Materials Science Forum Vols. 706-709

Abstract: The effect of friction stir processing (FSP), on the microstructure and mechanical properties of a magnesium alloy AZ61 has been analyzed. This is a widely used wrought magnesium alloy provided in the form of rolled and annealed sheets with a grain size of 45 μm. The FSP was performed with an adequate cooling device in order to increase the heat extraction and reduce the processing temperature. The final microstructure showed a noticeable grain size refinement down to values close to 1.8 μm and an important change in texture. The change in texture favors basal slip during tensile testing leading to an increase of ductility and a decrease in yield stress. The stability of the grain size and the creep behavior at high temperatures were investigated. The optimum conditions for superplastic forming were determined; however, the presence of a large amount of cavities precludes the achievement of high superplastic elongations. Additionally, these results are compared with those obtained by severe hot rolling.

Abstract: The evolution process of ultrafine grains during hot severe plastic deformation (SPD) was studied in several aluminum alloys. The structural changes can be characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be categorized into the three stages irrespective of deformation mode and temperature: i.e. i) an incubation period for new grain evolution in low strain; ii) a grain fragmentation by frequent development of MSBs and subsequently new grains in medium strain, and iii) a full development of fine grains in large strain. Temperature effect on the new grain formation in aluminum alloys is also analysed in detail and the mechanism operating is discussed.

Abstract: We studied the crystallographic features, especially the orientation relationship with respect to austenite, of martensite in a steel transformed from coarse-grained equiaxed austenite (35 μm), ultrafine-grained equiaxed (2.5 μm) or lamellar (300nm) austenite fabricated by sever plastic deformation. With decreasing the grain size of equiaxed austenite, the orientation relationship changed from Kurdjumov - Sachs relationship to Greninger - Troiano relationship. We inferred that this change of orientation relationship could be attributed to the small size of martensite plate transformed from the ultrafine-grained equiaxed austenite. The martensite transformed from the ultrafine-grained lamellar austenite did not have a definite orientation relationship with austenite. We considered that a high density of dislocations or a high density of low angle boundaries within the ultrafine lamellar austenite grain resulted in the large deviation of orientation relationship.

Abstract: In order to investigate roles of grain boundaries on the improved fracture tough-ness in ultrafine-grained metals, interactions between crack tips, dislocations, and disclinationdipoles at grain boundaries are performed to aluminium bicrystal models containing a crackand h112i tilt grain boundaries using molecular dynamics simulations. A proposed mechanismto express the improved fracture toughness in ultrafine-grained metals is the disclination shield-ing effect on the crack tip mechanical field. The disclination shielding can be activated whena transition of dislocation sources from crack tips to grain boundaries and a transition of thegrain boundary structure into a neighbouring energetically stable boundary by emitting dis-locations from the grain boundary occur. The disclination shielding effect becomes large asdislocations are continuously emitted from the grain boundary without dislocation emissionsfrom crack tips. This mechanism can further shield the mechanical field around the crack tipand obtain the plastic deformation by dislocation emissions from grain boundaries, hence itcan be expected that the disclination shielding effect can improve the fracture toughness inultrafine-grained metals

Abstract: The present contribution is aimed at investigating the microstructure evolution of commercially pure silver under severe plastic deformation conditions. ECAP billets have been produced by using a die with channels intersecting at 90° and straining the samples at room temperature. The evolution of the microstructure as a function of imparted strain was evaluated by scanning electron microscopy as well as X-ray diffractometry. Furthermore, tensile properties were measured from ECAP billets in order to evaluate the strengthening and work hardening behaviour of silver as a function of structure evolution. Comparison in terms of grain structure and corresponding properties are also drawn by considering published data about Al-Mg-Si alloy samples ECAP-processed by identical routes and parameters.

