Structure imaging and vanadium substitution in cubic TiCr₂Laves phase

Abstract

Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr 2 Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces. IntroductionLaves phases, a group of topologically close-pack hard intermetallic compounds with AB 2 stoichiometry, are known to form at a particular radius ratio of A and B (r A /r B = 1.22) atoms [1]. However, a considerable deviation from ideal radius ratio (r A /r B = 1.05-1.67) [2] is observed in practice and the phase can often accommodate non-stoichiometry [3]. There are three allotropic polymorphs of Laves phase i.e. cubic (cF24), hexagonal (hP12) and double-hexagonal (hP24) [2]. Since the discovery of this phase, more than a thousand binary and ternary Laves phase forming compounds with a range of interesting properties and varying crystal chemistry have been reported in the literature [4]. These attributes could be exploited to design single-phase or multiphase

Phase Formation and Microstructural Evaluation in V-Ti-Cr System Using Advanced Microscopy Analysis

“…Comparisons of the experimental and simulated electron intensity profiles confirm that V preferentially substitutes Ti rather than Cr from the regular lattice sites. This further confirms, V-doped TiCr2 exists as a pseudobinary Laves phase with a modified stoichiometry of (Ti,V)Cr2 [6]. The calculated dopant formation energies using first principla calculations for both the Ti (0.29 eV) and the Cr (0.40 eV) site substitutions also suggest that V prefer to replace Ti rather than Cr from its regular lattice sites which is in confirmation with the microscopy observations.…”

Characterizing Li in partially lithiated layer materials using atomic‐resolution imaging, modeling, and simulation

“…These changes can then be compared with the theoretically calculated structures to identify the inclusion of even a few atoms of the foreign element within a single atomic column parallel to the electron beam. This approach may be used as a method for determining the nanoscale chemistry in the localized domains 35,52,53,54 …”

“…This approach may be used as a method for determining the nanoscale chemistry in the localized domains. 35,52,53,54 In the present study, the image simulations have been performed for a C S -corrected condition with 0 nm defocus in order to reduce the TEM-induced artifacts to its maximum possible extent, which is otherwise inevitable for the phase-contrast imaging. Under negative sphericalaberration (C S ) imaging (NCSI) conditions, the contrast increases due to the constructive rather the destructive superposition of the phase and the amplitude part of the exit wave function.…”

Understanding Laves phase precipitation induced embrittlement of modified 9Cr–1Mo steel

“…This finding now opens doors for a more directional and constructive research towards combating the premature failure of P91 steel based components; a strategy targeting ductilization of the Laves phase. One most promising way of doing this is by bringing the synchro-shear mechanism in effect [9][10][11], i.e. tailoring the defect structure via materials chemistry.…”

“…Similarly, their complex crystal structure can offer absorption of massive quantities of deleterious elements such as phosphorous and hydrogen [7], which often acts to weaken the grain boundaries leading to premature failure of structures. Additionally, ductilization of the otherwise hard and brittle phase has been recorded possible by suitably controlling the chemistry and the defect structure [9][10][11]. Nevertheless, little practical knowledge has been elucidated from Laves phase study so far, particularly from a material of this kind.…”