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[en] The effect of the ternary element chromium in B2 ordered NiAl based alloys was studied by atom probe field ion microscopy. Samples of the compositions Ni50Al48Cr2, Ni49Al49Cr2, and Ni48Al50Cr2 were investigated after different heat treatments. The chromium concentrations of the NiAl matrix, measured by atom probe, increased with increasing Ni/Al ratio and heat treatment temperature. The site preference of solute Cr atoms for the Al sublattice was determined by layer resolved measurements. An antiphase boundary observed in Ni50Al48Cr2 showed a high local Cr concentration with Cr atoms substituting for Al atoms. Superdislocations in the quasi-stoichiometric Ni49Al49Cr2 were not dissociated into partial dislocations and showed no Cr segregation. These results are discussed in view of activation of a<1 1 1> slip systems
Journal
Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing; ISSN 0921-5093; 
; CODEN MSAPE3; v. 353(1-2); p. 87-91
; CODEN MSAPE3; v. 353(1-2); p. 87-91
[en] The micro- and nano-structure of a NiAl-Re alloy with 1 at.% Re has been investigated by means of atom probe field ion microscopy (APFIM) and by scanning electron microscopy. The effect of heat treatments on this alloy were studied for annealing temperatures at 1000 deg. C in a time interval of 10-1000 h, and at 1300 deg. C for 1000 h. The results were compared with those from the as-cast samples. Rhenium contents of the NiAl matrix were determined by APFIM. It was found that the Re concentration in the primary NiAl crystals decreases with increasing annealing time. Layer-resolved atom probe measurements were performed in order to determine the site occupancy of Re atoms in the NiAl superlattice. The results clearly showed a site preference for the Ni sublattice
Journal
Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing; ISSN 0921-5093; ; CODEN MSAPE3; v. 353(1-2); p. 80-86
; CODEN MSAPE3; v. 353(1-2); p. 80-86
[en] The constitution of the quasi-binary eutectic NiAl-Re section of the ternary Al-Ni-Re phase diagram was reinvestigated by using differential thermal analysis, metallography and scanning electron microscopy. The alloys studied were stoichiometric NiAl and various NiAl-Rex (0.75< x<2.5 at.%) alloys. The microstructure of the as-cast NiAl-Re eutectic is of irregular fibrous morphology. Ingots with Re concentrations ≥1.25 at.% reveal non-equilibrium microstructures which consist of primary Re crystals, NiAl crystals, and eutectic. An ideal eutectic microstructure was achieved by unidirectional solidification of a NiAl-Re1.25 alloy using Bridgman technique at a low growth velocity of 30 mm h-1. The melting temperature of the stoichiometric NiAl compound is 1674 deg. C. The eutectic point of the quasi-binary section of the NiAl-Re system is determined to be at 1.25 at.% Re and a temperature of 1668 deg. C. The solidus temperatures of the NiAl-Re ingots exhibit a slight dependence on the Re concentration. The solid solubility of Re in NiAl is 0.6 at.% at the eutectic temperature and 0.2 at.% at 1300 deg. C
Journal
Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing; ISSN 0921-5093; ; CODEN MSAPE3; v. 343(1-2); p. 301-307
; CODEN MSAPE3; v. 343(1-2); p. 301-307
[en] The site occupancies of the transition metals Cr, Fe, and Re dissolved in NiAl of stoichiometric composition have been determined by atomic layer resolved atom probe field-ion microscopy (APFIM). The investigations were supported by X-ray diffraction studies to evaluate the lattice parameters. These are influenced by atomic size effects and constitutional lattice defects like Ni antistructure atoms in the Al sublattice and vacancies in the Ni sublattice. The APFIM results were compared with ALCHEMI data and calculated site preference energies published in the literature. Chromium additions to stoichiometric NiAl with 0.8 at% in solid solution exhibit a strong preference for Al sites. The lattice parameter of NiAl(Cr) solid solution is decreased. Iron atoms dissolved in higher concentrations of 5 at % in NiAl are almost equally distributed within both sublattices. They are possessing a weak preference for Al sites, which causes a lattice expansion of NiAl(Fe) solid solution. ALCHEMI results and site preference energy data show a strong site preference of Cr atoms for the Al sublattice. In contrast, iron atoms exhibit a weak site preference for Ni sites depending upon the stoichiometry of the NiAl host lattice. Re solutes in low concentrations of about 0.2 at % in NiAl possess a strong site preference for the Ni sublattice. The increase of the lattice parameter of NiAl(Re) is due to the pronounced size effect of Re atoms. For these species no ALCHEMI and site preference energy data are available in the literature
Journal
[en] Texture measurements were carried out on a Fe-26Al-5Cr alloy that was deformed by warm rolling at temperatures above and below the stress anomaly temperature in order to obtain information about a possible change of slip systems around that temperature. Special attention was paid to the initial microstructure, temperature during rolling and recrystallisation problems. Textures were determined by automatic crystal orientation mapping (ACOM) using electron backscatter diffraction (EBSD), which allows separation of measurement data from recrystallised and unrecrystallised fractions in the microstructure. A clear difference in texture evolution appears above and below the stress anomaly temperature; the most preferred orientations after rolling at 600 and 400 deg. C are ∼{5 5 7}<1 1 0> and ∼{1 1 1}<2 1 1>, respectively. This result suggests that the shear fraction of the active {1 1 2}1 1 1> slip systems increases with respect to that of the {1 1 0}<1 1 1> slip systems with increasing temperature around the stress anomaly temperature
Journal
Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing; ISSN 0921-5093; ; CODEN MSAPE3; v. 387-389(2-3); p. 950-954
; CODEN MSAPE3; v. 387-389(2-3); p. 950-954
[en] Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15-30)wt%Mn alloys with additions of aluminium and silicon have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α''- and ε-martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy γfcc and the free energy ΔGγ→ε for the γ→ε phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase γfcc and therefore strongly suppress the γ→ε transformation while silicon decrease γfcc and sustains the γ-ε transformation. The γ→ε phase transformation takes place in alloys with γfcc ≤ 20 mJ/m2. The stacking fault energy of the Fe-25Mn-3Si-3Al alloy was calculated as a function of temperature and related with microstructural changes of the strained sample at different temperatures. These steels with reduced density of about 7,3 g/cm-3 combine high tensile ductility up to 80% at high strain rates with true tensile strength of about 1000 MPa. The excellent plasticity induced by twinning and additional phase transformation up to extremely high strain rates of about ε = 103 s-1 results in an extraordinary shock resistance and enables deep drawing and backward extrusion operations of parts with complex shapes and high production rates. (orig.)
Journal
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