E.P. George

Room-temperature mechanical behavior of FeAl: Effects of stoichiometry, environment, and boron addition

Abstract The intrinsic ductility of FeAl (in ultrahigh vacuum) decreases with increasing Al content, from around 16% in Fe–37Al to zero in Fe–48Al. The sharpest decline occurs around the composition where the fracture mode changes from transgranular to intergranular. Boron shifts this ductile–brittle transition to higher Al levels by segregating to the grain boundaries and suppressing grain-boundary fracture. However, its effectiveness decreases with increasing Al concentration, even though the amount of B segregating to the grain boundaries remains the same, independent of alloy stoichiometry. Consequently, even the B-doped alloys become brittle and fracture intergranularly as the stoichiometric composition is approached. Low-pressure hydrogen embrittles FeAl, although not as severely as atmospheric moisture. Environmental embrittlement is most noticeable in Fe-rich FeAl; with increasing Al concentration, the grain boundaries become intrinsically weak, and brittle fracture persists even after environmental effects are eliminated.

Size effect, critical resolved shear stress, stacking fault energy, and solid solution strengthening in the CrMnFeCoNi high-entropy alloy

High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, equatomic CrMnFeCoNi with the face-centered cubic (FCC) structure is noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. Here we report for the first time by single-crystal micropillar compression that its bulk room temperature critical resolved shear stress (CRSS) is ~33–43u2009MPa, ~10 times higher than that of pure nickel. CRSS depends on pillar size with an inverse power-law scaling exponent of –0.63 independent of orientation. Planar ½u2009<u2009110u2009>u2009{111} dislocations dissociate into Shockley partials whose separations range from ~3.5–4.5u2009nm near the screw orientation to ~5–8u2009nm near the edge, yielding a stacking fault energy of 30u2009±u20095u2009mJ/m2. Dislocations are smoothly curved without any preferred line orientation indicating no significant anisotropy in mobilities of edge and screw segments. The shear-modulus-normalized CRSS of the HEA is not exceptionally high compared to those of certain concentrated binary FCC solid solutions. Its rough magnitude calculated using the Fleischer/Labusch models corresponds to that of a hypothetical binary with the elastic constants of our HEA, solute concentrations of 20–50 at.%, and atomic size misfit of ~4%.

Tensile ductility, slow crack growth, and fracture mode of ternary B2 iron aluminides at room temperature

Abstract B2 iron aluminides containing 45 at% Al and microalloying additions of 0.2 at% B and 0.1 at% Zr were alloyed with 5 at% of the 3 d transition elements Ti, V, Cr, Mn, Co, Ni, and Cu. The extruded and annealed alloys were essentially single phase except for isolated stringers which appeared to be borides. Their strength, tensile ductility in air and dry oxygen, and slow crack growth behavior in air were determined at room temperature. Consistent with water vapor-induced embrittlement, the tensile ductility depended on the environment, and the crack growth resistance on the crack velocity. The binary iron aluminide Fe-45 at% Al exhibited ductilities of 8 and 19% in air and dry oxygen, respectively. Ternary alloying increased in most cases the yield strength, and always reduced the ductility. Different alloying additions influenced the slow crack growth behavior in different ways. For example, the crack growth resistance of Fe-45Al-5Cr (at%) did not depend significantly on the crack velocity, whereas that of Fe-45Al-5Mn (at%) decreased strongly as the crack velocity decreased. Alloying additions which are to the left of iron in the periodic system, such as Ti, Cr, and Mn, were associated with intergranular fracture, whereas Co, Ni, and Cu additions, to the right of iron, were associated with substantial amounts of transgranular fracture. For a given alloy, the fraction of intergranular fracture increased with decreasing crack velocity. The experimental results are rationalized in terms of the environmental sensitivity of the grain boundaries, the increase in yield strength upon alloying, and the site occupation of the ternary alloying additions. This does, however, not preclude the influence of additional factors such as differences in the chemical composition and strength of the grain boundaries in the different alloys.

Atomic displacement in the CrMnFeCoNi high-entropy alloy – A scaling factor to predict solid solution strengthening

Although metals strengthened by alloying have been used for millennia, models to quantify solid solution strengthening (SSS) were first proposed scarcely seventy years ago. Early models could predict the strengths of only simple alloys such as dilute binaries and not those of compositionally complex alloys because of the difficulty of calculating dislocation-solute interaction energies. Recently, models and theories of SSS have been proposed to tackle complex high-entropy alloys (HEAs). Here we show that the strength at 0 K of a prototypical HEA, CrMnFeCoNi, can be scaled and predicted using the root-mean-square atomic displacement, which can be deduced from X-ray diffraction and first-principles calculations as the isotropic atomic displacement parameter, that is, the average displacements of the constituent atoms from regular lattice positions. We show that our approach can be applied successfully to rationalize SSS in FeCoNi, MnFeCoNi, MnCoNi, MnFeNi, CrCoNi, CrFeCoNi, and CrMnCoNi, which are all medium-entropy subsets of the CrMnFeCoNi HEA.

An Auger investigation of the fracture behavior of PST TiAl alloys

Abstract Samples of two polysynthetically twinned (PST) TiAl alloys (Ti-49 at% Al and Ti-49 at%Al-0.6 at%Cr) were fractured under different environmental conditions at room temperature, and the fracture surfaces were analyzed using scanning Auger and scanning electron microscopy. These techniques were employed to determine the fracture mode and the chemical composition of the fracture surfaces of the TiAl alloys. Our study clearly indicates that all of the fracture surfaces exposed by bend testing were α 2 , except when fractures occurred across the lamellae. No chromium enrichment was observed on any of the exposed surfaces. From these observations, it was concluded that the fracture mode for these materials is cleavage through the α 2 -phase.

