Microstructure and mechanical properties of Mg-Zn-Mn-Sn-Nd wrought alloys

The microstructure and mechanical properties of as-cast and extruded Mg-6Zn-1Mn-4Sn-xNd(x=0, 0.5, 1.0, 1.5) alloys were investigated by means of optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM) and tensile test. The results showed that the dendrites sizes of these alloys were decreased by the addition of Nd. The phase compositions of the as-cast Mg-Zn-Mn-Sn-Nd alloys were dendritic α-Mg, MgZn2, Mg2 Sn, T phase and MgSnNd ternary phase. The mechanical properties of the as-extruded alloys were improved due to the refined equiaxed grains and dispersive Mg2 Sn and MgSnNd second phases. The comparison of the theoretical yield strength with the experimental yield strength revealed that the yield strength model of the as-extruded alloys should be modified as σys =σMg +σgb +σss +σsp.     

Tensile Work Hardening Behavior of Thin-Section Plate and Thick-Section Tubeplate Forging of 9Cr-1Mo Steel in the Framework of One-Internal-Variable Kocks–Mecking Approach

Comparative tensile flow and work hardening behavior of normalized and tempered plate and quenched and tempered tubeplate forgings of 9Cr-1Mo steel have been examined in the framework of one-internal-variable Kocks–Mecking approach at temperatures ranging from 300 K to 873 K (27 °C to 600 °C). Detailed analysis in terms of the variations of instantaneous work hardening rate, θ (θ = dσ/dε _p = dσ _p/dε _p, where σ is the true stress, σ _p is the plastic flow stress component, and ε _p is the true plastic strain) with σ and σ _p indicated two-stage work hardening behavior, and three distinct temperature regimes in the variations of work hardening parameters, θ ? σ and θ ? σ _p, with temperature. The influence of initial microstructures associated with different product forms of the steel is reflected in the systematic variations in work hardening parameters at temperatures ranging from 300 K to 873 K (27 °C to 600 °C). Tubeplate forging exhibited improved work hardening characteristics in terms of higher plastic component of flow stress because of microstructural softening than that of the plate material in the steel.     

The role of plastic deformation on the competitive microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy

The role of plastic deformation prior to artificial aging on the microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy designated AF/C 458 was investigated. Induced plastic deformation ranged from a non-stretched or 0% stretch condition to an 8% stretch, with intermediate stretches of 2%, 4% and 6%. Tensile properties, fractography and quantitative precipitate analysis were acquired from specimens that were water quenched from a solution heat treatment, immediately stretched and artificially aged at 150°C. Fractography was investigated through scanning electron microscopy (SEM). Quantitative transmission electron microscopy (TEM) determined the variation in precipitate type, number density, size and volume fraction of the major strengthening precipitates Al₂CuLi (T₁), Al₂Cu (θ″/θ^′) and Al₃Li (δ^′).Age hardening curves for each level of mechanical stretch illustrated the enhanced aging kinetics of plastically deformed material. Quantitative TEM indicated that increasing amounts of pre-age stretch were found to greatly affect the competitive precipitation kinetics of T₁ and θ″/θ^′ in AF/C 458 augmenting the volume fraction of fine matrix T₁ plates and dramatically decreasing the volume fraction of θ″/θ^′ for isochronal treatments. A quantitative microstructural comparison of specimens exhibiting a given strength demonstrated that the imposed level of cold work dictated the density, size and volume fraction of the competing precipitates. The tensile data indicated a trend of increasing ductility for equivalent yield strengths with the increasing amount of pre-age mechanical stretch and therefore shorter artificial aging times. The quantitative precipitate data were used with a computer simulation for yield strength determination. The theoretical simulation reported calculated yield strengths in good accord with experimental results and can thus be used to predict the optimum microstructural configuration for high strength.     

