The effect of roll gap geometry on microstructure in cold-rolled aluminum

Microstructure and texture are analyzed through the thickness of two aluminum plates cold-rolled 40% with different roll gap geometries. It is found that both texture and microstructure are strongly affected by the rolling geometry. After rolling with intermediate-size draughts a rolling-type texture is developed throughout the plate thickness. In this case, grains are subdivided by extended planar dislocation boundaries preferentially aligned at an angle of 40 ± 15° to the rolling direction. In the plate rolled with small draughts, shear texture components appear in the intermediate layers. In these layers, extended planar dislocation boundaries are frequently found to be inclined closely to the rolling direction. The subsurface and central layers of this plate exhibit microstructures similar to those in the plate rolled with intermediate draughts. It is suggested that the development of different textures and microstructures at different depths is related to the activation of different slip systems due to through-thickness strain gradients.     

Intrinsic and extrinsic size effects in the deformation of metallic glass nanopillars

Nanosized pillars with diameters ranging from 90 to 600 nm of four amorphous alloys, Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁, Zr₅₀Ti_16.5Cu₁₅Ni_18.5, Zr_61.8Cu₁₈Ni_10.2Al₁₀ and Al₈₆Ni₉Y_5, were fabricated and tested in situ in a transmission electron microscope. The major consideration when varying the composition was the change in bulk modulus and Poisson’s ratio, which may affect the deformation mode and ductility of metallic glasses (MGs) at the nanoscale. Differences between the deformation behavior of tapered (1.5-3°) and taper-free systems were also investigated. The yield stress of all the MGs measured through the in situ experiments is found to be essentially size independent, irrespective of tapering. With increasing size, all the MGs examined show a ductile-to-brittle transition under compression; the transition point, however, depends on the chemical composition of the specific MG investigated. The lower the μ/B ratio, the larger the pillar diameter above which more brittle behavior occurs. Al₈₆Ni₉Y₅ taper-free MG showed a transition threshold to brittle behavior at the largest pillar diameter of 300 nm. A micromechanical model is presented to explain the various dependencies.     

Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al–Mg thin films

Using in situ nanoindentation in a transmission electron microscope (TEM) the indentation-induced plasticity in ultrafine-grained Al and Al–Mg thin films has been studied, together with conventional quantitative ex situ nanoindentations. Extensive grain boundary motion has been observed in pure Al, whereas Mg solutes effectively pin high-angle grain boundaries in the Al–Mg alloy films. The proposed mechanism for this pinning is a change in the atomic structure of the boundaries, possibly aided by solute drag on extrinsic grain boundary dislocations. The mobility of low-angle boundaries is not affected by the presence of Mg. Based on the direct observations of incipient plasticity in Al and Al–Mg, it was concluded that solute drag accounts for the absence of discrete strain bursts in indentation of Al–Mg.     

Influence of interfaces on the plastic deformation in Ti–Al

The alloy Ti–48.6Al–1.9Cr–1.9Nb–1B with a near γ microstructure, obtained by heat treatment at 1200°C for 4 h, and with a lamellar microstructure, obtained by heat treatment at 1380°C for 1 h, is characterized by compression tests and transmission electron microscopy. A lower activity of superdislocations and a more frequent pinning of ordinary dislocations are detected in the lamellar Ti–Al specimens in comparison with the non-lamellar ones during deformation at room temperature. The lower activity of superdislocations and the more frequent pinning of ordinary dislocations is responsible for the higher yield stress in lamellar Ti–Al compared with non-lamellar Ti–Al. The different deformation mechanism in the lamellar structure is explained by the effect of the lamellar interfaces, which can transform superdislocations into ordinary dislocations, and where a very high density of locked ordinary interfacial dislocations has been found. The lamellar interfaces influence the activity of superdislocations and the pinning of ordinary dislocations and modify the mechanical properties.     

