Advanced Materials Research Vols. 89-91

Abstract: The Forming Limit Diagrams (FLD) are widely used in the formability analysis of sheet metal to determine the maximum strain, which gives the Forming Limit Curve (FLC). It is well known that these curves depend on the strain path during forming and hence on the test method used to calculate them. In this paper, different stretching tests such as the Nakajima and the Marciniak tests were performed, with different sample geometries to obtain points in different areas of the FLD. An optical analysis system was used, which allows following the strain path during the test. The increasing use of advanced high-strength steels (AHSS) has created an interest in determining the mechanical properties of these materials. In this work, FLCs for a TRIP steel were determined using Nakajima and Marciniak tests, which revealed different strain paths depending on the type of test. Determination of the FLCs was carried out following the mathematical calculations indicated in the ISO 12004 standard and was also compared with an alternative mathematical method, which showed different FLCs. Finally, the tests were verified by comparing the strain paths of the Nakajima and Marciniak tests with a well-known mild steel.

Abstract: A comparative study of two analytic solutions for the classical plasticity of incompressible materials and a plasticity model for powder materials is made to reveal an effect of plastic compressibility on qualitative behaviour of the velocity field in the vicinity of maximum friction surfaces.

Abstract: Magnesium alloys are expected to be used widely as structural materials because of their lowest density (1.8g/cm3) among all practical alloys and superior specific strength. However, magnesium alloys exhibit poor ductility due to its hcp structure and inactiveness of non-basal slip systems below 523K. Accordingly, magnesium alloy sheets had to be rolled at elevated temperature to avoid edge cracking and fracture during rolling. The present authors succeeded in single pass large draught rolling of AZ31 magnesium alloy sheets below 473K without heating rolls by rolling at the speed higher than 1000m/min. The rolled and quenched sheets had fine recrystallized microstructure and exhibited excellent mechanical properties. It was found that the high speed rolling is a promising method not only for increasing productivity but also for controlling microstructures and improving mechanical properties. If the above mentioned advantages of high speed rolling can be drawn from the rolling at the speed lower than 1000m/min, it is possible to mass-produce magnesium alloy sheets having superior mechanical properties at lower cost. In this study, we tried to determine the lower limiting rolling speed at which we can obtain advantages of high speed rolling. We revealed that the thickness could be reduced about 60% by single pass operation even at 250m/min without heating rolls. The rolled and quenched sheets had equiaxed fine recrystallized microstructure. For example, the mean grain size of 2.1m was obtained in the AZ31B sheet rolled at 250m/min at room temperature to the reduction of 60%.

Abstract: The effect of Al addition on the elastic modulus and aging behavior in Ti-10 Cr alloys was investigated by means of electrical resistivity, X-ray diffractometry and Vickers hardness measurements. All of the alloys used were formed from solutions treated at 1173 K for 3.6 ks and then quenched into ice water (STQ). Following STQ, all Al-containing specimens exhibited a  phase, with the athermal  appearing only for alloys with zero Al addition. The elastic modulus was found to decrease with addition of Al from approximately 80 to 70 GPa, due to the suppression of the athermal . The specimens following STQ were isothermally aged at 573 K, 673 K and 773 K. The addition of Al was found to retard the onset of precipitation of the isothermal  phase and decrease the upper limit temperature for precipitation of this phase. On the other hand, as the Al content was increased, precipitation of the  phase was accelerated in the presence of an existing isothermal  phase. By contrast, this precipitation was suppressed under single  phase conditions. Surface modification for osteointegration was also performed. When the modified specimens were immersed in simulated body fluid, the surface modification was found to promote the deposition of HAp.

Abstract: The study relates to joints fabricated by solid state bonding between alumina and nickel alloy HAYNESTM214®, using an intermediate nickel metallic foil. Experimentally, damages and cracks often are observed close to the metal/ceramics interface. Consequently, the residual stresses distributions in the specimen were characterized experimentally using X-ray diffraction (XRD) and indentation techniques and predicted by Finite Element Analysis (FEA) calculations using an elastic-plastic-creep model. We demonstrate that a good correlation between FEA calculations and experimental results is obtained. Then, the effect of elaboration and geometrical parameters has been studied in order to minimize the residual stresses in alumina close to the metal-ceramics interface. However, the Al2O3/Ni/HAYNESTM214® system always leads to high residual stresses. To solve this problem, we show that the use of a multi-layer Cu/Ni/Cu joint, associated with the Direct Copper Bonding method (DCB), by pre-oxidation of copper, allows reducing significantly the tensile residual stresses in ceramics.

Abstract: The evolution of stored energy and associated thermal behaviour was investigated for an ultrafine grained Ti-IF steel severely deformed by Equal Channel Angular Pressing (ECAP) followed by cold rolling at ambient and liquid nitrogen temperatures. Bulk stored energy measurements by Differential Scanning Calorimetry (DSC) returned 350-600 whereas local stored energy estimates from microhardness, Electron Back-Scattering Diffraction (EBSD) and X-ray line profile analysis resulted in 5-140 . Higher bulk stored energy values correspond to the enthalpy release from all sources of strain in the material volume as well as Ti precipitation during annealing while the lower local stored energy range alludes only to dislocation content or internal stresses. An apparent activation energy of 500-550 suggests sluggish recrystallisation due to excess of Ti in solid solution.

Abstract: Recently identified re-solidification behaviour of heavily alloyed single crystal super¬alloys has been incorporated into the Bridgman method. The process variables of resolidifcation temperature and mould withdrawal rate have been optimized. The microstructure, microporosity and eutectic phase fraction of re-solidified specimens were analyzed and compared with conventionally cast single crystal specimens. The potential advantage of this modified method has been illustrated by performing solution heat treatments on both re-solidified and normally cast specimens followed by eutectic phase fraction analysis.

Abstract: This work deals with the development of residual stresses during nitriding of steels. The main features of a chemico-thermo-echanical model of nitriding are presented. A micro-macroapproach is applied based on volume change computation in agreements with thermochemical modifications. Results are correlated with the characterization of a ternary Fe-C-3w.%Cr alloy nitrided at 550°C for various time. Residual stress-depth analyses are carried out by X-rays diffraction. Residual stress generation is deeply dependant on chemical and thermodynamical evolutions during the treatment, taking advantage on microstructural effects.

Abstract: In this article a numerical model to describe the mechanical behaviour of nanophased singlecrystalline Ti3SiC2 is proposed. The approach is a two dimensional finite element periodic unit cell consisting of an elastic matrix interlayered with shear deformable slip planes which obey the Hill’s yield criterion. The periodic unit cell is used to predict compression material behaviour of Ti3SiC2 crystals with arbitrary slip plane orientations. Stress strain relationships are derived for Ti3SiC2, and the effect of slip plane volume fraction as well as orientation of the slip planes are investigated. The two main deformation mechanisms of the material namely; ordinary slip and so called kinking are considered in the study.

Abstract: In the present study, the process modelling of AMCs linear friction weldment was carried out. Four major stages of the process (Part 1: Warm-Up; Part 2: Osci-Forging; Part 3: Forging; Part 4: Cool-Down) were identified and simulated consecutively to generate the temperature field and residual strains distribution within the model. An eigenstrain model calibrated by the neutron diffraction results was also employed to capture the permanent deformation distribution. Good agreement between the process modelling and the experimental measurements was found.