Increasing toughness by promoting discontinuous precipitation in Cu–Ni–Si alloys

The influence of precipitation morphology, including continuous precipitation (CP) and discontinuous precipitation (DP), on the mechanical behaviour of Cu–Ni–Si alloys was studied. The Cu–6Ni–1.5Si (in wt%) alloy was solution heat treated at 980 °C for 2 h and aged at 500 °C for 0.5 and 3 h to produce CP and DP structures. The DP specimen showed an abnormal increase in tensile ductility with increasing strain rate, unlike the CP counterpart. The impact toughness of the DP specimen was 1.6 times greater than that of CP specimen. The fracture mode in DP specimen was mostly dimpled rupture, while the mixed mode of cleavage fracture and dimpled rupture was noted on the CP specimen.     

Dynamic precipitation at elevated temperatures in a dual-phase AlCoCrFeNi high-entropy alloy: an in situ study

ABSTRACT

The effect of a possible phase transformation or precipitation of the face-centred cubic (FCC) phase on intermediate-temperature deformation of a dual-phase AlCoCrFeNi high-entropy alloy has been studied using in situ tensile testing at 550°C. Electron backscatter diffraction (EBSD) results showed localised precipitation of the FCC phase during the intermediate-temperature deformation. The overall fracture behaviour and crack propagation of the material was not altered much compared to the room-temperature behaviour, namely brittle trans-granular fracture. Deformation at higher temperatures (above 750°C) is suggested as a way to enhance the dynamic FCC phase precipitation, in order to improve the ductility or deformability of the alloy.     

Limb Segment Load Inhibits the Recovery of Soleus H-Reflex After Segmental Vibration in Humans

We investigated the effects of vertical vibration and compressive load on soleus H-reflex amplitude and postactivation depression. We hypothesized that, in the presence of a compressive load, limb vibration induces a longer suppression of soleus H-reflex. Eleven healthy adults received vibratory stimulation at a fixed frequency (30 Hz) over two loading conditions (0% and 50% of individual's body weight). H-reflex amplitude was depressed ～88% in both conditions during vibration. Cyclic application of compression after cessation of the vibration caused a persistent reduction in H-reflex excitability and postactivation depression for > 2.5 min. A combination of limb segment vibration and compression may offer a nonpharmacologic method to modulate spinal reflex excitability in people after CNS injury.     

Creep of micron-sized aluminium columns

Micron-sized aluminium columns, produced by focused-ion beam milling, were subjected to compressive creep in a nanoindentation machine using a flat-ended tip to study their time-dependent deformation behaviour at room temperature. At constant load, their strain increased almost linearly with time, with sporadic but large strain bursts superimposed. This represents a form of creep behaviour not known beforehand for aluminium. Strain bursts were also observed to occur during reloading at stresses higher than the first yield stress during initial loading.     

Networking amorphous phase reinforced titanium composites which show tensile plasticity

Titanium matrix composites reinforced by an in situ formed networking amorphous phase have been prepared directly in the form of as-cast ingots. The composites show significant compressive and tensile plasticity. In contrast to the highly localized plasticity for most monolithic amorphous alloys, the tensile deformation of the current composites distributes uniformly over the entire specimen as a consequence of substantial work (strain) hardening. The formation mechanism of the networking structure, as a result of well-separated two-step solidification of the current alloy melts, have possible implications for composite synthesis in other alloy systems.     

Influence of heat treatment on the microstructural evolution of Al–3 wt.% Cu during high-pressure torsion

Solution-treated, peak-aged and overaged samples of the model alloy Al–3?wt.% Cu, obtained by selective heat treatments of the pre-material, have been subjected to high-pressure torsion at room temperature and at 200?°C. The mechanical behaviour of the samples was investigated with torque measurements during deformation and microhardness measurements after deformation. Irrespective of the initial material condition, in the saturation regime a comparable equilibrium microstructure was found consisting of ultrafine aluminium grains stabilized by precipitates formed at grain boundaries.     

