Key Engineering Materials Vols. 345-346

Abstract: The choice of a kinematically admissible velocity field has a great effect on the predictive capacity of upper bound solutions. It is always advantageous, in addition to the formal requirements of the upper bound theorem, to select a class of velocity fields satisfying some additional conditions that follow from the exact formulation of the problem. In the case of maximum friction law, such an additional condition is that the real velocity field is singular in the vicinity of the friction surface. In the present paper this additional condition is incorporated in the class of kinematically admissible velocity fields chosen for a theoretical analysis of two - layer cylinders subject to compression and twist. An effect of the angular velocity of the die on process parameters is emphasized and discussed.

Abstract: The paper deals with the deformation and transformation behaviour of thermomechanically (TM) treated low alloyed Si-Mn TRIP steel. The aim of this work was to investigate the contribution of the factors governing the deformation and transformation process of conditioned austenite. Variation in strain and temperature parameters of TM treatment of TRIP steel samples resulted in formation of different complex microstructures. The deformation behaviour of TRIP specimens of different multiphase structures was tested in incremental neutron diffraction in situ tensile testing. It was proved that neutron diffraction technique is very convenient method for retained austenite (RA) transformation of the retained austenite with respect to monitoring of transformation quantification of retained austenite and rising internal stress in structural phases.

Abstract: The article focuses on the results from recent experimental of severe plastic deformation of low carbon (LC) steel and medium carbon (MC) steel performed at increased temperatures. The grain refinement of ferrite respectively ferrite-pearlite structure is described. While LC steel was deformed by ECAP die (ε = 3) with a channel angle φ = 90° the ECAP severe deformation of MC steel was conducted with die channel angle of 120° (ε = 2.6 - 4). The high straining in LC steel resulted in extensively elongated ferrite grains with dense dislocation network and randomly recovered and polygonized structure was observed. The small period of work hardening appeared at tensile deformation. On the other side, the warm ECAP deformation of MC steel in dependence of increased effective strain resulted in more progressive recovery process. In interior of the elongated ferrite grains the subgrain structure prevails with dislocation network. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrain and submicrocrystalline structure. The straining and moderate ECAP temperature caused the cementite lamellae fragmentation and spheroidzation as number of passes increased. The tensile behaviour of the both steels was characterized by strength increase however the absence of strain hardening was found at low carbon steel. The favourable effect of ferrite-pearlite structure modification due straining was reason for extended work hardening period observed at MC steel.

Abstract: In the present study, a new approach is conducted to evaluate dislocation structure induced by cyclic plasticity. First, cyclic plastic loading tests are carried out up to 100 cycles with three different small strain amplitudes on SUS316L stainless steel at room temperature. The test result presents the dependence of the strain amplitude on cyclic hardening and softening behaviors. Specifically, it is found that the cyclic loading test with strain amplitude of 0.25% shows both cyclic hardening and cyclic softening, while the cyclic loading tests with strain amplitudes of 0.75% and 1.0% show no cyclic softening. Secondly, the dislocation structures of the specimens after cyclic loading are observed by using a transmission electron microscope (TEM), and this observation reveals that the dislocation structure after cyclic loading test depends on the strain amplitude. Finally, a quantitative evaluation method of the dislocation structure is also proposed. The TEM images are converted into binary images and the resolution dependence of the generated binary image is used to visualize the characteristics of the dislocation structure. The relationship between strain amplitudes of cyclic plasticity and dislocation structure organization is clarified by the evaluation method. Finally, the heterogeneity of the dislocation structure is discussed.

Abstract: A constitutive equation of rubber is derived by employing a nonaffine molecular chain network model for an elastic deformation behavior and the reptation theory for a viscoelastic deformation behavior. The results reveal the roles of the individual springs and dashpot, and the strain rate dependence of materials in the monotonic and cyclic deformation behaviors, particularly softening and hysteresis loss, that is, the Mullins effect, occurring in stress-stretch curves under cyclic deformation processes of carbon black filled rubber..

