolled Fe–28Mn–6Si–5Cr–0.1Ta alloy.

ChemInform Abstract: An XPS Study of the Corrosion Behavior of Sputter-Deposited Amorphous Cr-Nb and Cr-Ta Alloys in 12 M HCl Solution.

Abstract Sputter-deposited amorphous Cr-Nb and Cr-Ta alloys possessing higher corrosion resistance than that of alloy constituents were spontaneously passivated in 12 M HCl solution. Their open circuit potentials were higher than those of alloy constituents in 12 M HCl solution, and the passive current densities of amorphous Cr-Nb and Cr-Ta alloys were lower than those of niobium and tantalum metals, respectively, and decreased with increasing chromium concentration. XPS analysis revealed that air-formed films and passive films on the amorphous Cr-Nb and Cr-Ta alloys were composed of double oxyhydroxides of chromium and niobium or tantalum. The higher corrosion resistance of the amorphous Cr-Nb and Cr-Ta alloys in comparison with their constituent elements is attributed not only to the uniformity and homogeneity of the amorphous alloys but also to the formation of the double oxyhydroxide film of chromium and niobium or tantalum, both of which act synergistically in improving the corrosion resistance, partly because the double oxyhydroxide films possess high activity for cathodic oxygen reduction with a consequent ennoblement of the open circuit potential.

Fabrication of SmCo$_{5}$-CrTa Granular Films

A Sm-Co-CrTa granular layer between a Sm-Co continuous layer and a Cu underlayer was prepared in order to improve magnetic properties and magnetic reversal process of Sm-Co perpendicular film. A phase of CrTa alloy in Sm-Co-CrTa layer has a role in control of Cu partial interlayer diffusion between the Sm-Co continuous layer and the Cu underlayer. The Cu diffusion provides the partial crystallization of SmCo5 in the continuous layer. Although the perpendicular coercivity of films decreased slightly by introducing the CrTa alloy into Sm-Co layer, the magnetic intergranular exchange interaction in the films was drastically improved. Size of average magnetic cluster became smaller and magnetization reversal process was changed from a wall-motion to a coherent rotation for increasing the thickness ratio of Sm-Co-CrTa/Sm-Co layers. We confirmed that an addition of Sm-Co-CrTa layer is effective to reduce the intergranular exchange interaction in Sm-Co perpendicular films without deterioration of perpendicular magnetic anisotropy.

Effects of tantalum on the temporal evolution of a model Ni–Al–Cr superalloy during phase decomposition

The effects of a 2.0 at.% addition of Ta to a model Ni–10.0Al–8.5Cr (at.%) superalloy aged at 1073 K are assessed using scanning electron microscopy and atom-probe tomography. The γ′(L1 2)-precipitate morphology that develops as a result of γ-(fcc)matrix phase decomposition is found to evolve from a bimodal distribution of spheroidal precipitates, to {0 0 1}-faceted cuboids and parallelepipeds aligned along the elastically soft 〈0 0 1〉-type directions. The phase compositions and the widths of the γ′-precipitate/γ-matrix heterophase interfaces evolve temporally as the Ni–Al–Cr–Ta alloy undergoes quasi-stationary state coarsening after 1 h of aging. Tantalum is observed to partition preferentially to the γ′-precipitate phase, and suppresses the mobility of Ni in the γ-matrix sufficiently to cause an accumulation of Ni on the γ-matrix side of the γ′/γ interface. Additionally, computational modeling, employing Thermo-Calc, Dictra and PrecipiCalc, is employed to elucidate the kinetic pathways that lead to phase decomposition in this concentrated Ni–Al–Cr–Ta alloy.

Chromium and tantalum site substitution patterns in Ni3Al(L12) γ′-precipitates

The site substitution behavior of Cr and Ta in the Ni3Al(L12)-type γ′-precipitates of a Ni–Al–Cr–Ta alloy is investigated by atom-probe tomography (APT) and first-principles calculations. Measurements of the γ′-phase composition by APT suggest that Al, Cr, and Ta share the Al sublattice sites of the γ′-precipitates. The calculated substitutional energies of the solute atoms at the Ni and Al sublattice sites indicate that Ta has a strong preference for the Al sites, while Cr has a weak Al site preference. Furthermore, Ta is shown to replace Cr at the Al sublattice sites of the γ′-precipitates, altering the elemental phase partitioning behavior of the Ni–Al–Cr–Ta alloy.

The effects of Ta additions on the phase compositions and high temperature properties of Pt base alloys

Pt–Al alloys attain high creep strength by precipitation hardening of the face-centred cubic matrix phase by the L1 2 ordered intermetallic γ′ phase Pt 3Al (analogous to Ni base superalloys). Along with superior environmental resistance these alloys have a potential as high-temperature materials. In this investigation the effect of Ta on strength, phase composition, lattice misfit and γ′ volume fraction are assessed. Starting from the composition with 12 at.% Al, 5 at.% Cr and Pt balance, precipitation hardened Pt–Al–Cr–Ta alloys were analysed. Ta contents higher than 9 at.% lead to a decrease of γ′ volume fraction. Ta increases the lattice parameters of both phases, but decreases the lattice misfit. By substitution of Al by Ta more Al partitions to the γ′ phase, while Ta shows an inverse tendency.

