Interface characteristics in an {\alpha}+{\beta} titanium alloy

The alpha/beta interface in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) is investigated via centre of symmetry analysis, both as-grown and after 10% cold work. Semi-coherent interface steps are observed at a spacing of 4.5 +/-1.13 atoms in the as-grown condition, in good agreement with theory prediction (4.37 atoms). Lattice accommodation is observed, with elongation along [-1 2 -1 0]alpha and contraction along [1 0 -1 0]alpha . Deformed alpha exhibited larger, less coherent steps with slip bands lying in {110}beta. This indicates dislocation pile-up at the grain boundary, a precursor to globularisation, offering insight into the effect of deformation processing on the interface, which is important for titanium alloy processing route design.Comment: Revised after revie

A nickel based superalloy reinforced by both Ni3Al and Ni3V ordered-fcc precipitates

A nickel based superalloy has been designed where the fcc γ Ni matrix is reinforced by two different ordered-fcc intermetallic compounds, γ' L12 Ni3Al and γ'' D022 Ni3V. Primary ageing at 900–1000 °C precipitated spherical L12 Ni3Al, whose volume fraction and size were controlled by altering the ageing temperature and time. Secondary ageing at 700 °C for 1–1000 h precipitated D022 Ni3V laths. The duplex precipitation increased hardness by up to 85 HV, with ∼ 500 MPa compressive proof strength maintained at 800 °C. Electron microscopy studied the Ni3Al precipitation and confirmed the form of the secondary Ni3V precipitates and their long term stability

Precipitate dissolution during deformation induced twin thickening in a CoNi-base superalloy subject to creep

The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal γ ′ distribution has been examined at 800 ∘C and 300 MPa. The rupture life of the alloy is comparable to that of RR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealed the presence of continuous γ ′ precipitates and M 23C 6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary γ ′ precipitates occurred at the expense of the fine tertiary γ ′. Long planar deformation bands, free of γ ′, were also observed to traverse individual grains. Examination of the deformation bands confirmed that they were microtwins. Long sections of the microtwins examined were depleted of γ ′ stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolution of the precipitates is suggested based on the Kolbe reordering mechanism

Interface characteristics in an α+β titanium alloy

The α/β interface in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) was investigated via center of symmetry analysis, both as-grown and after 10% cold work. Semicoherent interface steps are observed at a spacing of 4.5±1.13 atoms in the as-grown condition, in good agreement with theory. Lattice accommodation is observed, with elongation along [1210]α and contraction along [1010]α. Deformed α exhibited larger, less coherent steps with slip bands lying in {110}β. This indicates dislocation pile-up at the grain boundary, a precursor to globularization during heat treatment. Atom probe tomography measurements of secondary α plates in the fully heat-treated condition showed a Zr excess at the interface, which was localized into regular structures indicative of Zr association with interface defects, such as dislocations. Such chemo-mechanical stabilization of the interface defects would both inhibit plate growth during elevated temperature service and the interaction of interface defects with gliding dislocations during deformation

Alloying and the micromechanics of Co-Al-W-X quaternary alloys

The lattice misfit and diffraction elastic constants in hot rolled polycrystalline Co-7Al-5W-2Ta and Co-6Al-6W-2Ti (at.%) are measured using neutron and synchrotron X-ray diffraction. The misfit in the two alloys was found to be +0.67 and +0.59%, using neutron diffraction at HRPD. The misfit was found to increase with temperature, as in Ni superalloys. This implies that the amount of coherency strengthening increases with temperature. The diffraction elastic constants measured show that the γ ' phase is less stiff than the γ matrix in all orientations, which means that load shedding will occur to the γ phase

Femtosecond quantification of void evolution during rapid material failure

Understanding high-velocity impact, and the subsequent high strain rate material deformation and potential catastrophic failure, is of critical importance across a range of scientific and engineering disciplines that include astrophysics, materials science, and aerospace engineering. The deformation and failure mechanisms are not thoroughly understood, given the challenges of experimentally quantifying material evolution at extremely short time scales. Here, copper foils are rapidly strained via picosecond laser ablation and probed in situ with femtosecond x-ray free electron (XFEL) pulses. Small-angle x-ray scattering (SAXS) monitors the void distribution evolution, while wide-angle scattering (WAXS) simultaneously determines the strain evolution. The ability to quantifiably characterize the nanoscale during high strain rate failure with ultrafast SAXS, complementing WAXS, represents a broadening in the range of science that can be performed with XFEL. It is shown that ultimate failure occurs via void nucleation, growth, and coalescence, and the data agree well with molecular dynamics simulations

High-temperature

Interim results from the development of a polycrystalline Co-Al-W based superalloy are presented. Cr has been added to provide oxidation resistance and Ni has then been added to widen and stabilise the γ′ phase field. The alloy presented has a solvus of 1010 °C and a density of 8.7 g cm−3. The room temperature flow stress is over 1000 MPa and this reduces dramatically above 800 °C. The flow stress anomaly is observed. A microstructure with both ∼ 50 nm γ′ produced on cooling and larger 100–200 nm γ′ can be obtained. Isothermal oxidation at 800 °C in air for 200 h gave a mass gain of 0.96 mg cm−2. After hot deformation in the 650–850 °C temperature range, both anti phase boundaries (APBs) and stacking faults could be observed. An APB energy of 71 mJ m−2 was measured, which is comparable to that found in commercial nickel superalloys

Effect of precipitation on mechanical properties in the β-Ti alloy Ti-24Nb-4Zr-8Sn

Tensile testing and cyclic tensile loading measurements were performed on heat-treated samples of annealed Ti-2448 and cold-rolled Ti-2448. Quenching from above the β-transus produces an alloy that is highly superelastic has ultra-low elastic modulus (10-25. GPa) and exhibits hysteresis on loading-unloading cycles. On repeated cycling the strain energy absorbed in each cycle decreases. Annealed Ti-2448 exhibits a stable hysteresis loop. Peaks from the α″ phase are observed in X-ray diffraction (XRD) patterns, thus the material is quite lean in β-stabilising additions. The alloy is shown to be highly unstable when heat-treated. A combination of small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) was employed to relate the thermally induced microstructural evolution to the change in mechanical properties. A heat-treatment of 80. °C to the cold-rolled material precipitated the ω phase causing embrittlement. Increasing the ageing temperature from 80 to 300. °C increased the stiffness, made the elastic regime more linear, and further embrittled the alloy. The low temperature heat-treatments precipitate both ω and α″ phases. A higher temperature ageing treatment at 450. °C increased the yield strength to over 1. GPa and caused embrittlement, indicating co-precipitation of α and ω phases