Texture Development in the Ni47Ti44Nb9 Shape Memory Alloy During Successive Thermomechanical Processing and Its Effect on Shape Memory and Mechanical Properties

For improving the shape memory performance and mechanical properties of shape memory alloys (SMAs), crystallographic texture and second phase are generally induced in SMAs by suitable thermomechanical processing. For this purpose, the development of texture in the Ni₄₇Ti₄₄Nb₉ SMA during successive processing (e.g., hot forging, hot rolling, cold rolling, and heat treatment) and the effects of texture, grain size, and β-Nb particle precipitation on recoverable strains and tensile properties were studied. In the hot-forged and hot-rolled Ni₄₇Ti₄₄Nb₉ alloy rods, intense 〈111〉 fibers are formed, and water quenching from 873 K and 1123 K (600 °C and 850 °C) leads to the decrease in intensity of 〈111〉 fiber in the hot-rolled rod. When the hot-forged rod is hot-rolled into sheet, intense {001} and weak {123} fibers appear, but grain growth leads to the disappearance of {001} fiber and {110}〈001〉 becomes the strongest component. Cold-rolling deformation of the hot-rolled sheet promotes the development of γ-fiber and the convergence of {332} and {123} fibers to {233}〈110〉 and {123}〈121〉 components, respectively, and the intense component is turned into {111}〈110〉; in this case, the recoverable strain (ε _SRS) and tensile yield strength (σ _YS ) exhibit an anisotropy. When the quenching temperature is 1123 K (850 °C), some weaker components appear, the anisotropy of ε _SRS disappears, and the difference level in σ _YS along the rolling direction (RD) and transverse direction (TD) becomes smaller. Therefore, an appropriate heat-treatment temperature should be selected to maintain the deformation texture and also to obtain fine grains for different thermomechanical processing.     

Effects of Different Modes of Hot Cross-Rolling in 7010 Aluminum Alloy: Part II. Mechanical Properties Anisotropy

The influence of microstructure and texture developed by different modes of hot cross-rolling on in-plane anisotropy (A _IP) of yield strength, work hardening behavior, and anisotropy of Knoop hardness (KHN) yield locus has been investigated. The A _IP and work hardening behavior are evaluated by tensile testing at 0 deg, 45 deg, and 90 deg to the rolling direction, while yield loci have been generated by directional KHN measurements. It has been observed that specimens especially in the peak-aged temper, in spite of having a strong, rotated Brass texture, show low A _IP. The results are discussed on the basis of Schmid factor analyses in conjunction with microstructural features, namely grain morphology and precipitation effects. For the specimen having a single-component texture, the yield strength variation as a function of orientation can be rationalized by the Schmid factor analysis of a perfectly textured material behaving as a quasi-single crystal. The work hardening behavior is significantly affected by the presence of solute in the matrix and the state of precipitation rather than texture, while yield loci derived from KHN measurements reiterate the low anisotropy of the materials. Theoretic yield loci calculated from the texture data using the visco-plastic self-consistent model and Hill’s anisotropic equation are compared with that obtained experimentally.     

The effect of matrix strength on the fracture resistance of an alpha-Beta titanium alloy,corona-5

This paper presents the results of a study of the effect of matrix yield strength, at constant Widmanstätten α microstructure, on the fracture resistance of an α-β Ti alloy, CORONA-5. Fracture initiation resistance,J _q, and the stable crack growth resistance,T, were evaluated by the single specimen, unloading compliance method for four different microstructures and three yield strengths. The microstructures involved coarse or fine Widmanstätten α particles in a heat treated β-matrix; the yield strength ranged from 765 to 1018 MPa. It was found thatJ _q/σ_Y, where σ_Y is the effective yield strength, decreased with increasingσ _Y.T/σ _Y also decreased with increasing σ_Y for fine structures. For the coarse α structures, however, T/σ_Y revealed intermediate maxima. Coarser structures, in general, revealed higher values ofJ _q/σ_Y andT/σ _Y. The cause was found primarily to be due to the effect of increased α particle thickness. The effect of grain size was secondary. J_q/σ_Y increased with increasing tensile strain hardening rate, obtained at the onset of void nucleation. T/σ_Y was found to decrease with increasing tensile void growth rate. In general, J_q/σ_Y and T/σ_Y revealed different relationships with microstructure. Fatigue precrack front- and the stable crack length-tortuosities did not yield any general relationship to fracture resistance at different yield strengths.     

