Strengthening mechanisms in a high-strength bulk nanostructured Cu–Zn–Al alloy processed via cryomilling and spark plasma sintering

A bulk nanostructured alloy with the nominal composition Cu–30Zn–0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (～200–400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)–O–N, Al–O–N and Al–N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.     

In situ TEM study of deformation twinning in Ni–Mn–Ga non-modulated martensite

The straining of non-modulated (NM) Ni–Mn–Ga martensite was studied by in situ transmission electron microscopy (TEM). Initially, the self-accommodated NM martensitic structure consists of internally twinned domains. During straining, the detwinning process starts within these domains. The internal twin variant more favorably oriented to the stress grows at the expense of the other one. In the detwinned, single-variant domain, a new twin variant can form, gradually replacing the existing variant via the twinning process. Both processes—detwinning and new twinning—proceed by the same mechanism, namely by the movement of twinning dislocations along the twin boundary. Lattice dislocations are also created in the detwinning process. While the boundaries between the internal twins are coherent and mobile, the boundaries between the internally twinned domains are incoherent, strained and not mobile. The planes of the coherent twin boundary are {2 0 2) planes and the Burgers vectors of the twinning dislocations are parallel to the 〈1 0 1] direction. The magnitude of the Burgers vector determined from the TEM observations disagrees with the calculation from the lattice constant measurement by X-ray diffraction. Possible reasons for this discrepancy are discussed.     

Quasicontinuum study of incipient plasticity under nanoscale contact in nanocrystalline aluminum

Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with 〈1 1 0〉-tilt GBs. The contact pressure–displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.     

Nanoquasicrystalline Al–Fe–Cr-based alloys. Part II. Mechanical properties

Nanoquasicrystalline Al-based alloys show considerable promise for elevated temperature applications compared with commercial Al-based alloys. In particular, a group of Al–Fe–Cr-based alloys-containing Ti, V, Nb or Ta have outstanding thermal stability. In the present work, the elevated temperature mechanical properties of these nanoquasicrystalline alloys were studied by tensile tests at a constant strain rate. Tests were designed in order to compare the mechanical behaviour at different test temperatures. Fractographic analysis was also carried out. The apparent activation energy for plastic deformation was found to be close to that for lattice self-diffusion for pure Al in the Al–Fe–Cr ternary alloy and in the Ti-containing alloy, and for grain boundaries diffusion for pure Al in the V-containing alloy, whereas the activation energy of the alloy with Ta additions was three times higher. All of the alloys showed similar sensitivity of plastic deformation to the strain rate in the range of 10^?3–5 × 10^?6 s^?1 at 350 °C. The apparent true stress exponent was n_app ～ 7, which can be associated with a deformation process controlled by dislocation mechanisms.     

Microscopic description of plasticity in computer generated metallic nanophase samples: a comparison between Cu and Ni

Simulations are reported on the plastic behavior of two model f.c.c. metals, Ni and Cu, with different stacking fault energies, and average grain sizes in the range of 3–12 nm. A change in deformation mechanism is observed: at the smallest grain sizes all deformation is accommodated in the grain boundaries. At higher grain sizes intragrain deformation is observed. Analysis of the atomic configurations shows that intrinsic stacking faults are produced by motion of Shockley partial dislocations generated and absorbed in opposite grain boundaries. In Cu the stacking faults are observed at smaller grain sizes than in Ni (8 nm in Cu, 12 nm in Ni) which is attributed to the lower stacking fault energy. Shockley partial dislocations appear on slip systems that are not necessarily those favored by the Schmid factor. Atomic displacement analysis shows that deformation starts at triple points, with grain boundary sliding followed by the creation of intragrain partial dislocations.     

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.     

Void initiation in fcc metals: Effect of loading orientation and nanocrystalline effects

It is shown, through molecular dynamics simulations, that the emission and outward expansion of special dislocation loops, nucleated at the surface of nanosized voids, are responsible for the outward flux of matter, promoting their growth. Calculations performed for different orientations of the tensile axis, [0 0 1], [1 1 0] and [1 1 1], reveal new features of these loops for a face-centered cubic metal, copper, and show that their extremities remain attached to the surface of voids. There is a significant effect of the loading orientation on the sequence in which the loops form and interact. As a consequence, the initially spherical voids develop facets. Calculations reveal that loop emission occurs for voids with radii as low as 0.15 nm, containing two vacancies. This occurs at a von Mises stress approximately equal to 0.12G (where G is the shear modulus of the material), and is close to the stress at which dislocation loops nucleate homogeneously. The velocities of the leading partial dislocations are measured and found to be subsonic (～1000 m s^?1). It is shown, for nanocrystalline metals that void initiation takes place at grain boundaries and that their growth proceeds by grain boundary debonding and partial dislocation emission into the grains. The principal difference with monocrystals is that the voids do not become spherical and that their growth proceeds along the boundaries. Differences in stress states (hydrostatic and uniaxial strain) are discussed. The critical stress for void nucleation and growth in the nanocrystalline metal is considerably lower than in the monocrystalline case by virtue of the availability of nucleation sites at grain boundaries (von Mises stress ～0.05G). This suggests a hierarchy of nucleation sites in materials, starting with dispersed phases, triple points and grain boundaries, and proceeding with vacancy complexes up to divacancies.     

