Showing papers on "Laves phase published in 2013"

Mechanical properties of low-density, refractory multi-principal element alloys of the Cr–Nb–Ti–V–Zr system

Abstract: Room temperature and elevated temperature mechanical properties of four multi-principal element alloys, NbTiVZr, NbTiV 2 Zr, CrNbTiZr and CrNbTiVZr, are reported. The alloys were prepared by vacuum arc melting followed by hot isostatic pressing and homogenization. Disordered BCC solid solution phases are the major phases in these alloys. The Cr-containing alloys additionally contain an ordered FCC Laves phase. The NbTiVZr and NbTiV 2 Zr alloys showed good compressive ductility at all studied temperatures while the Cr-containing alloys showed brittle-to-ductile transition occurring somewhere between 298 and 873 K. Strong work hardening was observed in the NbTiVZr and NbTiV 2 Zr alloys during deformation at room temperature. The alloys had yield strengths of 1105 MPa and 918 MPa, respectively, and their strength continuously increased, exceeding 2000 MPa after ∼40% compression strain. The CrNbTiZr and CrNbTiVZr alloys showed high yield strength (1260 MPa and 1298 MPa, respectively) but low ductility (6% and 3% compression strain) at room temperature. Strain softening and steady state flow were typical during compression deformation of these alloys at temperatures above 873 K. In these conditions, the alloys survived 50% compression strain without fracture and their yield strength continuously decreased with an increase in temperature. During deformation at 1273 K, the NbTiVZr, NbTiV 2 Zr, CrNbTIZr, and CrNbTiVZr alloys showed yield strengths of 58 MPa, 72 MPa, 115 MPa and 259 MPa, respectively.

New insight on the solidification path of an alloy 625 weld overlay

Abstract: Ni-based alloys are a special class of engineering material with excellent corrosion resistance in harsh environments. However, microstructural changes due to the solidification, may result in solidification cracks and reduction in the corrosion resistance. Knowing the microchemical and microstructural evolutions during the solidification of these alloys is essential for the understanding of the relationship between the metallurgical aspects and the properties. This research presents a new insight on the solidification path of alloy 625 weld metals deposited by the TIG cold wire process on the C–Mn steel plates. After the welding, samples having been extracted and evaluated by the scanning electron microscope, transmission electron microscopy and energy dispersive X-ray spectroscopy techniques. The results showed the presence of two types of secondary phases, identified as Laves phase and complex nitride/carbide particles. Due to the presence of nitrides particles, stable in the solid state during the melting of the alloy, during the solidification it is noted the occurrence of a complex nitride/carbide precipitation. This implies in a new route to explain the solidification of the aforementioned alloy.

Progress in bulk MgCu2-type rare-earth iron magnetostrictive compounds

Abstract: Studies of bulk MgCu2-type rare-earth iron compounds with Laves phase are reviewed. The relationship between magnetostriction and structural distortion and the consequent crystallographic method for measuring magnetostriction are introduced at first. Then we review recent progress in understanding bulk magnetostrictive Laves phase materials, especially the magnetostriction and the minimization of the anisotropy of the light rare-earth Pr- and Sm-based compounds. Finally, a summary and outlook for this kind of compounds are presented.

Phase transformations and mechanical properties in heat treated superaustenitic stainless steels

Abstract: A microstructure–properties relationship study in two superaustenitic stainless steels (S31254 and S32654) was carried out, following exposure at elevated temperatures for various ageing times. Due to high temperature ageing, most stainless steel grades suffer the formation of various precipitates, directly affecting their properties. The full characterization of those precipitates and the correlation with the mechanical behavior of the steels is the primary aim of this study. Samples of the steel grades studied, were exposed to isothermal heat treatments within the temperature range of 650–950 °C, for ageing times varying between 0.5 h and 3000 h, followed by water quenching at room temperature. Microstructural examination indicated the formation of four different secondary phases, sigma phase (σ), chi phase (χ), Laves phase and β-Cr2N nitride, which were characterized by transmission electron microscopy (TEM) and electron diffraction. The results obtained permitted the construction of the time–temperature–precipitation (TTP) plots. In addition, tensile and Vickers hardness testing were utilized and the modulus of toughness was calculated. The kinetics of the formation of various precipitates with increasing temperature and aging duration was also observed. It was found that various precipitates had a significant effect on all mechanical properties studied.

