Key Engineering Materials Vols. 345-346

Abstract: Tensile deformation behavior of the high-nitrogen austenitic Fe-18Cr-14Mn-4Ni-3MoxN steel with various nitrogen contents has been studied. The nitrogen content of the steel varied from 0.28 to 0.88 wt. %. Nitrogen atoms in high nitrogen steel (HNS) make an interstitial solid solution by being scattered in the steel constituting a short-range order. They strengthen the austenite matrix without deteriorating ductility of the steel. The present investigation was carried out to elucidate the hardening and plasticizing role of the nitrogen in the HNS by analyzing tensile deformation behavior of the steel containing various nitrogen contents. Tensile tests of the steel specimens were performed at room temperature with a constant strain rate of 5x10-5/sec. Microstructure of the tested specimens was analyzed to explore the deformation mechanism of the HNS as a function of nitrogen contents. The flow stress of the steel increased with the increase of the nitrogen content; however, the specimen with the highest nitrogen content (0.88 wt. %) showed saturated strength and reduced ductility. The superior mechanical property of the HNS was explained by the low stacking fault energy and the twin-induced plasticity provoked by the nitrogen.

Abstract: In the multi-stage deep drawing processes of a beta titanium alloy sheet, the formability has been investigated. The beta titanium alloy sheets have sufficient ductility at room temperature, whereas a seizure tends to occur during deep drawing due to high reactivity with other materials. To prevent the seizure, the beta titanium alloy sheet was treated by oxide coating heating, because the coated sheet was not in direct contact with the die during deep drawing due to the existence of the oxide layer. The blank used was the commercial beta titanium alloy Ti-15V-3Cr-3Sn-3Al. The effect of the coating condition on the formability in the multi-stage deep drawing process was examined. It was found that long drawn cups with a height sixfold that of the diameter were successfully formed by oxide coating heating.

Abstract: The effect of deformation route on the mechanical properties of grade-1 CP-Ti deformed by equal-channel angular pressing (ECAP) was studied using tensile testing, TEM observation, Xray pole figure measurement, and ODF calculation. Route Bc showed high yield stress and comparatively large uniform elongation, thus high ultimate tensile strength, because of its fine grain structure with high angle grain boundary. The reason for this was because of the occurrence of prismatic slip in route Bc, in addition to the commonly occurring basal slip. Route C showed surprisingly high yield stress despite of its unfavorable grain structure because of the significant contribution of texture hardening.

Abstract: In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals, c-axis compression was performed. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression is bigger than that of a-axis tensile. {10-13} twin and {11-24} twin occurred at 77
293K and 77
473K respectively. Fracture surface at 77
293K was {11-24} and at 473K was {11-22}.

Abstract: Gradient-dependent plasticity considering the microstructural effect is introduced into Johnson-Cook model to calculate the nonuniform temperature distribution in adiabatic shear band (ASB) and the evolutions of average and peak temperatures in ASB. Effects of initial static yield stress, strain-hardening coefficient, strain-hardening exponent, strain-rate parameter and thermal-softening parameter are numerically investigated. The calculated peak temperature in ASB considering both the plastic work and the microstructural effect is always greater than the average temperature calculated only using the plastic work. For much lower flow shear stress, the peak temperature approaches two times the average temperature. The occurrence of phase transformation in ASB is easier in metal material with higher initial static yield stress, strain-hardening coefficient, strain-rate parameter and thermal-softening parameter. At much lower flow shear stress or much higher average plastic shear strain, the phase transformation occurs more easily in material with a lower strain-hardening exponent. Traditional elastoplastic theory without the microstructural effect underestimates the peak temperature in ASB so that the experimentally observed phase transformations cannot be explained.

Abstract: Generally, plastic deformation of magnesium alloys is difficult at room temperature. In order to improve formability of magnesium, impurity elements in magnesium were reduced by vapor deposition technique. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was applied to the determination of trace elements in refined magnesium. To investigate influence of impurity element to deformation behavior, high purity magnesium single crystals were prepared. When the magnesium single crystals are stretched in <11-20> direction, {11-22} <-11-23> pyramidal slips were activated just after yielding in the range of 77K to 293K. The yield stress of high purity magnesium was a half of the stress in raw magnesium.

Abstract: The study of girder-to-column joints under experiment and numerical analysis was carried out to evaluate change of the flexural capacity of the joints with the 2-layer upper reinforcement of girder within rectangular section and the single-layered upper reinforcement at the girder flange. The distribution of the upper reinforcement of girder within the effective flange width was the variable in the experiment and numerical study. The ultimate strength of the specimen with 2-layer upper reinforcements of girder was larger than that of the single-layered specimen by maximum 17.4%. Based on the results of this experimental study, for the single-layered tension reinforcement it is recommended to provide 15% additional reinforcement at twice slab thickness on each side of the main girder to attain the flexural capacity equal to the 2-layered case.

Abstract: The stress-strain characteristics and plastic behavior of aluminum alloy were examined by tensile test and infrared thermo-viewer. This thermo viewer is a device for converting a thermal radiation pattern from an object into visible images. Al-Mg alloy plates with different crystal grain size subjected to a uni-axial tensile load were measured continuously by this thermo viewer. The strain concentration pattern was analyzed by the differential thermal image and the macroscopic estimation method by the thermal image processing was proposed and the propriety of FE simulation based on polycrystalline plasticity model was shown.

Abstract: Autofrettage is used to introduce advantageous residual stresses into cylinders. The Bauschinger effect can produce less compressive residual hoop stresses near the bore than are predicted “ideal” autofrettage solutions. A723 steel is used for compound cylinder. This paper extends the analysis to material the addition of pressure or of shrink-fitting to the cylinders, providing associated residual stress profiles following various amounts of further yielding due to a net external pressure. The Bauschinger effects for “realistic” – Bauschinger effect dependent autofrettage are obtained. The 2-D analysis is performed via the finite element method. The Bauschinger effect is found to significantly lower the beneficial stress due to autofrettage.

Abstract: The time-temperature dependence of the compressive behavior of polypropylene (PP) foam was investigated to make predictions about what sort of behavior for wide ranges of temperature and strain rate. Compressive stress relaxation tests were conducted at 213 K and 373 K. Compression tests were also conducted. The strain rate was 2×10-3 1/s at 213 K and 373 K. The compressive stress-strain curves were roughly linear and dependent on temperature until the maximum stress was reached. The maximum stress occurred at 5% strain regardless of temperature. The plateau stresses decreased as temperature increased. By plotting compressive behavior of the PP foam at the master curve of the stress relaxation modulus, its temperature dependence could be explained by the thermo-viscoelastic properties. Therefore, the behavior of PP foam at different strain rates could be approximately predicted from the stress relaxation modulus with the timetemperature equivalence principle.