Fundamental differences in the plasticity of periodically twinned nanowires in Au,Ag, Al,Cu, Pb and Ni

The role played by nanoscale twins is becoming increasingly important in order to understand plasticity in nanowires synthesized from metals. In this paper, molecular dynamics simulations were performed to investigate the synergistic effects of stacking fault energy and twin boundary on the plasticity of a periodically twinned face-centered cubic (fcc) metal nanowire subjected to tensile deformation. Circular nanowires containing parallel (1 1 1) coherent twin boundaries (CTBs) with constant twin boundary spacing were simulated in Au, Ag, Al, Cu, Pb and Ni using different embedded-atom-method interatomic potentials. The simulations revealed a fundamental transition of plasticity in twinned metal nanowires from sharp yield and strain-softening to significant strain-hardening as the stacking fault energy of the metal decreases. This effect is shown to result from the relative change, as a function of the unstable stacking fault energy, between the stress required to nucleate new dislocations from the free surface and that to overcome the resistance of CTBs to the glide of partial dislocations. The relevance of our predictions to realistic nanowires in terms of microstructure, geometry and accuracy in predicting the generalized planar and stacking fault energy curves is also addressed. Our findings show clear evidence that the plastic flow of twinned nanowires under tension differs markedly between fcc metals, which may reconcile some conflicting observations made in the past.     

Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

High-temperature strength and deformation of γ/γ′ two-phase Co–Al–W-base alloys

The high-temperature strength and deformation behavior of γ/γ′ two-phase Co–Al–W-base alloys have been studied with polycrystalline and single-crystal materials. The ternary, quaternary and higher-order alloys containing Ta, Cr and/or Re exhibit flow stress anomalies above 873 K due to slip of pairs of 1/2〈1 1 0〉 superpartial dislocations on {0 0 1} planes, in addition to {1 1 1} planes, in the γ′ precipitates. Compression tests on the single-crystal specimens reveal a true anomalous peak temperature of 1073 K for both ternary and Ta-containing quaternary alloys. Above the peak, the ternary alloy exhibits a rapid decrease in strength with temperature, as 1/2〈1 1 0〉 dislocations bypass the γ′ precipitates without significant shearing. Conversely, the Ta-containing quaternary alloy sustains strength to higher temperatures due to the activation of 1/3〈1 1 2〉 partial dislocation slip that introduces a high density of stacking faults in the γ′ precipitates.     

Effect of Er additions on ambient and high-temperature strength of precipitation-strengthened Al–Zr–Sc–Si alloys

The effect of substituting 0.01 at.% Er for Sc in an Al–0.06Zr–0.06Sc–0.04Si (at.%) alloy subjected to a two-stage aging treatment (4 h/300 °C and 8 h/425 °C) is assessed to determine the viability of dilute Al–Si–Zr–Sc–Er alloys for creep applications. Upon aging, coherent, 2–3 nm radius, L1₂-ordered, trialuminide precipitates are created, consisting of an Er- and Sc-enriched core and a Zr-enriched shell; Si partitions to the precipitates without preference for the core or the shell. The Er substitution significantly improves the resistance of the alloy to dislocation creep at 400 °C, increasing the threshold stress from 7 to 10 MPa. Upon further aging under an applied stress for 1045 h at 400 °C, the precipitates grow modestly to a radius of 5–10 nm, and the threshold stress increases further to 14 MPa. These chemical and size effects on the threshold stress are in qualitative agreement with the predictions of a recent model, which considers the attractive interaction force between mismatching, coherent precipitates and dislocations that climb over them. Micron-size, intra- and intergranular, blocky Al₃Er precipitates are also present, indicating that the solid solubility of Er in Al is exceeded, leading to a finer-grained microstructure, which results in diffusional creep at low stresses.     

The effect of nickel and silicon addition on some oxidation properties of novel Co-based high temperature alloys

Isothermal oxidation was carried out on γ′-strengthened Co–Al–W–B–Ni and Co–Al–W–B–Si superalloys at 800, 900, and 1000 °C. Ni-addition to Co–Al–W–B alloys leads to an absence of B within the inner oxide layer and at the grain boundaries, as indicated by ToF-SIMS. As a result, also inner Al₂O₃-formation is inhibited, which leads to inferior oxidation properties compared to Ni-free alloys. In contrast, Si-addition enhances formation of protective Al₂O₃ at 900 and 1000 °C due to enhanced selective Al-oxidation. Si-containing phases at the oxide/alloy interface, at the grain boundaries, and within precipitates are of subordinate importance but may further improve oxidation resistance.     

