Nanocrystalline (nc) Al-Pb two phase mixtures of different Pb concentrations were made by two different routes using high energy ball milling. The microhardness measurements show a softening in nc Al-Pb composites with the increase in Pb content, contradicting the previous results reported in the literature. We conclude that interaction of Pb atoms with nanocrystalline Al grain boundaries is responsible for the softening of the nc Al matrix observed in the current study. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
This work gives an overview of the theoretical background and of the numerical modelling framework used to describe the mechanical properties and the response of materials on scales ranging from the atomistic, through the microstructure and all the way up to the macroscale. In order to describe the dual nature of the structure of matter, which is continuous when viewed at large length scales and discrete when viewed at the atomic scale, plus the interdependence of these scales, multiscale modelling is required to complement the continuum and the atomistic models. More specifically, what we aim for in this review is to present and discuss the following basic conceptual models, as well as the methodologies that accompany them: (a) discrete models such as ab initio, atomistic / molecular, mesoscopic; (b) continuum mechanics models (CMM) comprising pure CMM, non-local elasticity CMM, higher-order strain gradient and higher-order nonlocal strain gradient elasticity CMM, and surface elasticity CMM; (c) multiscale material models (MMM). Since the field of nanomechanics is currently a rapidly expanding research area, the presented state-of-the art is by no means exhaustive. It simply outlines the research efforts that go behind formulating numerical models for the solution of problems in nanomechanics. Despite the advantages that boundary element methods (BEM) have in solving problems at the physical scale, either as stand-alone or in combination with finite element methods (FEM), their application to multiscale modelling is still limited, despite the promise they seem to hold.
17-4 PH stainless steel and Al2O3 ceramic were successfully vacuum brazed using graphene reinforced Ag-Cu-Ti brazing alloy. In this paper, the effects of operating parameter such as brazing temperature, holding time and graphene amount on the microstructure and mechanical properties of the brazed joints were systematically studied. The experiment revealed that with the elevation of brazing temperature and holding time, the shear strength of 17-4 PH stainless steel and Al2O3 ceramic joints brazed with Ag-Cu-Ti filler alloy increased first, getting the maximum 170 MPa at 880 °C for 10 min and then declined. When the joint brazed with 0.1 wt % graphene reinforced Ag-Cu-Ti composite filler, the maximum shear strength reached 212 MPa that was 25% higher than the joint with single Ag-Cu-Ti filler metal. Moreover, fracture path of the 17-4 PH stainless steel and Al2O3 ceramic joint with 0.1 wt % graphene reinforced Ag-Cu-Ti was changed from the ceramic to the reaction layer that was closed to Al2O3 substrate. The improvement of the joint shear strength was chiefly benefited from the formation of TiC particle that reinforced brazing seam and the refinement of joint microstructure.
Interdiffusion in single-phase diffusion couples of TiAl and Ti3Al has been investigated over the temperature range of 1118 to 1583 K. Concentration distributions after annealing were measured by an electron probe microanalyzer (EPMA). Interdiffusion coefficients were calculated according to the Boltzmann-Matano method. No significant concentration dependence of the interdiffusivities in both compounds was detected. The temperature dependence of the diffusivities follows Arrhenius laws with a pre-exponential factor D0 = 2.8 × 10−4 m2 s−1 and an activation enthalpy Q = (295 ± 10) kJ mol−1 for TiAl, and D0 = 1 × 10−3 m2 s−1 and Q = (312 ± 6) kJ mol−1 for Ti3Al, respectively. The present results are discussed together with Ti self-diffusion data for TiAl and Ti3Al from tracer-diffusion experiments.
Solid state actuation characteristics of CuAlMn shape memory alloys (SMAs) are reported at extreme low temperatures (
During the past decades particle reinforced light metal matrix composites have established a strong position as candidate materials for structural parts requiring a high stiffness-to-weight ratio. More recently, steel matrix composites have been proposed as inexpensive wear resistant materials as substitutes for the more expensive WC-Co cermets. However, steel is a polymorphous metal and, contrary to aluminium, therefore also solid state phase transformations ought to be considered. Since technologically interesting steel matrix composites have in many cases martensitic matrix structures, the effect of particle reinforcement on austenite decomposition and martensite formation in a steel matrix composite is studied in this work.