Atomic structure of hardening precipitates in an Al–Mg–Zn–Cu alloy determined by HAADF-STEM and first-principles calculations: relation to η-MgZn2

Abstract: The structures of two nanoscale plate precipitates prevalent at maximum strength and over-aged conditions in a 7449 Al–Mg–Zn–Cu alloy were investigated. Models derived from images of high angle annular dark field scanning transmission electron microscopy were supported by first-principles calculations. Both structures are closely linked to the η-MgZn2 Laves phase through similar layers of a rhombohedral atomic subunit. The finest plate contains one such layer together with a layer of an orthorhombic unit. The second plate contains rhombohedral layers only, normally four, but rotated relatively to form different stacking variants, one of which may be likened to η. For both structures, the same atomic planes describe the main interface with Al. Both plates could be described in space group P3. The unit cells comprise interface and arbitrary numbers of {111}Al (habit) planes. Eight Al-planes were included in the first-principles calculations. The enthalpy indicates high layer/unit stability. The plate thickness can be understood by a simple mismatch formulation.

Phase Composition of a CrMo0.5NbTa0.5TiZr High Entropy Alloy: Comparison of Experimental and Simulated Data (Postprint)

Abstract: Microstructure and phase composition of a CrMo0.5NbTa0.5TiZr high entropy alloy were studied in the as-solidified and heat treated conditions. In the as-solidified condition, the alloy consisted of two disordered BCC phases and an ordered cubic Laves phase. The BCC1 phase solidified in the form of dendrites enriched with Mo, Ta and Nb, and its volume fraction was 42%. The BCC2 and Laves phases solidified by the eutectic-type reaction, and their volume fractions were 27% and 31%, respectively. The BCC2 phase was enriched with Ti and Zr and the Laves phase was heavily enriched with Cr. After hot isostatic pressing at 1450 °C for 3 h, the BCC1 dendrites coagulated into round-shaped particles and their volume fraction increased to 67%. The volume fractions of the BCC2 and Laves phases decreased to 16% and 17%, respectively. After subsequent annealing at 1000 °C for 100 h, submicron-sized Laves particles precipitated inside the BCC1 phase, and the alloy consisted of 52% BCC1, 16% BCC2 and 32% Laves phases. Solidification and phase equilibrium simulations were conducted for the CrMo0.5NbTa0.5TiZr alloy using a thermodynamic database developed by CompuTherm LLC. Some discrepancies were found between the calculated and experimental results and the reasons for these discrepancies were discussed.

Cyclic oxidation behaviour of the titanium alloys Ti-6242 and Ti-17 with Ti–Al–Cr–Y coatings at 600 and 700 °C in air

Abstract: Oxidation protective intermetallic Ti–Al–Cr–Y coatings of 10 μm thickness were deposited on specimens of the near-α alloy Ti-6242 and the (α + β) alloy Ti-17 using magnetron sputtering. The oxidation behaviour of the coated samples was studied at 600 °C and 700 °C under cyclic oxidation conditions in air. The coatings exhibited very good adhesion to the substrate materials and provided effective oxidation protection to the titanium alloys at both temperatures. A thin alumina scale formed on top of the coatings which showed a multi-phase microstructure consisting of the Ti(Cr,Al)2 Laves phase and, dependent on exposure temperature and time, the γ-TiAl, r-TiAl2 and/or AlCr2 phases.

Effect of retained austenite and high temperature Laves phase on the work hardening of an experimental maraging steel

Abstract: Maraging steels including the experimental alloy studied here show atypical stress–strain behaviour during tensile testing. In particular, there is a gradual decrease in the ability of the sample to support a stress following a small fraction of the total plastic strain to failure. It is demonstrated here that this phenomenon is not associated with the early onset of a necking instability, and that a large amount of the plastic strain beyond the peak stress is uniform. Investigations of microstructure and retained austenite content reveal that the intrinsic microstructure of maraging steels has a poor ability to work harden. The work hardening capacity can, as expected, be improved by introducing retained austenite, but there is an associated reduction in strength. Experiments have been designed to control the retained austenite content in such a way that clear comparisons can be made and conclusions reached on both the role of the austenite and of Laves phase generated at different temperatures.