Effects of processing on the microstructure and mechanical behavior of binary Cr–Ta alloys

The microhardness, and tensile and fracture-toughness properties of drop-cast and directionally-solidified Cr-9.25 at.% (atomic percent) Ta alloys have been investigated. Directional solidification was found to soften the alloy, which could be related to the development of equilibrium and aligned microstructures. It was observed that the tensile properties of the Cr–Ta alloys at room and elevated temperatures could be improved by obtaining aligned microstructures. The directionally-solidified alloy also showed increased fracture toughness at room temperature. This trend is mainly associated with crack deflection and the formation of shear ribs in the samples with aligned microstructures. The sample with better-aligned lamellae exhibits greater fracture toughness.

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al 2S 3 or Cr 2S 3 layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al 3+ ions into NbS 2 and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al 2S 3 layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr 2O 3 layer in the oxide scales is formed on Cr-rich Cr–Nb and Cr–Ta alloys due to outward diffusion of Cr 3+ ions. In contrast, continuous alumina layer cannot be formed on the Al–Nb and Al–Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.

Exchange stiffness constants of CoCr-alloy thin films

Exchange stiffness constants ( A) are determined for a CoCr(Ta, Pt)-alloy system by measuring the temperature variations of saturation magnetization of single-crystal specimens. The A value decreases with increasing Cr concentration. Co–Cr–Pt alloy has a higher A value than that of Co–Cr–Ta alloy with similar Cr concentration.

Oxidation resistance and mechanical properties of Laves phase reinforced Cr in-situ composites

Two-phase Cr(X)–Cr2X (X=Nb, Ta) in-situ composites are of interest for high-temperature applications due to their high melting points and potential for high-temperature strength. A six cycle, 120 h, 1100°C cyclic oxidation screening test was used to evaluate potential for high-temperature oxidation resistance of several Cr(X)–Cr2X in-situ composites. Alloys based on the Cr–Ta system near the Cr(Ta)–Cr2Ta eutectic exhibited superior oxidation resistance compared to corresponding alloys based on the Cr–Nb system. The binary Cr–Ta alloys were also found to exhibit a moderate degree of room-temperature fracture toughness, in the range of 9–10 MPa√m. It was concluded that the Cr(Ta)–Cr2Ta alloys are a promising base for future high-temperature intermetallic alloy development efforts.

The corrosion behavior of sputter-deposited amorphous Fe–Cr–Ni–Ta alloys in 12 M HCl

Amorphous Fe–Cr–Ni–Ta alloys containing 7–75 at.% tantalum were successfully prepared by the D.C. magnetron sputtering method by using a target of commercial Type 304 stainless steel and high purity tantalum discs. The corrosion behavior of the Fe–Cr–Ni–Ta alloys in 12 M HCl at 30°C was investigated by immersion tests, electrochemical measurements and XPS analysis, and compared with that of Cr–Ta binary alloys. The corrosion rate of the sputter-deposited Fe–Cr–Ni–Ta alloys containing more than 19 at.% tantalum was more than six orders of magnitude lower than that of bulk Type 304 stainless steel and even lower than that of sputter-deposited tantalum metal, although the corrosion rate of the alloys was higher than that of the Cr–Ta alloys. The open circuit potentials of the Fe–Cr–Ni–Ta alloys are located in the passive regions of both chromium and tantalum, and all the alloys are spontaneously passivated. XPS analysis revealed that the passive films formed on the alloys by immersion in 12 M HCl were significantly rich in tantalum and chromium cations. The films were deficient in iron cation and the formation of homogeneous double oxyhydroxides containing mainly chromium and tantalum should be responsible for the high corrosion resistance of the alloys.

Sulfidation- and oxidation-resistant alloys prepared by sputter deposition

In order to tailor novel alloys resistant to high temperature corrosion in multicomponent sulfidizing–oxidizing environments, amorphous or nanocrystalline Al-refractory metal alloys with and without silicon and Cr-refractory metal alloys have been prepared by sputter deposition. The sulfidation and oxidation behavior of the alloys has been studied as a function of temperature in sulfur vapor pressure of 10 3 Pa and in oxygen of 2×10 4 Pa, respectively. The sulfidation of these alloys generally follows a parabolic rate law, being thus diffusion controlled. The sulfidation rates of Al-refractory metal and Cr-refractory metal alloys are several orders of magnitude lower than those of conventional high temperature alloys and comparable to or even lower than those of the corresponding refractory metals. The sulfide scales formed on these alloys consist of two layers, comprising an outer Al 2 S 3 or Cr 2 S 3 layer and an inner refractory metal disulfide layer. The formation of the inner layer is attributed to the excellent sulfidation resistance of these alloys. The oxidation resistance of Al-refractory metal alloys is not sufficiently high, but the addition of silicon improves remarkably their oxidation resistance by synergistic effect of aluminum and silicon. Although the CrMo alloys possess poor oxidation resistance, due to the formation of volatile molybdenum oxide, the oxidation resistance of the CrNb and CrTa alloys is as high as that of typical chromia-forming alloys.

Electrochemical and XPS studies of the passivation behavior of sputter-deposited Cr–Ta alloys in 12 M HCl

The sputter-deposited Cr–Ta alloys show extremely high corrosion resistance in 12 M HCl. The open circuit potentials of the Cr–Ta alloys are located in the passive regions of both chromium and tantalum, and all Cr–Ta alloys are spontaneously passivated. XPS analysis indicates that the composition and thickness of the air-formed film are the same as those of spontaneously passivated film, and the composition of the passive films becomes constant after prolonged immersion. The film consists of a double oxyhydroxide of chromium and tantalum cations. The formation of the homogeneous double oxyhydroxide film consisting of both chromium and tantalum cations by air oxidation is responsible for the extremely high corrosion resistance of the Cr–Ta alloys in comparison with the corrosion resistance of chromium and tantalum metals.