In situ (Al3Ti + Al2O3)/Al composites were fabricated from powder mixtures of Al and TiO2 using hot pressing, forging and subsequent multiple-pass friction stir processing (FSP). The reactive mechanism and mechanical properties of the FSPed composites were investigated. Four-pass FSP with 100% overlapping induced the Al–TiO2 reaction, as a result of the enhanced solid diffusion and mechanical activation effect caused by the severe deformation of FSP. Decreasing the size of TiO2 from 450 to 150 nm resulted in the formation of more Al3Ti and Al2O3 particles. The formation mechanisms of Al2O3 and Al3Ti during FSP are understood to be a deformation-assisted interfacial reaction and deformation-assisted solution-precipitation, respectively, based on detailed microstructural observations. The microhardness, Young’s modulus and tensile strength of the FSPed composites were substantially enhanced compared with those of FSPed pure Al with the same processing history, and increased as the TiO2 size decreased from 450 to 150 nm. The strengthening mechanisms of the FSPed composites included load transferring, grain refinement and Orowan strengthening, among which Orowan strengthening contributed the most to the yield strength of the composites.
Evolution of nano-precipitates after Cryorolling (CR) followed by warm rolling (WR) at rolling temperatures of 100 °C, 145 °C and 175 °C on the microstructure and strengthening of Al-6061 alloy has been investigated. The enhanced yield (401 MPa) and tensile strength (415 MPa) with 6% ductility was achieved after CR plus 80% WR. The evolution of various precipitates such as G.P. zones, Cluster/co-cluster, β' and β" after CR plus WR was investigated, by studying the thermal behavior using Differential scanning calorimetry studies (DSC). Time of the optimized artificial ageing temperature (125 °C for 45 Hrs) was obtained using hardness testing and DSC study. Significant improvement in ductility (9%) of CR plus 80% WR Al 6061 alloy after peak ageing showing 450 MPa tensile strength was obtained, due to the evolution of highly dense coherent nano-precipitates (β") responsible for uniform flow localization. A graphical model is proposed explaining the effect of solution treatment, deformation temperature, dislocations and peak ageing on evolution of precipitates.
Present work deduces the development of Nd3Pr3Fe67Co3Nb3Ti1B20 nanomagnets produced through magnetic annealing the amorphous precursors prepared through rapid solidification technique. Changes in structure and magnetic properties were investigated at as-cast and annealed stages. Results revealed that magnetic field annealing stimulates the kinetics of crystallization and modifies the structure and magnetic properties of the Nd3Pr3Fe67Co3Nb3Ti1B20 nanomagnets. Thermal analysis showed a glass transition temperature at 845 K and a crystallization temperature at 890 K for the alloy. X-ray diffraction studies demonstrated that as-cast alloy has amorphous structure while annealed magnet has multi-phase crystalline structure. Phase analysis elucidated that optimal annealed structure is compose of 34% Nd2Fe14B, 32% Pr2Fe14B, 21% α-Fe and 13% Fe3B phases. The HRTEM studies provoked that magnet microstructure consists of 55 nm Nd2Fe14B, 50 nm Pr2Fe14B, 24 nm α-Fe and 20 nm Fe3B magnetic grains which were interacted through ultra-thin grain boundaries. Henkel plot showed that grains of hard magnetic Nd2Fe14B (Pr2Fe14B) phase are coupled to grains of soft magnetic α-Fe (Fe3B) phase. Magnetic properties of nanocomposite magnets depend critically on the mass fraction of alloy constituent elements, casting conditions and heat treatment parameters. A small deviation of ±30 K from the ideal annealing temperature may affect the morphology of phases in the microstructure, which, in turn, influences the resultant magnetic properties of the final magnetic product. Optimal annealed Nd3Pr3Fe65Co3Nb3Ti1B20 rod magnet enunciated coercivity of 630 kA/m, remanence of 0.83 T and magnetic energy product of 84.3 kJ/m3. The present research work opens a new way to manufacture high performance magnetic components for advanced electronic devices, magnetic systems and magnetic recording media.