Abstract: Recrystallization and grain growth behavior of alloy 718 casting were investigated to obtain homogeneous microstructure during hot forging. For this purpose, compression tests were carried out for cylindrical specimens at the temperature range of 1000 to 1150°C and the strain rate of 10-1 and 10sec-1. The dynamic recrystallization behavior caused by the hot compression was investigated in terms of the recrystallized area fraction and average grain size. Reheating was followed to the hot compressed samples at the temperature range of 1050 to 1150°C for 100, 600 and 1800sec, and the static recrystallization behavior caused by the reheating was also investigated. As hot deformation temperature increased from 1000 to 1150°C, both the area fraction and average grain size of dynamically recrystallized grains increased. When higher strain rate of 10 sec-1 was used, the area fraction of dynamically recrystallized grains increased substantially, but the average grain size was not affected. When reheating the hot compressed samples at 1050°C for 100, 600 and 1800sec, respectively, microstructural change including grain growth was not noticed. On the other hand, when reheating the samples at higher temperatures, 1100°C and 1150°C, both the area fraction and the average grain size of the statically recrystallized grains increased considerably as the holding time increased from 100 to 1800sec.

Abstract: To study the effect of grain size on texture and tensile properties of equal channel angular pressed commercially pure Ti, specimens were made to have the grain size ranging from 4 m to 60 m with the severe plastic deformation followed by recrystallization annealing. In this initial condition all the specimens exhibited a strong bimodal split basal texture. During subsequent repressing at 350°C, the texture pattern became randomized through crystal rotations, the phenomenon being more pronounced in coarse-grained specimens. The microstructure of deformed specimens, as examined by electron back scattered diffraction, showed formation of mechanical twins and microstructural inhomogeneity in the coarse-grained specimens. The room temperature tensile properties of the re-pressed specimens showed that the yield strength was remarkably enhanced regardless of the grain size whereas the elongation was reduced as compared to the initial condition, particularly in coarse-grained specimen. It was concluded that microstructural refinement during the severe plastic deformation was the main cause of the improved yield strength while the twinning and microstructural inhomogeneity were responsible for the texture randomization and the impaired ductility.

Abstract: The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.

Abstract: Both tensile and strain controlled low cycle fatigue (LCF) tests were conducted for 316L and 316LN at 550oC and 600oC to investigate the nitrogen effect on the deformation behavior of type 316L stainless. The waveform of LCF was a symmetrical triangle with a constant strain rate of 4×10-3/s was employed for most tests. It shows that the addition of nitrogen in the alloy results in an increase in tensile strength but a decrease in ductility. Both the alloys exhibited cell structure after severe tensile deformation. However, after low cycle fatigue, only planar slip band is observed in 316LN, whereas cell structure is still the main feather of microstructure in 316L. This is due to the strong interstitial-substitutional Mo-N pairs and various stress strain conditions.

Abstract: The aims of this study are to investigate the effect of Y (yttrium) addition in the Mg-Al- Ca alloys on microstructure and mechanical properties. In additions, the alloys were solution treated in order to achieve a better understanding of the precipitation mechanisms. The as-cast microstructure of Mg-5Al-3Ca alloy and Mg-5Al-3Ca-xY alloys contains dendritic α-Mg matrix and eutectic intermetallic compound at grain boundary. The hardness values of Mg-Al-Ca alloy with Y additions were slightly increased than that of Mg-Al-Ca with no Y addition. It is because of reduction of α-Mg phase and presence of (Mg,Al)2Ca and Al-Y rich intermetallic phase at grain boundary and α-Mg matrix grains. Also, hardness value of yttrium (Y) containing alloys was increased with increasing Y contents. Compared to Mg-5Al-3Ca alloy, maximum strength and yield strength of the alloys with Y additions have slightly increased with increasing Y additions in the case of as-cast samples.