Microstructure and mechanical properties of a 2091 AlLi alloy—I. Microstructure investigated by SAXS and TEM

The microstructure of the 2091 alloy (AlLiCuMgZr) alloy has been studied and compared to that of two simpler AlLi and AlCuMg alloys. The SAXS, in situ SAXS and TEM techniques have been used for heat treatments at room temperature and at 150°C. The time evolution of the size and of the volume fraction of the δ′ precipitates and of the Cu-rich GPB zones have been determined. The δ′ precipitates are shown to follow a classical Lifschitz-Slyosov-Wagner law. Their interfacial energy has been estimated. S and S′ precipitates have also been characterized, respectively on grain boundaries and on helical dislocations, by TEM.     

Microstructure and mechanical properties of a 2091 AlLi alloy—III. Quantitative analysis of portevin le chatelier instabilities and relation to toughness in AlLi,AlCuMg and AlLiCuMg (2091) alloys

The microstructure and mechanical properties of a 2091 alloy are studied and compared to simpler AlLi and CuMg alloys. For ageing times between 6 and 24 h at 150°C, the 2091 alloy exhibits a toughness drop and a simultaneous change in PLC characteristics (as evidenced by a combination of local and total strain measurements), but no significant change in microstructure, except for the size of δ′ precipitation. SEM in situ tests show that plastic instabilities are always related to extra damage. A quantitative model accounts for the toughness drop, based on plastic dissipation by PLC active bands.     

Microstructure and mechanical properties of a NiAlMo eutectic composite

Post-solidification heat treatments are shown to have significant effects on tensile properties and high cycle fatigue resistance of a NiMo(31.2 wt%-Al(6.3 wt%) aligned eutectic. In the as-directionally solidified condition retained blocky γ is noted in the γ/γ′ matrix. Solution heat treatment at 1270°C produces a regular cubic network of γ/γ′. Subsequent aging at 850°C produces elliptical precipitates identified as αMo in the γ/γ′ matrix. These precipitates are responsible for a sharp drop in tensile ductility, accompanied by improved resistance to high cycle fatigue.     

Cold work hardening in stages IV and V of F.C.C. metals—II. Model fits and physical results

The paper presents a common concept for work hardening of f.c.c. metals at temperatures below 0.5 T_m up to very high strains. The concept considers statistical dislocation dynamics in terms of screw and edge dislocations and their specific interactions, and also allows for deformation-induced vacancies. By fitting the concept equations to experiment and using measured interaction parameters, very good agreement with strengthening data and dislocation densities of Cu and Al is achieved. The resulting fit parameters exhibit the expected order of magnitude and temperature dependence of dislocation storage and annihilation, for each dislocation type. The annihilation parameter of edge dislocations yields vacancy migration enthalpies of ≈0.2 eV for CU and Al indicating core migration of vacancies. From this the vacancy concentration is calculated for all strains and temperatures in question. Up to ≈0.3 T_m, the saturation concentration of vacancies decreases, but reincreases beyond this temperature following the thermal vacancy concentration.     

Large strain behaviour of a superplastic copper alloy—II. Cavitation and fracture

A quantitative study of cavitation damage and fracture of a superplastic copper alloy, Coronze 638, has been made. Cavities are found to nucleate at large particles present in the form of stringers. The size and shape of cavities, as well as the level of damage up to fracture are essentially independent of strain rate over regions I and II of the σ−ϵ curve, as are the true strain to fracture and the development of t instabilities. As the strain rate increases into region III, the level of damage to failure decreases, while the true failure strain increases and necks become sharper. Extensive cavity coalescence is observed up to strains of about 1.5, producing a number of large (> 100 μm) cavities which exhibit a high stability, and little tendency to coalescence. This allows the sample to sustain a very high level of cavitation without failure. The mechanism of cavity growth for small isolated cavities (< 10 μm) is thought to be diffusive growth constrained by matrix creep at low strain rates, with a transition to plasticity controlled growth at large strain rates. For larger cavities growth appears to be entirely creep controlled. Final fracture occurs by the material exhaustion in the ligaments between voids once the reduction in the cross section exceeds about 30%. No large instability either in flow or damage seems to be involved in this process.     

Some effects of microcracks on the mechanical properties of brittle solids—II. Microcrack toughening

The stress-strain characteristics of a microcracking material are used as the basis for computations of the fracture toughness, by applying a line integral formulation, pertinent to frontal and steady-state microcrack process zones. The calculated toughness is used to predict the trends in toughness with grain size and specimen geometry. The trends are correlated with experimental data.     

Processing,Microstructure, and Mechanical Properties of B4C ― TiB2 Particulate Sintered Composites. Part II. Fracture and Mechanical Properties

The fracture response of pressureless sintered boron carbide ceramics containing 5-25 vol.% TiB₂ phase produced via the in-situ chemical reaction between B₄C, TiO₂ and elemental carbon was studied. Both strength and fracture toughness depend on TiB₂ volume fraction, reaching their maximum values of 500 MPa and 4.6 MPa·m^1/2, respectively, at 15 vol.% TiB₂. The observed increase in strength and fracture toughness was ascribed to the interaction between the propagating crack front and local thermal mismatch stress associated with TiB₂ particles. Induced circumferencial microcracking and crack impedance are discussed as the major toughening mechanisms. Spontaneous circumferencial microcracking due to thermal mismatch stress in TiB₂ particles was found to occur when the particle size exceeds its critical value. The theoretical interpretation of spontaneous circumferencial microcracking, toughening via induced microcracking, and crack impedance was justified experimentally.     