Using the nonlinear Lamb wave, this work investigated the damage resulting from microstructure evolution during the creep of 9Cr1Mo steel at 873 K in detail. At eight creep times, specimens of this steel were firstly prepared by the interrupted test to simulate various creep states, respectively. Then, the measurement of acoustic nonlinearity parameter, and microstructure observation were employed to characterize the resultant damage. The experiment results showed that the measured value of this parameter is strongly dependent upon the creep state. Meanwhile, the hierarchical microstructure of 9Cr1Mo steel evolves during creep. In order to conduct the quantitative assessment, both the analytical model of acoustic nonlinearity, and the kinetics of microstructure evolution were applied to calculating the acoustic nonlinearity parameter during creep. By comparison of the measured and calculated results, the predominated component of acoustic nonlinearity during the present creep of 9Cr1Mo steel was clarified successfully by the modeled microstructures. Above all, the present work provided an effective way to monitor and evaluate the creep damage quantitatively.
Factors limiting the strain rate of superplastic deformation in ceramic materials are discussed on the basis of existing models and experimental results concerning high-temperature plastic deformation, intergranular cavitation and dynamic grain growth. From the discussion, it is indicated that simultaneously fulfilling the following conditions is essential for attaining high-strain-rate superplasticity (HSRS) in ceramic materials: reduction in the initial grain size, enhanced diffusivity, suppressed dynamic grain growth, a homogeneous microstructure and a reduced number of residual defects. In the light of these conditions, explanations are given for HSRS attained in earlier studies on some oxide materials. It is also shown that HSRS can be intentionally attained in doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and composites synthesized from ZrO2, Al2O3 and MgO2; the tensile ductility of these composites reached 300–2500% at a strain rate of 0.01–1.0 s−1. The postdeformation microstructure indicates that some secondary phases may suppress cavitation damage and thereby enhance HSRS.
Deformation at the notch tip of ice single crystals was studied by in-situ X-ray topography. The plastic deformation of ice is mainly caused by the characteristic behavior of the dominant dislocations which lie on the basal plane. Short segment of dislocations on the non-basal plane play an important role in the generation of screw dislocations on the basal plane. The dislocation/crack tip interaction mechanism depends strongly on the crystal orientation relative to the loading direction and the crack plane, i.e. the slip system with the highest Schmid factor is the most active in single crystal ice. No dislocation free zone (DFZ) was found in any of the specimens. Etch pit and replication method has the limitation for observing dislocations in ice crystals.
In a number of papers serrations on the tensile curve during the Portevin-Le Chatelier (PLCh) deformation (1) (often referred to as jerky flow) were used as the feature of the phenomena from which the information about its nature may be deduced. It is noticeable however, the regardless the conditions of deformation (temperature, strain and strain rate), the amount and frequency of serrations depend on structure of material, especially grain size. It seems, therefore, necessary that the mechanical parameters of the test to be supplemented by appropriate observation of slip pattern in the material and this is the purpose of these studies.
In any composite development effort it is important to demonstrate that modifications in constituents or in the fabrication process do not adversely affect the other constituents or the resulting composite properties. Previous work has demonstrated that the extracted fiber strength distribution in as-consolidated composites is different than typical virgin fiber strength distributions and that composite heat-treatments in an inert environment may further degrade fiber strength. The degree to which a composite can withstand elevated temperature exposure depends on the kinetics of the fiber-matrix interaction which is related to the matrix chemistry and microstructure. Smith et al. have characterized the reaction kinetics of several Ti-Al-Nb alloys with SiC fiber, but the effect of the reaction on fiber strength had not been measured. The goal of this work was to establish the effect of composite consolidation in a Ti-22Al-23 Nb (atomic percent) matrix on the fiber strength distribution. The consequences of post-consolidation heat treatments, as well as prolonged high temperature exposure, on fiber strength were also evaluated.
Numerical simulations of the dynamics of discrete dislocations exhibit chaotic behavior. The effect of chaos on the computed overall tensile stress–strain response is small, but the effect on the crack growth resistance is significant.
The grain boundary diffusivities sδDb of 48V in binary Fe-Cr alloys with 8–12 wt.% Cr, in a ternary Fe-8Cr-0.17C alloy and in two commercial modifications of 8% Cr ferritic steels were measured by the serial sectioning method in the temperature range 743 – 1103 K. A variation of the Cr concentration in the range 8–12 wt.% does not influence the value of sδDb. The GB diffusivity of vanadium in steels of commercial purity is about four times slower than in the materials prepared of pure constituents.