Texture-induced enhancement of the magnetocaloric response in melt-spun DyNi2 ribbons

Abstract: The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group Fd3¯m) exhibiting a saturation magnetization and Curie temperature of MS = 157 ± 2 A m2 kg−1 and TC = 21.5 ± 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change |ΔSMpeak| = 13.5 J kg−1 K−1, and a refrigerant capacity RC = 209 J kg−1. Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications.

β-Mn-Type Co8+xZn12–x as a Defect Cubic Laves Phase: Site Preferences, Magnetism, and Electronic Structure

Abstract: The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co–Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co–Zn phases crystallize in the space group P4132 [Pearson symbol cP20; a = 6.3555(7)–6.3220(7)], and their stoichiometry may be expressed as Co8+xZn12–x [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the sp...

Investigation on long-term creep rupture properties and microstructure stability of Fe–Ni based alloy Ni–23Cr–7W at 700 °C

Abstract: Long-term creep rupture properties and microstructural stability of Fe–Ni based alloy Ni–23Cr–7W (HR6W, ASME Code Case 2684) were experimentally investigated. Crept specimens at 700 °C for durations up to 37,667 h were chosen, the microstructure evolution during creep was characterized. Besides the MC and M 23 C 6 carbides found in the as-received sample, the formation of α -W phase, α -Cr phase and Laves phase in crept samples were confirmed with scanning/transmission electron microscopes. Statistical quantitative image analysis was used to evaluate the precipitation behavior and growth kinetics for the main strengthening precipitate, Laves phase. The alloy was proven to have good microstructural stability without observable coarsening of strengthening precipitates during long-term creep up to around 37,667 h. It was also verified that the growth kinetics of Laves phase can be well described by the Johnson–Mehl–Avrami–Kolmogorov equation.

Microstructure characterization and mechanical properties of a Laves-phase alloy based on Cr2Nb

Abstract: Microstructure and mechanical properties of a binary Cr 2 Nb-based alloy containing the Laves phase Cr 2 Nb and Cr phase have been studied in the arc-melted condition. Microstructure consisting of fully coupled eutectic was firstly observed in this hypereutectic alloy. The Cr 2 Nb primary phase experienced a faceted to nonfaceted transition as the solidification distance increased. Based on the maximum interface growth temperature criteria, the microstructure evolutions in this alloy were explained successfully by means of the competitive growth between the β-Cr 2 Nb phase and eutectic. Meanwhile, the yield strength and fracture toughness of the samples taken from the bottom of the ingot attained 1986 MPa and 4.3MPa⋅m 1/2 respectively, which were considerably enhanced over the monolithic Cr 2 Nb. The enhancements were explained in terms of the second Cr phase toughening and grain boundary toughening mechanisms. Furthermore, the variation of mechanical properties in different position of arc-melted ingot was proven to result in different eutectic fracture mechanisms.

Effect of Thermal Aging on Ductile-Brittle Transition Temperature of Modified 9Cr-1Mo Steel Evaluated with Reference Temperature Approach Under Dynamic Loading Condition

Abstract: The effect of thermal aging on the ductile-brittle transition behavior has been assessed for a modified 9Cr-1Mo steel (P91) using the reference temperature approach under dynamic loading condition (T 0 dy ). The steel in normalized and tempered (NT) condition and in different levels of subsequent cold work (CW) was subjected to thermal aging at temperatures of 873 K and 923 K (600 °C and 650 °C) for 5000 and 10,000 hours. For the NT and all the cold work conditions of the starting material, a drastic increase in T 0 dy has been noticed after aging at 923 K (650 °C) for 10,000 h. A moderate increase was observed for the NT steel aged at 873 K (600 °C) for 5000 hours and for the 10 pct CW steel aged at 873 K (600 °C) for 10,000 h. A detailed transmission electron microscope (TEM) study of the embrittled materials aged at 923 K (650 °C)/10,000 hours and 873 K (600 °C)/10,000 hours has indicated presence of hexagonal Laves phase of Fe2(Mo,Nb) type with different size and spatial distributions. The increase in the T 0 dy is attributed to the embrittling effect of a network of Laves phase precipitates along the grain boundaries.