Grain-boundary segregation of impurities in iridium and effects on mechanical properties

Impurity effects were investigated in an Ir alloy of nominal composition Ir–0.3 W–0.006 Th–0.005 Al1. The impurities that were added included Fe, Ni, Cr, Al, and Si, at levels ranging from 50 to 5000 ppm. Of the elements investigated (impurities as well as alloying additions), only Si and Th were found to segregate to the grain boundaries. In alloys doped with low levels of Si (<200 ppm), both Si and Th segregated to the grant boundaries. In addition, Th-containing precipitates were identified in these alloys, but not Si-containing precipitates. When added at higher bulk levels, Si displaced (beneficial) Th from the boundaries. This caused the segregated Th at the boundaries to decrease (to practically zero) when the amount of grain-boundary Si increased (to its saturation level). Silicon also reacted with Th to form Ir–Th–Si precipitates and promoted the formation of a low-melting eutectic on the grain boundaries of the highest Si alloys. As for the other elements, not only did they not segregate to the grain boundaries, but they also did not affect Th segregation. Together, these results explain why Fe, Ni, Cr, Al (and Si at low levels) do not embrittle iridium, whereas Si at high levels causes severe embrittlement.

Deformation and fracture of iridium: microalloying effects

Abstract The effects of Ce and Th doping (20–50 wppm) on the mechanical properties of Ir alloys were investigated. At both low (∼10 −3 s −1 ) and high (∼10 3 s −1 ) strain rates, the Ce+Th doped alloys undergo a transition from brittle intergranular (plus some transgranular) fracture at low temperature to ductile transgranular fracture at elevated temperature. The ductile–brittle transition temperature (DBTT) is ∼400 K higher at the higher strain rate. Grain size and grain-boundary cohesion both affect the ductility of Ir alloys. Cerium and thorium, when added together, refine grain size more effectively than when Th is added by itself (especially at high temperatures). Their effect on grain-boundary cohesion is similar to that of Th by itself. At low strain rates (∼10 −3 s −1 ), the strength and ductility of alloys doped with Th were similar to those of alloys doped with both Th and Ce. The brittle–ductile transition appears to be related to a change from high work hardening rate at low temperature to low work hardening rate at elevated temperature.

Microstructures and mechanical properties of V-V3Si eutectic composites

Abstract V–V3Si eutectic alloys were directionally solidified in a high temperature optical floating zone furnace. Depending on the solidification conditions, several microstructures were observed, such as well-aligned broken lamellar, fibrous, or cellular microstructures. The interphase spacings increased with decreasing solidification rates in agreement with the Jackson-Hunt theory. The mechanical properties of the individual phases were investigated by nanoindentation. It was found that the modulus and nanoindentation hardness of V3Si are 214 and 13.8 GPa, respectively, and those of the V solid-solution are 165 and 3.4 GPa, respectively. The high-temperature strength was examined by tensile testing at elevated temperatures. Preliminary results show that the ductile-to-brittle transition temperature is about 800 °C for this composite, and its strength is significantly higher than conventional V solid-solution alloys.

An Auger investigation of the grain-boundary chemistry in Ni3(Si, Ti) alloys

Abstract Three different Ni3(Si,Ti) alloys were cathodically charged with hydrogen and fractured under high vacuum conditions. Scanning Auger microscopy (SAM) and scanning electron microscopy (SEM) were used to determine the chemical compositions of the fracture surfaces and the mode of fracture. The compositions of the transgranular regions were compared with the intergranular regions in order to determine the amount of grain-boundary segregation or depletion of the various constituents. In all the alloys studied, Si and Ti were depleted on the grain boundaries, while Ni was enriched relative to the bulk. Correlations were made between the grain-boundary compositions and the previously reported environmental embrittlement of these alloys. An embrittlement factor defined as [(Si+Ti)/Ti] on the grain boundaries is suggested to be related to the degree of environmental embrittlement at ambient temperatures. Some possible reasons for this correlation are discussed.

Strength, ductility, and fracture mode of ternary FeAl alloys

The iron aluminides Fe-45Al-5X-0.2B-0.1Zr (at. %), where X stands for the first row transition metals Ti, V, Cr, Mn, Fe, Co, Ni, Cu, were examined at room temperature for strength, ductility, environmental sensitivity, and fracture mode. The extruded materials were annealed at 1273 K to produce similar grain sizes and then at 673 K to reduce the quenched-in vacancies. All alloys were essentially single phase. Their solid-solution strengthening was found to correlate with the atomic size misfit from the lattice parameters. The ``binary`` alloy Fe-45Al-O.2B-0.1Zr exhibited predominantly transgranular fracture. Ternary alloying additions with atomic numbers less than that of Fe tended to enhance intergranular fracture, whereas those with atomic numbers higher than that of Fe favored substantial amounts of transgranular fracture. Tensile testing in a partial pressure of dry oxygen increased the ductilities of the ternary alloys only slightly, whereas the ductility of the binary alloy increased from about 8 to about 19%. The ductilities in air correlated inversely with the yield strength. However, those alloys exhibiting substantial amounts of transgranular fracture always showed higher ductilities than those fracturing intergranularly. The fracture results are interpreted in terms of yield strengths and ternary element site occupations.