A powder metallurgy nickel-based superalloy EP962NP was subjected to solid solution treated and cooled in four media (water, oil, air and furnace) followed by aging treatment to investigate the correlation between secondary γ′ (γs′) and tensile properties of the experimental alloy at 750 °C. Results show that the size of γs′ increases sharply with the decreasing cooling rate, and morphologies of γs′ depend on the cooling regime and present various characteristics including spherical, rod-shaped, cubic, etc. By implementing the heat treatment process of solid solution treatment (1210 °C/2 h, air cooling/AC) and two-step aging treatment (i.e., 870 °C/8 h, AC + 760 °C/16 h, AC), superior comprehensive mechanical properties can be obtained with ultimate tensile strength, yield strength and elongation of 1251 MPa, 1079 MPa and 13.3%, respectively, due to the high density of stacking faults. Interaction between γ′ and dislocations indicates that the deformation mechanism of the alloy exhibits stacking faults on (1‾11‾) [121] and microtwins, which depends on the size of γs′. The increasing size of γs′ precipitates drives a transition in the dominant deformation mechanism from stacking faults shearing to microtwinning.
The Ti–Al–Cr ternary system is one of the most important systems to studying the titanium alloys. Some experimental data of this ternary system are available and a few partial thermodynamic assessments are reported. However, no full thermodynamic descriptions were published. In this study, the previous work on the Ti–Al–Cr system and its related binary systems are reviewed. Based on the thermodynamic descriptions of the Ti–Al, Ti–Cr and Al–Cr systems and the ternary experimental data in literature, the Ti–Al–Cr ternary system is assessed by means of the Calphad method. Several isothermal sections from 1073 K to 1573 K and some invariant reactions are calculated, which are in good agreement with the most of the experimental results.
Gas tunnel-type plasma spraying (GTPS) was employed to deposit ZrO2/SiO2 bioinert ceramic composite coatings with an appropriate thickness on SUS 304 substrate. Zirconia and fused silica powders, with equal wt%, have been mixed together in ceramic pot for 30 min and internally fed in the plasma jet. The composite coatings were sprayed at two different gas flow rates (120 and 150 l/min) and constant vortex arc current of 450 A and gun current of 50 A. The microstructure of as-sprayed coatings was examined by scanning electron microscope. Elemental analysis was achieved for the composite coatings using EDS analysis unit which is attached to SEM. Phase structure was investigated by X-ray diffraction. The hardness and abrasive wear test of the coatings were investigated. The biological property of the coatings was examined by immersing the as-sprayed coatings in simulated body fluid (SBF) solution for 20 days at 36.5 ∘C. The growth of apatite (HA) on the coatings surfaces was observed by SEM and EDX analysis.
Fe-30Mn-6Si belongs to a multi-functional material and is one of the most attractive alloys for use in engineering applications, such as pipe joints, repairing bone defects in load-bearing areas, etc. The present work focuses on the effect of face-centered cubic (FCC) grain size on the FCC → hexagonal close-packed (HCP) martensitic transformation and mechanical properties of the Fe-30Mn-6Si biodegradable alloy (BA). The specimens which were fully recrystallized with different FCC grain sizes ranging from 8.2 to 15.0 μm were obtained by different annealing treatments following rolling. According to the results, Hall-Petch relationship of the Fe-30Mn-6Si BA was fitted as: σy = 58.9 MPa + 1080.4 MPa•μm1/2 dγ–1/2. The ultra-high K value (1080.4 MPa•μm1/2) indicates that the Fe-30Mn-6Si BA may exhibit ultra-high strength in ultra-fine grain structure. Remarkably, the Fe-30Mn-6Si BA annealed at 900 °C for 5 min overcame the strength-ductility trade-off which resulted in the higher yield strength, ultimate tensile strength and fracture elongation than the alloy annealed at 1000 °C for 5 min despite with a smaller grain size. In addition, compared to Mg- and Zn-based BAs, the Fe-30Mn-6Si BA exhibits superior mechanical properties.