The effects of hydrogen on the deformation and fracture of β-titanium

The hydrogen-induced ductile–brittle transition in the BCC β-titanium alloy, Timetal® 21S, occurs abruptly at a critical hydrogen concentration that decreased with decreasing tensile test temperature. Mechanical property tests showed that solute hydrogen reduced the yield strength of ductile specimens and decreased the fracture stress of brittle specimens. To identify the operative mechanism a series of experiments were performed to test the applicability of the stress-induced hydride mechanism, the hydrogen-enhanced plasticity mechanism, and the decohesion mechanism of hydrogen embrittlement. The experiments showed that no hydrides were associated with the fracture process, indicating that the stress-induced hydride mechanism was not responsible for the observed sharp ductile–brittle transition. In situ straining experiments in a controlled environment transmission electron microscope showed that hydrogen enhanced the mobility of dislocations in both uncharged and hydrogen charged alloys, showing that the hydrogen-enhanced localized plasticity mechanism cannot account for the observed behavior. The experimental results are, however, fully consistent with the decohesion mechanism of hydrogen embrittlement.     

γ′-cutting as rate-controlling recovery process during high-temperature and low-stress creep of superalloy single crystals

In the present study the pairwise cutting of the γ′-phase after high-temperature and low-stress shear creep deformation of superalloy single crystals was investigated using weak-beam and high-resolution transmission electron microscopy. Recently, a cutting process in the single-crystal superalloy CMSX-6 was observed [Acta mater., 45 (1997) 4251] where two γ-channel dislocations with different Burgers' vectors (b) jointly shear the γ′-phase in forming a superdislocation with an overall Burgers' vector of a[010]. This type of high-temperature and low-stress γ′-phase cutting mechanism was also observed for CMSX-4 in the present work, indicating that this mechanism is relevant for superalloy single crystals in general. Two different configurations have been observed associated with the pure edge a010 and the 45° a001 dislocations. The cores of these superdislocations are not compact, but rather are composed of two different a/2110 dislocations. The distance between the leading and the trailing superpartial dislocation for the pure edge a010 configuration is of the order of 25 Å. In all cases observed in the present study, the common superpartial is associated with the crystallographic slip system that is directly loaded (Schmid factor 1). The striking feature of the movement of the superdislocations in the γ′-phase is that the two superpartials need to move by a combined process of glide and climb. This requires diffusional exchange of atoms/vacancies between the leading and the trailing superpartial, in which case the process is self-fed and the overall vacancy equilibrium is not disturbed. It is also possible that one dislocation pair produces or absorbs vacancies so that its movement must be coupled to events which maintain overall vacancy equilibrium. Minimum creep rates can be rationalized on the basis of the fluxes associated with the movement of superdislocations in the γ′-phase.     

Pristine-to-pristine regime of plastic deformation in submicron-sized single crystal gold particles

Pristine single crystalline gold particles with sizes ranging from 300 to 700 nm have been fabricated through high-temperature (1150 °C) liquid de-wetting of gold thin films atop a specially designed SiO₂/Si substrate for in situ transmission electron microscopy testing. Quantitative compression tests showed that these particles display cataclysmic structural collapse immediately following elastic loading to very high stresses (over 1 GPa), resulting in a nearly pristine postmortem microstructure despite the large plastic deformation experienced by the particle. This distinct class of dislocation plasticity behavior is attributed to the very high degree of structural perfection of the initial sample, resulting from high-temperature formation or annealing around the melting point. Temporally correlated dislocation nucleation from the contact interface together with the inability to form stable junctions inside is proposed to explain the pristine-to-pristine structural collapse. Upon further compression, once the contact diameter d increases to above a critical value (∼250 nm), continuous plastic deformation begins to set in under relatively low flow stress with the postmortem microstructure containing a high density of tangled dislocations, suggesting that a critical dislocation tangling volume under multiple slip is needed for the onset of dislocation storage (robust dislocation jamming) and more conventional plasticity.     

Fatigue crack propagation in an aluminium alloy at 223 K

The fatigue crack growth behaviour of a naturally aged aluminium alloy has been investigated at 223 K. Crack growth rates in a cold environment are shown to be slower than in ambient air, which is associated with a crystallographic crack path. Results are discussed on the basis of work previously proposed [Petit J, Hénaff G, Sarrazin-Baudoux C. Fracture of materials from nano to macro. In: Milne I, Ritchie RO, Karihaloo B, editors. Comprehensive Structural Integrity, vol. 6. Oxford: Elsevier; 2003. p. 211.].     