Microstructure,Texture, and Mechanical Behavior of As-cast Ni-Fe-W Matrix Alloy

This article describes the tensile properties, flow, and work-hardening behavior of an experimental alloy 53Ni-29Fe-18W in as-cast condition. The microstructure of the alloy 53Ni-29Fe-18W displays single phase (fcc) in as-cast condition along with typical dendritic features. The bulk texture of the as-cast alloy reveals the triclinic sample symmetry and characteristic nature of coarse-grained materials. The alloy exhibits maximum strength (σ_YS and σ_UTS) values along the transverse direction. The elongation values are maximum and minimum along the transverse and longitudinal directions, respectively. Tensile fracture surfaces of both the longitudinal and transverse samples display complete ductile fracture features. Two types of slip lines, namely, planar and intersecting, are observed in deformed specimens and the density of slip lines increases with increasing the amount of deformation. The alloy displays moderate in-plane anisotropy (A_IP) and reasonably low anisotropic index (δ) values, respectively. The instantaneous or work-hardening rate curves portray three typical stages (I through III) along both the longitudinal and transverse directions. The alloy exhibits dislocation-controlled strain hardening during tensile testing, and slip is the predominant deformation mechanism.     

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.     

Modeling of Strain Hardening in the Aluminum Alloy AA6061

In this paper, the evolution of work-hardening and dynamic recovery rates vs the flow stress increase (σ ? σ _y ) in Al-Mg-Si alloys is presented. The experimental data have been extracted from stress–strain curves. All curves show an initial very rapid decrease in slope of the σ–ε curve, which is associated with the elastic–plastic transition. After the elastic–plastic transition, there are typically two distinctive behaviors. For underaged alloys, there is an approximately linear decrease of work-hardening rate as (σ ? σ _y ) increases. However, for overaged alloys after elastic–plastic transition, there is a plateau in the work-hardening rate followed by an almost linear decrease. The maximum work-hardening and dynamic recovery rates are found to be dependent on the aging state. In order to investigate these phenomena, a model has been employed to simulate the work-hardening behavior of Al-Mg-Si alloys. The model is based on a modified version of Kocks–Mecking–Estrin (KME) model, in which there are three main components: (1) hardening due to forest dislocations, grain boundaries, and sub-grains; (2) hardening due to the precipitates; and (3) dynamic recovery. The modeling results are discussed and compared with the experimental data.     

Modeling Growth and Dissolution Kinetics of Grain-Boundary Cementite in Cyclic Carburizing

In vacuum carburizing of steels, short-time carburizing is usually followed by a diffusion period to eliminate the filmlike cementite (θ _GB ) grown on the austenite (γ) grain boundary surface. In order to obtain the θ _GB amount during the process, the conventional model estimates the amount of cementite (θ) with the equilibrium fractions for local C contents within a framework of the finite difference method (FDM), which overestimates the amount of θ _GB observed after several minutes of carburizing. In our newly developed model, a parabolic law is assumed for the growth of θ _GB and the rate controlling process is considered to be Si diffusion rejected from θ under the isoactivity condition. In contrast, the rate constant for the dissolution of θ _GB is considered to be controlled by Cr diffusion of θ. Both rate coefficients (α) were validated using multicomponent diffusion simulation for the moving velocity of the γ/θ interface. A one-dimensional (1-D) FDM program calculates an increment of θ _GB for all grid points by the updated diffusivities and local equilibrium using coupled CALPHAD software. Predictions of the carbon (C) profile and volume fraction of cementite represent the experimental analysis much better than the existing models, especially for both short-time carburization and the cyclic procedure of carburization and diffusion processes.     

Structure and Certain Properties of Hot-Pressed Boron Carbide-Based Ceramic with Calcium-Silicon Additive

The influence which the composition of powder mixtures, the treatment conditions which the mixtures are subjected to, and the conditions under which the hot-pressed composite materials B₄C – (5-10 mass%) calcium-silicon are fabricated exert on the structure, nature of failure, and mechanical properties of these materials is investigated. Optimum properties are possessed by material containing 10 mass% of addition. It is shown that the structure, morphology, and dispersivity, as well as the nature of the distribution of the components that are added to the composite material (secondary phase) vary as the temperature of hot pressing changes. Maximal mechanical characteristics of the composite material (σ_bend = 560 MPa, K _1c = 4.7 MPa·m^1/2, HV = 37 GPa) are attained at hot-pressing temperatures in the range 2000-2100°C.     