Effect of applied stress on precipitation of Guinier–Preston zones in a Cu–0.9 wt.% Be single crystal

A stress-oriented formation of plate-shaped Guinier–Preston (GP) zones in single crystals of a Cu–0.9?wt.%?Be alloy aged at 200°C has been found. A compressive stress applied in the [001] direction assists the formation of the GP zones perpendicular to the [001] axis, while a tensile stress results in the preferential formation of the GP zones parallel to the stress axis. The applied stress strongly affects the nucleation of the GP zones. Free ageing at 275°C after compressive-stress ageing at 200°C produces plate-shaped γ′′ precipitates perpendicular to the stress axis, indicating a continuous change in the structure of the GP zones into that of the γ′′ phase. From length-change measurements during ageing, the misfit strains of the GP zone or γ′′ precipitate in directions parallel and perpendicular to the plate plane are estimated, respectively, as ?0.01 and ?0.10 for the GP zone and ?0.01 and ?0.11 for the γ′′ precipitate.     

Plastic deformation behaviour of decagonal Al70Ni15Co15 single quasicrystals

High-temperature deformation experiments have been performed on decagonal Al₇₀Ni₁₅Co₁₅ single quasicrystals at a constant strain rate of 10^-5s^-1 in the temperature range between 700 and 860°C. The samples were deformed in compression with the compression axis in different orientations, parallel to, inclined by 45° and perpendicular to the tenfold symmetry axis. Stress relaxation tests and temperature changes were carried out to determine thermodynamic activation parameters. The flow stress and the activation enthalpy were found to depend on the sample orientation whereas dependences of the activation volume and the stress exponent on the orientation were not observed. Additionally, deformation tests were performed on samples of the basic Co-rich modification of the decagonal phase at the temperature of 860°C in the same three orientations. The deformation behaviours of the two different modifications of the decagonal phase are discussed.     

Deformation twinning of Bi–Sb solid alloy formed under a strong gravitational field

We performed an ultracentrifuge experiment on Bi–Sb alloy. Deformation twins with misorientations of about 90° were observed in the low-gravitational region where grain refinement had not occurred. The twins were thicker than the conventional deformation twins and their thickness was proportional to the gravitational field. We found that the minimum gravitational field required for grain refinement was 0.17 × 10⁶ G at 240°C for periods <10 h.     

TEM study of defects generated in 4H-SiC by microindentations on the prismatic plane

The deformation microstructure of single crystals of 4H-SiC resulting from microindentations on a prismatic surface was investigated by TEM. Indentations were performed at 400 and 675°C, i.e. below the brittle to ductile transition temperature of 4H-SiC (temperature close to 1100°C). TEM analysis reveals dissociated dislocations as well as extended stacking faults in the basal plane. In addition, perfect edge dislocations are observed on prismatic planes. From the observations, it is assumed that perfect dislocations are nucleated in the prismatic plane and cross-slip on the basal one where they dissociate.     

Effect of aspect ratio on the compressive deformation and fracture behaviour of Zr-based bulk metallic glass

The compressive deformation and fracture features of Zr₅₉Cu₂₀Al₁₀Ni₈Ti₃ bulk metallic glassy samples with aspect ratios in the range of 0.67–2.00 have been investigated. The compressive plastic strain of the glass monotonically increases with decreasing aspect ratio, but the maximum strength almost maintains a constant value of 1.77–1.88?GPa. All the compressive shear-band angles are equal to?～40° if modified by the rotation of the primary shear bands.     

Recrystallization of amorphous ion-implanted silicon carbide after thermal annealing

Single crystals of 6H–SiC were implanted at room temperature with 4-MeV Au ions to a fluence of 10^15?cm^?2. Raman spectra showed that full amorphization was achieved. The recrystallization process was studied by micro-Raman spectrometry after isochronal thermal annealing between 700 and 1500°C. The spectra permitted the evolution upon annealing of Si–C bonds, and also of Si–Si and C–C bonds, to be followed. Amorphous phase relaxation takes place below 700°C; then recrystallization of the 6H polytype sets in at 700°C. At 900°C crystallites with different crystalline states are formed. Moreover, Raman spectra provide evidence of graphitic nanocluster formation at 1500°C.     

Observations of copper clustering in a 25Cr-7Ni super duplex stainless steel during low-temperature aging under load

Atom-probe tomography was used to investigate phase separation and copper (Cu) clustering in the ferrite phase of a 25Cr-7Ni super duplex stainless steel. The steel was subjected to a tensile load during aging at 325°C for 5800?h. The degree of phase separation into α (Fe-rich) and α′ (Cr-rich) was small, but still, it was the highest in the steel subjected to the highest load. Cu was found to cluster, and the number density of clusters increased with increasing load. In the material subjected to the highest load, Cu was enriched in regions that were neither Fe-rich nor Cr-rich. These regions also had the highest number density of Cu clusters.     