Plastic properties of Cu2O,mechanical tests and transmission electron microscopy—II. High temperature

Creep tests have been performed on Cu₂O at temperature from 700°C (0.64 T_M) to 1060°C (0.88 T_M), stresses from 2 MPa (σ/θ ∼- 2 × 10⁻⁴) to 24 MPa (σ/θ ∼- 2.4 × 10⁻³) and oxygen partial pressures from 10⁻¹ Pa to 10⁴ Pa. A creep law has been established for polycrystals and single crystals compressed along 〈001〉, 〈011〉 and 〈111〉. Microstructural observations including transmission electron microscopy, show the build up of a polygonized substructure typical of high temperature steady state creep. This allows to conclude in favour of a creep kinetics controlled by a dislocation climb recovery of the microstructure, the elementary process being the oxygen diffusion.     

Microstructural evolution and mechanical properties of a new Ni-based heat-resistant alloy during aging at 750℃

Microstructural evolution and mechanical properties of a new candidate Ni-based heat-resistant alloy for advanced ultra-supercritical( A-USC) steam turbine rotors were investigated during aging at 750 °C up to 10 000 h. The evolutions of γ' particles inside austenitic grain and M_(23)C_6 carbides along grain boundaries were characterized according to their morphologies,distributions,and growth kinetics. Mean radius of the γ' spherical particles grew from 20. 3 to 90. 0 nm after aging for 10 000 h,and the corresponding coarsening behavior was conformed to the law of Lifschitz-Slyosovd-Wagner( LSW). The weight fraction of γ' particles slightly increased from 10. 0 to 12.0wt. % after aging of long duration at 750 ℃. The Cr-rich M_(23)C_6 carbides discontinuously precipitated along grain boundaries,while other detrimental phases were not formed during the aging treatment,and hence the strength of grain boundary was enhanced by these discontinuously distributed carbides. The critical size of γ' had a direct influence on the maximum hardness of this alloy. Moreover,this alloy presented a good impact toughness for the safety after long time aging at high temperature.     

Cold work hardening in stages IV and V of F.C.C. metals—I. Experiments and interpretation

The paper presents numerous measurements which confirm stages IV and V to be general ranges of cold work deformation. Analogous to stage II, stage IV exhibits a linear athermal hardening with constant strain rate sensitivity and activation enthalpy. In stage IV the dislocation cell size is constant, while the dislocation density growth rate is markedly reduced compared with stages II and III. Features of stage V are analogous to stage III, the increase of strain rate sensitivity (decrease of activation enthalpy) indicating the onset of thermally activated dislocation annihilation. In stage V, the mechanism is identified as dislocation climb from observing subgrain formation and saturation in density of deformation induced vacancies. Comparisons with recent investigations of stage IV and V at high temperatures suggest a common picture of low and high temperature deformation which only requires principles of storage and annihilation for both screw and edge dislocations.     

Microstructure and mechanical properties of RS NiCrAl melt-spun alloys

The microstructure and mechanical properties of three melt-spun NiCrAl alloy ribbons have been studied in the as-cast condition as well as after thermal treatments. The microstructure of the alloys is dendritic-microcellular in as-cast condition and phases present for 10 at.% Al and 30 at.% Al alloys are as is predicted by the equilibrium phase diagram. In the 20 at.% Al alloy, γ' has frozen in metastable form and partial ordering takes place during cooling in the solid state. After thermal treatments the ribbons generally maintain a refined microstructure; α phase precipitates are always found in β and γ' phases in 20 and 30 at.% Al alloys. The hardness of the alloys increases with aluminum content. The tensile strength at room temperature is related to the phases present in the material for each state of treatment. The alloys are brittle, a higher ductility always being obtained in the as-cast condition.     

Heat treatment,microstructure and mechanical properties of a metastable β titanium alloy timetal® 21s

Microstructure and mechanical properties of a metastable β titanium alloy were investigated in different heat treatment conditions. The alloy was melted by consumable vacuum arc melting followed by conventional forging and rolling. Microstructure and mechanical property evaluation were carried out in solution treatment and three different aging conditions. While low temperature aging resulted in increase in strength and decrease in ductility as compared to solution treatment condition, a double aging cycle provided considerably higher elongation values with a marginal increase in strength. Maximum fracture toughness was obtained in the double aging condition.     

On the tensile properties of a fiber reinforced titanium matrix composite—II. Influence of notches and holes

The effects of holes and notches on the ultimate tensile strength of a unidirectionally reinforced titanium matrix composite have been examined. During tensile loading, a narrow plastic strip forms ahead of the notch or hole prior to fracture, similar to that observed in thin sheets of ductile metals. Examination of the fibers following dissolution of the matrix indicates that essentially all the fibers within such a strip are broken prior to catastrophic fracture of the composite. The trends in notch-strength have been rationalized using a fracture mechanics-based model, treating the plastic strip as a bridged crack. The observations suggest that the bridging traction law appropriate to this class of composite is comprised of two parts. In the first, the majority of fibers are unbroken and the bridging stress corresponds to the unnotched tensile strength of the composite; in the second, the fibers are broken and the bridging stress is governed by the yield stress of the matrix, with some contribution derived from fiber pullout. This behavior has been modeled by a two-level rectilinear bridging law. The parameters characterizing the bridging law have been measured and used to predict the notch strength of the composite. A variation on this scheme in which the fracture resistance is characterized by an intrinsic toughness in combination with a rectilinear bridging traction law has also been considered and found to be consistent with the predictions based on the two-level traction law.