On the relationship between ductility and fracture toughness in an Al-6.0% Zn-2.5% Mg alloy

The fracture toughness (K_IC) of an Al-6.0%Zn-2.5%Mg alloy heat-treated variously was examined in relation to tensile properties using the notched and unnotched specimens. As usual trends, the fracture strain (ϵ_f) showed minimum at the peaks of yield stress (σ_0.2) and ultimate tensile stress (σ_uts). The K_IC and ϵ_f when aged isothermally varied similarly, but when isochronally aged K_IC and σ_0.2 (and σ_UTS) showed a similar trend. An intimate relationship was found between the dimple size at the portion fractured intergranularly and the fracture strain (ϵ_f). An equation showing the relation between k_ic and tensile properties such as σ_0.2. ϵ_UTS (the strain up to ultimate tensile stress) and n (the work hardening parameterl could be introduced, based on a new model of the plastic zone. The model was verified experimentally.     

Effect of heat treatment on the phase composition,the texture,and the mechanical properties of a V1461 (Al–Cu–Li) alloy

The heterogeneity of the phase composition, the texture, and the mechanical properties in various zones and directions of plates (thickness T = 80 mm) in a V1461 (Al–Cu–Li) alloy has been studied. It is noted that the strength characteristics are maximal in the median cross section (ultimate strength and yield strength are 570 and 540 MPa, respectively); in the cross section at 0.25T, these values are 530 and 490 MPa, respectively; in the height direction, they are only 490 and 440 MPa. The studies of texture show that an intense one-component texture, which is similar to the matrix and δ' phases, is observed in a medium plate layer of thickness (0.3–0.35)T; the {011} texture plane is parallel to the plate plane with the dominant “brass” {110}〈112〉 texture. Hardness is shown to increase from HRB70 after aging at 120°C for 20 h to HRB85 after three-step aging at 120°C, 20 h + 140°C, 24 h + 150°C, 24 h. It is shown that aging at 120 and 140°C is accompanied by the precipitation of the Θ' phase along with the δ' phase, and aging at 150°C also leads to the precipitation of the T₁ phase.     

Plastic anisotropy of sheets with continuously varying anisotropic parameters and flow stress

A continuum mechanics model has been developed on the basis of Hill's theory of orthogonal anisotropy for predicting global mechanical properties of sheets with a through-thickness texture gradient and strength gradient. By the present model, the globalr value and yield and flow stresses of the entire sheet can be predicted from the local anisotropic parameters, yield and flow stresses which are given as arbitrary functions of the through-thickness position of the sheet.     

The strengthening of ordered Zr3Al by fast neutron irradiation

The room temperature tensile properties have been measured of Zr₃Al irradiated by fast neutrons to a dose of 1.4 × 10²⁴n/m². Yielding occurs discontinuously and is accompanied by the propagation of a Lüders band within which the strain, ε_L^φ, is given by ε_L^φ = λ^φd^1/2, where λ^φ is an experimental parameter and d is the average grain size. The flow stress, σ_f^φ, including the yield strength, obeys an expression of the form σ_f^φ = σ₀^φ(ε) + k^φd^1/2 where σ₀^φ(ε) and k^φ are experimental parameters; σ₀^φ(ε) increases linearly with plastic strain, ε, while k^φ remains constant. The work-hardening rate is essentially independent of the grain size, as are the tensile strength and the ductility. Fracture occurs in a ductile mode. When compared with the behaviour of unirradiated material, the results show that the irradiation increases the yield strength and decreases the work-hardening rate, but has little effect on the tensile strength, ductility and fracture.     

Atomic-level elastic properties of interfaces and their relation to continua

The relationship between atomic structure and elastic properties of grain boundaries is investigated from both discrete and continuum points of view. The complete fourth-order tensors of both the atomic-level and the effective elastic moduli are defined for the discrete system, where the latter corresponds to sub-blocks from an infinite bicrystal and are calculated here for a relatively few atomic layers above and below the grain boundary. Then, a heterogeneous continuum model of the boundary is introduced where distinct phases are associated with individual atoms and possess their atomic-level moduli. Only estimates (upper and lower bounds) of the effective moduli can be determined for the continuum model. Comparison between the atomistic results and those for the continuum model establishes the validity of this definition of elastic properties for heterogenous structures at atomic scales. Furthermore, these comparisons as well as algebraic properties of the fourth-order tensor of moduli lead to criteria to assess the stability of a given grain boundary structure.     