In situ composite formation in the Ni–(Cu)–Ti–Zr–Si system

In situ composites were synthesized by arc melting Ni–(Cu)–Ti–Zr–Si alloys. The X-ray diffraction patterns of rapidly cooled cast strips show a primary Ni(Ti, Zr) B2 structure superimposed on the diffuse scattering maxima from the amorphous phase. Compression test results show that the composite starts to yield at 1200 MPa and fractures at 1900 MPa.     

Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

Sulphur grain boundary segregation during hot-compression of nickel (5.4 wt. ppm S) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550 °C/0, 550 °C/3 × 10^?5 s^?1, 550 °C/3 × 10^?4 s^?1 and 450 °C/3 × 10^?5 s^?1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ～10³ to a few 10⁵, depending on the deformation conditions. Very high levels of segregation (～0.8 monolayer of sulphur) are obtained after very low deformation (～5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly.     

On the role of plastic deformation during the mild wear of alumina

Polycrystalline -alumina was worn against Mg-partially stabilized zirconia (Mg-PSZ), using water lubrication, a sliding speed of 0.24 m/s and a load of 10 N. Differential wear between grains (maximum 33 nm) and fine (0.3–1.9 μm diameter) abrasive grooves were found on the worn surface. TEM of back-thinned samples indicated widespread dislocation flow at the surface, heterogeneously distributed between grains, and largely associated with abrasive grooves. Those grains standing proud of the surface invariably contained extensive dislocation damage. The dominant slip system was pyramidal ( , , and ) although occasional basal slip was also found. No prism slip was observed. The pyramidal slip planes were concentrated at angles of 6–33° to the worn surface. Basal slip was frequently associated with basal twinning on planes at 72–73° to the worn surface. Dislocation pile-ups at grain boundaries often coincided with grain boundary cracking. The extent of damage from abrasive grooves varied from grain to grain and was dictated by crystallographic orientation more than the grain height. No evidence of mechanical damage was found in those grains that had suffered the highest wear, indicating that material removal had been controlled by tribochemical mechanisms. The origin of the differential wear between grains is considered and the implications of the experimental observations on the time-dependent transition to severe wear in aluminas are discussed.     

The microstructure development in Fe32Cu20Ni28P10Si5B5 immiscible alloy and possibilities of formation of amorphous/crystalline composite

Durch Messungen mittels differentieller Thermal-Analyse und dazu komplementäre Untersuchungen der Mikrostrukturen an Fe-Cu Legierungen ergibt sich erstmals der Nachweis für das Auftreten von kritischer-Punkt Benetzung an einer vollständig metastabilen Mischungslücke. Die damit verbundene vollständige Benetzung tritt in dem Zusammensetzungsbereich von 50–65 at.% Fe nahe der kritischen Konzentration auf. Glaseingelagerte Proben zeigen die vollständige Benetzung des Glases durch die Cu-reiche Schmelze während des Unterkühlens bis zur metastabilen Mischungslücke. Im Zusammensetzungsbereich der vollständigen Benetzung erfolgt die Entmischung bei der zugehörigen Binodaltemperatur ohne zusätzliche Unterkühlung. Bei der Erstarrung der phasenseparierten Schmelze aus tiefer Unterkühlung führt dies zu einem grobskaligen Entmischungsgefüge. Im Gegensatz dazu zeigt die Entmischungsreaktion bei Zusammensetzungen außerhalb des Intervalls vollständiger Benetzung eine merkliche Unterkühlung unterhalb der jeweiligen Binodaltemperatur. Das zugehörige Erstarrungsgefüge zeigt die Phasentrennung auf einer deutlich kleineren Skala. Ohne Glaseinlagerung befinden sich die Proben in Kontakt zu dem Al₂O₃-Tiegel und zu einer Oberflächenschicht aus Eisenoxid. Unter diesen Bedingungen findet kritischer-Punkt Benetzung auf beiden Seiten der kritischen Zusammensetzung statt. Diese Ergebnisse demonstrieren daß kritischer-Punkt Benetzung unabhängig von der Probenumgebung erfolgt, daß jedoch die benetzende Phase oberflächenempfindlich ist und durch die Probenumgebung selektiert wird.