Abstract: The technical requirements to produce parts as light as possible with more complex geometries are ever increasing in recent years in automotive industries, rail vehicle and aerospace. Manufacturing technologies such as super-plastic metal forming, which had been considered as a niche technology earlier, is now gaining greater technical relevance for industrial size production. The parts that have been generated through super-plastic forming in innovative vehicle for example Mercedes SLS AMG reflect the industrial activity in this area. Manufacturability of thin-walled structures enables lightweight design. Also the tool and manufacturing costs are lower in comparison. However, the main challenge could be found in maintaining exact process parameters. There is a large research potential in super-plastic forming; from the determination of optimal material data through process simulation to manufacturing of real parts and their characterization. In the framework of the project international Bureau of the Federal Ministry of Education and Research UKR005/08 “Manufacture of parts with special properties” a preliminary research work has been carried out.

Abstract: The mechanical properties of two-phase Ti-6Al-4V titanium alloy with ultrafine grained microstructure were studied in the present work. Bulk ultrafine grained specimens of the alloy were produced by means of warm “abc” deformation. The final structure consisted of α/β particles with a size of 500 nm. Extensive studies of the mechanical properties of this material in comparison with conventionally heat-strengthened condition were conducted. A room-temperature strength and fatigue resistance of the ultrafine grained material was found to be 25-40% higher than that of heat-strengthened alloy. However such ductility related properties as tensile elongation and impact toughness noticeably decreased with decreasing grain size. Efficacy of ductility improvement and the strength/ductility balance optimization were analyzed.

Abstract: In this study ultrafine grain structure evolution during high pressure torsion (HPT) of commercial aluminium alloy AA6082 at increased temperature is presented. Two different initial structural states of the alloy were prepared by thermal treatment. The progress in structure refinement in dependence on the shear strain level strain was investigated by TEM of thin foils. The impact of different amount of strain (ε_ef) introduced was analyzed with respect to the effect of increased temperature. The microhardness results measured across the deformed discs pointed out that some data scattering. The results of microstructure analyses showed that ultrafine grain (ufg) structure was already formed in deformed disc upon the first turn, regardless the initial structure of alloy, resulting from prior thermal treatment. The observed heterogeneity in ufg structure formation across the deformed disc was observed, supporting microhardness results scattering. By increasing the strain level (number of turns N-2,4,6), more effectively homogenized ufg structure was observed across the deformed discs. The effect of increased deformation temperature became evident and dynamic recrystalization modified locally ufg structure.. The retardation of new grains growth and higher thermal stability of ufg structure was observed, when two steps thermal treatment of alloy (quenching and ageing) was executed prior deformation. Strength measurements results yielded form tensile tests showed that the effect of structure strengthening was degraded by local recrystallization. The results of torque measurement versus the time showed that the torque required to deform the sample was increasing until the first turn and then kept stable or even decreased.

Abstract: A series of carbon nanotube (CNTs) and stainless steel fiber (SSFs) filled nylon 6 (PA6) conductive composites were synthesized for electromagnetic interference (EMI) shielding applications. The materials were prepared by the melt blending method with CNTs weight fraction of 1 and 3 wt% and SSFs of 2, 4, 6, 8, 10, and 12 wt%. The shielding effectiveness, electrical resistance and crystallization behaviors were measured. The results indicate that the shielding effectiveness and electrical properties can be improved by increasing either SSFs or CNTs contents. Higher content of CNTs can bring forward the percolation threshold and enhance the shielding effectiveness to 51.8 dB. Due to the nanoconfinement/multiple nucleation effects, PA6 crystallization behavior is influenced by adding the CNTs and SSFs.

Abstract: For off-stoichiometric Ni₂MnGa ferromagnetic shape memory alloys, a large shape change could be induced through the rearrangement of martensitic variants under an external magnetic field. Insight into the orientation relationships of martensitic variants and the characteristics of variant boundaries is thus essential for understanding the magnetic shape memory performance. In this paper, a thorough crystallographic investigation was made on the incommensurate 7M modulated martensite in one polycrystalline Ni₅₀Mn₃₀Ga₂₀ alloy by means of X-ray diffraction and SEM electron backscattered diffraction (EBSD). Locally, there are four differently-oriented martensitic variants, being twin related to one another. The twin interface planes are coherent and they are in coincidence with the respective twinning planes (K₁). A primary exploration was performed to improve the microstructure by repeated magnetic field training during phase transition. The present investigation could offer useful guidance to develop specific technique for microstructure optimization.