Yield stress of a composite consisting of amorphous Ni78Si10B12 and μm sized WC particles

The yield strength and elastic modulus of amorphous Ni₇₈Si₁₀B₁₂ alloys, dispersed with 4–5 μm sized particles of WC, was measured as a function of the volume fraction (V_f). Within experimental error, Young's modulus (E_c) measured by an ultrasonic method, increased linearly with V_f according to the rule of mixtures: E_c = E_m (1 − V_f) + E_pV_f, where E_m and E_p are Young's modulus of the matrix and the second phase, respectively. The yield stress (σ_yc) of the composite, measured in bending, was found to increase linearly with V_f. For V_f = 18.2%, the highest volume fraction investigated, σ_yc was 2.2 times higher than the yield stress (σ_ym) of the nondispersed matrix material. Experimentally, the ratio of E_c to σ_yc was found to be independent of V_f and to be approximately 60. Thus, the yield stress, σ_yc of the dispersed alloy various with volume fraction approximately as: σ_yc = σ_ym {1 + V_f(E_p/E_m − 1)}. The above dependence of the yield stress on V_f corresponds to a rule of mixture in which the yield stress of the second phase must exceed the value of σ_ym(E_p/E_m) or about 1000 kg/mm². Comparison with the experimentally measured value for the compressive yield stress of sintered 3% Co-WC indicate that elastic stresses in excess of 1000 kg/mm² might be reached in small WC particles.     

Characterization of microstructure and precipitation behavior in Al-4Cu-xTiB2 in-situ composite

Al-4Cu-xTiB₂ (x=0, 2.5, 5, 10 wt %) in-situ composites were prepared by a mixed salt route technique. The composites were characterized by X-ray diffraction techniques, to confirm that no Al₃Ti has formed, which is the advantage of mixed salt route technique. The scanning electron microscopy (SEM) analysis was carried out to determine the size distribution of TiB₂ particles in the matrix. The results showed that there was no agglomeration of TiB₂ particles throughout the matrix. The differential scanning calorimerty (DSC) studies were performed on the alloys as well as on composites to identify and characterize the precipitation sequence G.P.(I)→G.P.(II)/θ″→ θ′→ stable θ. To understand the precipitation kinetics of these precipitates in the presence of TiB₂, the solutionized samples were heat treated at different temperatures of precipitation as indicated by the DSC Thermogram and subsequently quenched to room temperature to retain the precipitates that form at corresponding high temperatures. The TEM analysis was carried out to characterize the crystal structure and morphology of the different precipitates. The analysis suggested that the precipitation occur primarily on the dislocations in the matrix as well as in the TiB₂ particle/Al-Cu matrix interface dislocations. It is believed that these dislocations are generated to accommodate the strain due to the difference in the coefficient of thermal expansion (CTE) of the TiB₂ particles and the Al-Cu matrix. TEM results displayed that the interface contained large amount of dislocations which may possibly accelerate the precipitation sequence.     

Thermal and Mechanical Properties of ACoO3 (R = Sm, Tb, Dy, Ho, and Er)

The thermal and mechanical properties of orthocobaltates, ACoO₃ (A = Sm, Tb, Dy, Ho, and Er), have been investigated using the modified rigid ion model (MRIM) by incorporating the effect of lattice distortions. We have computed the variations of specific heat and thermal expansion coefficient for these orthocobaltates in wide temperature range of 1 K (?272 °C) ≤ T ≤ 1000 K (727 °C). The calculated results of specific heat, thermal expansion, bulk modulus, and other thermal and mechanical properties accord very well with the available experimental data, implying that MRIM represents properly the nature of the perovskite-type rare earth cobaltates. In addition, we have also reported the results on molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (?), Debye temperature (θ _D), and Gruneisen parameter (γ).