Effect of grain size on the damping capacity of quasicrystalline Al-Cu-Fe materials

Thick quasicrystalline (QC) Al-Cu-Fe coatings obtained by electron-beam physical vapour deposition (EB-PVD) on titanium substrates at different temperatures have been used for damping measurements. The measurements were performed by the method of free-decay vibrations with using flat cantilever specimens covered with the QC coatings in the strain amplitude range of 10^− 4-10^− 3 and in the temperature range of 290-620 K. We have studied the intrinsic damping capacity of the QC coatings with different grain sizes as a function of strain amplitude using a calculation procedure. The intrinsic damping capacity of all the QC materials is found to increase progressively with temperature in the whole strain amplitude range. It was found that decreasing of the QC grain size to nanoscale values leads to a significant increase of their damping capacity at temperatures above 520 K. Possible mechanisms of dissipation of mechanical energy in nanostructured quasicrystals at elevated temperatures are discussed.     

Equilibrium shape and interface roughening of small liquid Pb inclusions in solid Al

The shape of liquid Pb inclusions embedded in a solid Al matrix was investigated at temperatures between 300 and 500°C using in-situ electron microscopy. Inclusion shapes in the size range from a few nanometers to about 150 nm were found to depend on size, temperature and thermal history. During isothermal annealing after melting, small inclusions rounded off while larger inclusions remained faceted until the temperature was raised to about 500°C. During subsequent cooling, inclusions refaceted, although less strongly than during heating. The shape hysteresis between heating and cooling cycles was found to be due to the barrier of ledge nucleation necessary to advance the faceted interfaces. It is shown that this kinetic barrier can explain the observed dependence on size and temperature, and that the {1 1 1} interface undergoes a roughening transition at about 550°C. Even under conditions of kinetic limitation it was possible to measure local equilibrium by modeling kinetically limited inclusions as a droplet in a crevice. For this type of measurement, the hysteresis between heating and cooling cycles disappeared, and the true equilibrium shape could be derived. The anisotropy of interfacial energy was shown to be significantly smaller than previously reported, and at about 2%, similar to the anisotropy of the surface energy for fcc metals. From the width of facets on the equilibrium shape, the step energy was determined to be =1.9·10⁻¹¹ J/m at 350°C.     

Investigating the microstructure and composition of cold gas-dynamic spray (CGDS) Ti powder deposited on Al 6063 substrate

The compositional variation, morphology and microstructure of cold gas-dynamic spray are of great importance for its proper application. This study investigates titanium powder deposition on an Al 6063 substrate using light optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composition was examined using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF SIMS). Optical and electron microscopy revealed heavily deformed Ti powder particles penetrating 10 to 30 μm into the Al substrate. Examination using TEM did not reveal any evidence of second phases at the interface suggesting a sharp transition between the two metals. The presence of nanocrystals and grain refinement of both the coating and the substrate suggest the formation of a partial hetero-epitaxy condition near the interface. EDX results from a dedicated high-resolution scanning transmission electron microscope showed a sharp compositional change with a maximum inter-diffusion region of about 5 nm. Bonding of the coating to the substrate is therefore thought to be achieved by the particle/substrate interlocking and direct metal to metal bonding. However, it is most likely that the refine crystalline structure near the interface will be beneficial to the adhesion of the coating. XPS and ToF SIMS provided evidence of nitrogen pick-up during the spray process in the form of N and TiN even when utilizing Helium as the gas carrier. The presence of TiN suggests reaction of the Ti with the entrained air during spraying which explains the occurrence of flashing jet outside the nozzle. Investigation of the material properties using nanoindentation showed reasonably consistent hardness and elastic modulus values throughout the titanium coating and at the transition region. The hardness was slightly higher than typical commercially available bulk Ti.