Second-order stresses and strains in heterogeneous steels: Self-consistent modeling and X-ray diffraction analysis

Theoretical and experimetal methods have been developed to characterize the effect of mechanical loading on the mesoscopic and macroscopic mechanical state of polycrystalline materials. Ferritic and austenitic single-phase materials were first analyzed, then phase interaction was studied in a multiductile phase material (austeno-ferritic duplex steel) and a natural reinforced composite (pearlitic steel). The theoretical method is based on the self-consistent approach in which elastic and plastic characteristics of the phases have been applied through the micromechanical behavior of single-crystal-using slip systems and microscopic hardening. The effects of a crystallographic texture and phase interaction during loading and after unloading were studied. The elastic and plastic anisotropy of the grains having the same crystallographic orientation were assessed by diffraction strain analysis. The simulation was compared with the experiments performed using the X-ray diffraction technique. In the considered duplex and pearlitic steels, it was observed that the ferrite stress state is much lower than the austenite and cementite ones. The results of diffraction strain distribution have showed the pertinence of the models and give valuable information, for example, for the yield stress and the hardening parameters of each phase in a two-phase material.     

Study on TiAl2-based ternary (Fe or Ni) titanium aluminides

Several ternary alloys were designed to help understand the type of substitution and the nature of transformation in L1₂-type ternary (Fe or Ni) titanium aluminides. X-ray powder diffraction analysis, scanning electron microscopy and microhardness measurements were used to delineate the phases present in arc-melted alloys that were annealed at 1300 K for 10 days. The results of this study show that, in L1₂ type ternary (Fe, Ni) titanium aluminides, Fe or Ni substitutes for Ti, and these materials are TiAl₂-based, contrary to previous assumptions.     

Characterization of shock-hardened Al-8090 alloy

The structure and mechanical properties of Al-Li8090 alloy, that was dynamically deformed and then age hardened, were studied as a function of the changes in the nature and amount of precipitates produced. A comparison was made between two groups of samples, one group that was solution heat treated (SHT) and quenched from 530°C before the dynamic deformation and the other group that was dynamically deformed in the as-received (AR) condition. The higher values for microhardness and ultimate tensile strength observed (138 and 140 VHN, and 405 and 458 MPa, respectively), subsequent to shock treatment (ST), have been attributed to the increase in dislocation density and grain-boundary precipitation produced due to shock deformation. Dislocations and grain boundaries were assumed to act as precipitation sites and an increase in dislocation density, due to ST, was expected to increase precipitation density of (Al₃Li), S(Al₂CuMg), and T₁(Al₂CuLi) phases which, in turn, are expected to increase strength properties of the alloy. Differential scanning calorimetry showed that, for the species that precipitate below 180°C, (Al₃Li) and GP zones, an increase in the amount of deformation increased the precipitation temperatures. However, for the species that precipitate at 197°C, S(Al₂CuMg), an increase in the amount of deformation produced a decrease in its precipitation temperature. These results have been partially confirmed by the activation energy calculations for temperatures below 197 °C, which show a decrease of precipitation energies with an increase in the amount of deformation. Activation energies calculated from ageing curves showed that when ageing at low temperature (165–180 °C range), activation energies for the precipitation process are decreased upon increase in cold work. Shock treatment of SHT samples exhibited decreased activation energy values of precipitation, from 36.14 kcal mol^–1 for the SHT sample to 24.18, 24.08, and 21.00 kcal mol^–1 for SHT + ST samples 1, 2, and 3, respectively (corresponding to 1, 2, and 3 sheets of explosive). Activation energies of precipitation for AR + ST samples showed even lower values; 9.45, 9.95, and 8.21 kcal mol^–1 for samples 4, 5, and 6, respectively. These activation energies strongly corroborate the role of defect substructure on the age-hardening kinetics of this alloy.     

High-temperature crystallization behaviour of amorphous Fe80B20

Samples of 0.003 in. round Fe₈₀B₂₀ amorphous wires were annealed in vacuo for 1 sec to 8 h periods at 780° C and the crystallinity induced in these wires from this heat treatment was studied through X-ray diffraction and field-ion microscopy. X-ray diffraction studies indicate that complete crystallinity is produced following 1 sec anneal at 780° C. However, the initial product is a primitive-tetragonal Fe₃B phase unlike the body-centred tetragonal Fe₃B observed in low-temperature isothermal transformation studies with this alloy. The Fe₃B phase is seen to persist in the diffraction patterns for annealing durations of up to 15 min. Upon annealing for periods of up to 1 h, an intermediate three-phase structure consisting of -Fe, Fe₃B, and Fe₂B is seen to result with a gradual decrease in the Fe₃B phase corresponding to longer annealing durations. Anneals of more than 1 h at 780° C are seen to result in the disappearance of the Fe₃B phase producing a two-phase microstructure consisting of -Fe(b c c) and Fe₂B (b c t). Field-ion-microscopy with a pure neon imaging gas at 78 K likewise indicates that existence of a three-stage phase structural change during the isothermal anneals, and the atomic arrangement of the various species are quite readily discernible because of the different symmetries contained in the three distinct phases.     