Recrystallization in a low-density low-alloy Fe–Mn–Al–C duplex-phase alloy

The recrystallization behaviour of a cold-rolled, low-density, low-alloy duplex-phase alloy (Fe–6.57Al–3.34Mn–0.18C, wt.%) has been studied. Temperature-resolved X-ray diffraction and dilatometry showed that the alloy recrystallizes at 850?°C during continuous heating. However, electron back-scattered diffraction investigations using Kernel average misorientation revealed that during annealing ferrite recrystallizes at lower temperatures while austenite remains strained up to 1200?°C. This study underlines the complexity of recrystallization of a microstructure comprising of constituents with high and low stacking fault energy.     

The yield point of GaAs:Zn pre-deformed in the Peierls regime

In previous work by the present authors, the influence of pre-deformation at 600°C on the strain-rate and temperature dependence of the yield point of GaAs:Zn was investigated. Marked deviations in comparison with the behaviour of as-grown material were found in subsequent deformation at lower temperatures. In the present study the specimens were pre-deformed at 420°C, and the second tests were performed at temperatures between 270 and 390°C. This is the range in which the dislocation motion governing plasticity is dominated by a Peierls mechanism. Again, marked deviations from the properties of as-grown material were observed. They are mainly characterized by a distinctly smaller strain-rate dependence of the yield stress. Only close to the ductile-brittle transition in a rather limited temperature and strain-rate range does the behaviour of pre-deformed crystals approximate that of as-grown material.     

Secondary ageing in Al-Cu-Mg

Secondary ageing, that is microstructural evolution occurring at room temperature after short heating at temperatures above the metastable phase boundary of Guinier-Preston zones, has been studied for an Al-Cu-Mg alloy with a high Cu-to-Mg ratio. Combined data from positron annihilation spectroscopy, Vickers microhardness measurements and differential scanning calorimetry show that, on secondary ageing after 5 or 7min at 190°C, firstly, hardening takes place at a rate nine to 16 times slower than natural age hardening; secondly, vacancies slowly released by Cu-rich aggregates formed during the heat treatment at 190°C promote further formation of solute aggregates, with a time-dependent chemical composition; thirdly, the thermal stability of the structures formed during secondary ageing increases with increasing dwell time at room temperature; and, fourthly, solute aggregates formed at 190°C undergo a structural reorganization and possibly a change in the composition, leading to species with a different thermal stability. The slow release of vacancies from Cu-rich aggregates is proposed as one of the limiting factors of the hardening rate.     

Loss of coherency of the alpha/beta interface boundary in titanium alloys during deformation

The loss of coherency of interphase boundaries in two-phase titanium alloys during deformation was analyzed. The energy of the undeformed interphase boundary was first determined by means of the van der Merwe model for stepped interfaces. The subsequent loss of coherency was ascribed to the increase of interphase energy due to absorption of lattice dislocations and was quantified by a relation similar to the Read–Shockley equation for low-angle boundaries in single-phase alloys. It was found that interphase boundaries lose their coherency by a strain of approximately 0.5 at T = 800°C.     

Accelerating heterogeneous nucleation to increase hardness and electrical conductivity by deformation prior to ageing for Cu-4 at.% Ti alloy

ABSTRACT

Deformation bands, especially shear bands were intentionally formed in supersaturated Cu-4 at.% Ti alloy by groove-rolling with 75% area reduction. Cold deformation prior to the ageing process changed the precipitation behaviour of the alloy. The deformed structure with shear bands was maintained, without showing recovery and recrystallisation, even after prolonged ageing at 450°C for more than one day. The thermally stable shear bands act as nucleation initiation sites and prevent the expansion of discontinuous cellular precipitates. The hardness reached a value of 305?Hv and an electrical conductivity of 15% IACS (IACS?=?International Annealed Copper Standard. 100% IACS is defined as the conductivity corresponding to a volume resistivity at 20°C of 17.241?nΩ), compared to 283?Hv and 13% IACS for conventional solid-solutioned and peak-hardened alloy. In addition, the ageing time to reach the highest hardness was shortened from 720 to 180?min at 450°C.     

Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.     

Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Hf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti 2 Nitype phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H][M] =1.2). Pressure-composition isotherm studies for gasphase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at as lightly higher pressure and extends over a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.