Solute and vacancy segregation to a/4<1 1 1> and a/2<1 0 0> antiphase domain boundaries in Fe3Al

Segregation of solute atoms and vacancies to antiphase domain boundaries (APDBs) in Fe–Al alloys near the stoichiometry Fe₃Al (Fe–22–28 at.% Al) was studied using a phase-field model based on the Bragg–Williams approximation. Local equilibrium vacancy concentration was determined from experimental data for vacancy formation enthalpy and the configurational entropy of vacancies assuming that the formation enthalpy is independent of long-range order and chemical composition. Fe atoms and vacancies segregate to APDB with the phase-shift vector a/2<1 0 0>(D0₃-APDB) in crystals with stoichiometric composition (Fe–25 at.% Al) and with the Fe-rich composition, whereas both of them tend to be depleted in Al-rich crystals. On the other hand, Fe atoms and vacancies both segregate on APDBs with the phase-shift vector a/4<1 1 1>(B2-APDB) in all compositions studied. The effects of vacancy segregation on APDB energy and thickness is negligibly small; however, the vacancy concentration at the center of APDBs can be up to 80% larger than in the bulk, and therefore it is anticipated that the mobility of APDBs can be significantly affected by the segregation of vacancies as well as by that of solute atoms.     

Ductile tensile failure in metals through initiation and growth of nanosized voids

We here reveal the initiation of ductile failure in metals at the nanometer scale by molecular dynamics simulations coupled with a novel analytical model. This proceeds by the emission of a special type of dislocation shear loop, which can expand as a partial or perfect dislocation, evolve into a prismatic loop through reaction, or develop into twins. Molecular dynamics (MD) simulations predict a strong dependence of the stress required for the initiation of plastic flow at the surface of the void for both Cu (a model fcc metal) and Ta (a model bcc metal). The decrease in stress with increasing void size is also analyzed in terms of a new analytical approach based on the energetics of dislocation loop emission. For both fcc (copper) and bcc (tantalum) metals initiation of plastic flow in MD simulations takes place at voids as small as a tri-vacancy (radius R ≈ 0.1 nm). Extensive calculations for tantalum combined with the analytical model, which tracks the simulations, enable extrapolation to R ≈ 300 nm, in the realm of second phase particles and inclusions. Thus we conclude that this is a general mechanism of tensile failure in pure monocrystalline metals where other initiation sites are absent.     

Atom probe tomographic study of elemental segregation at grain boundaries for a peak-aged Al–Zn–Mg alloy

Atom probe tomography (APT) has been used to characterize the element segregation at the grain boundary (GB) for a peak-aged Al–Zn–Mg alloy with high stress corrosion cracking (SCC) susceptibility. The results show that Mg segregates along the GB with a peak concentration of 1.38 at.% and width of 3 nm. Zn does not segregate at GB. However, segregation of Zn and H atoms at oxide-containing clusters on GB has been observed. APT atom maps also reveal that Mg₂Si is the H trapping site, but MgZn₂ is not.     

On-chip laboratory suite for testing of free-standing metal film mechanical properties,Part II – Experiments

In this paper, we demonstrate the fabrication of electrostatically loaded, free-standing Al–0.5 wt.%Cu thin-film samples, realizing a near-zero compliance support post. We measure Young’s modulus E = 74 GPa using cantilevers, in good agreement with grain texture measurements. We measure residual stress σ_R ranging from 30 to 60 MPa using fixed–fixed beams and find that processing induces significant plastic straining, which leads to residual stress values significantly less than the as-deposited value. Strength of this alloy is at least 172 MPa if the film is not severely strained, and the material exhibits no room-temperature fatigue up to 1 billion cycles at this stress level. Notched devices that have been subjected to process-induced plastic straining of ∼4% are weaker and fatigue logarithmically with the number of cycles. We compare deformation processes on the samples using ex situ TEM. The mechanism for the high strength value is attributed to the grain size and the thin surface oxide which constrain dislocation glide, while fatigue of the highly strained material is associated with the appearance of persistent slip bands.     

Tailoring and patterning the grain size of nanocrystalline alloys

Nanocrystalline alloys that exhibit grain boundary segregation can access thermodynamically stable or metastable states with the average grain size dictated by the alloying addition. Here we consider nanocrystalline Ni–W alloys and demonstrate that the W content controls the grain size over a very broad range: ∼2–140 nm as compared with ∼2–20 nm in previous work on strongly segregating systems. This trend is attributed to a relatively weak tendency for W segregation to the grain boundaries. Based upon this observation, we introduce a new synthesis technique allowing for precise composition control during the electrodeposition of Ni–W alloys, which, in turn, leads to precise control of the nanocrystalline grain size. This technique offers new possibilities for understanding the structure–property relationships of nanocrystalline solids, such as the breakdown of Hall–Petch strength scaling, and also opens the door to a new class of customizable materials incorporating patterned nanostructures.