Annealing behaviour of amorphous Fe80B20 on continuous heating

Resistance measurements during direct heating of Fe₈₀B₂₀ amorphous alloys indicate phase changes occur at 395, 500, 720 and 840° C. Samples heated to these temperatures, and maintained for five minutes in a neutral atmosphere, show that a hardness maximum occurs at the crystallization temperature of 395° C and that annealing at 500° C produces a material with the same hardness. Above 500° C the microhardness is seen to drop below that of the amorphous alloy. Saturation magnetization measurements show a steady increase following each anneal, up to a temperature of 720° C, and the rate of increase is seen to drop in the range of 720 to 840° C. X-ray diffraction studies show that only a small fraction of the matrix is crystallized following the anneal at 395° C and the transformed phases are -Fe and Fe₃B. Following annealing at 500° C, an increased proportion of -Fe and Fe₃B are observed with complete crystallinity while samples heattreated at 720° C are seen to consist of a three-phase mixture of -Fe, Fe₂₃B₆ and Fe₂B. Annealing at 840° C is seen to produce an equilibrium phase mixture of -Fe and Fe₂B phases. Only in the sample annealed at 395° C is a fraction of the amorphous phase seen to persist, indicating that a 5 min anneal is not sufficient, at this temperature, to induce complete crystallization. These structural features are corroborated by field ion microscope analyses, made at liquid nitrogen temperature in a medium of pure neon, and scanning electron microscopy, and are also consistent with our earlier study involving the isothermal annealing, for various times, of Fe₈₀B₂₀ alloy at 780° C.     

Impact of variable frequency microwave and rapid thermal sintering on microstructure of inkjet-printed silver nanoparticles

The effect of thermal profile on microstructure is studied in the frame of thin films deposited by inkjet-printing technology. The role of sintering temperature and thermal ramp is particularly investigated. Fast heating ramps exhibit coarse grains and pores, especially when a hybrid microwave curing is performed. This enhanced growth is attributed to the quick activation of densifying sintering regimes without undergoing thermal energy loss at low temperature. Microstructural evolution of various sintered inkjet-printed films is correlated with electrical resistivity and with the Young’s modulus determined by nanoindentation. A strong link between those three parameters is highlighted during experiments giving credit to either a surface or a fully volumetric sintering, according to the process. Sintering is then mainly triggered by surface mass transfer or by grain boundary diffusion. Silver thin films with an electrical resistivity 4–5 times higher than the bulk, has been reached in a few minutes with a Young’s modulus of 38?GPa.     

A methodology for predicting regional freight traffic in developing countries

The prime concern of transportation planners in developing countries is how to collect and transfer data into models as simple and as effective as possible and obtain solutions in the shortest time.

In this study, a methodology which utilizes multivariate statistical analysis techniques for travel estimation is presented. The two simple and convenient techniques namely Cluster Analysis and Principal Component Analysis, are used for the evaluation of available data. The traffic demand is expressed as a function of principal components which are determined independently as a small set of variables to represent total system variability. A stepwise regression analysis is carried out to derive the traffic demand‐principal component relationship. The proposed methodology is then applied to Southeastern Anatolia Regional Development Project in Turkey.     

A new strain hardening model for rate-dependent crystal plasticity

This paper presents a crystal plasticity based finite element analysis employing the new microstructure-based strain hardening model recently presented by Saimoto and Van Houtte (2011) [7] to simulate formability and texture evolution in the commercial aluminum alloy 5754. Simulations are performed to compare the predictive capability of the new hardening model against the common work hardening models using a rate-dependent plasticity formulation. The parameters in the numerical models are calibrated using the X-ray data published by Iadicola et al. (2008) [9] for the aluminum sheet alloy 5754. The predictions of the model for balanced biaxial tension and in-plane plane-strain tests are compared against experimental observations presented in Iadicola et al. (2008) [9]. It is concluded that the new model provides the best predictions of the large strain behavior of Aluminum sheet alloy 5754 subjected to various strain paths.