Materials Science Forum Vols. 667-669

Abstract: Sheets of commercial purity aluminum and super saturated solid solution of 0.3% Sc were accumulatively roll bonded to form 64 alternating layers in a 0.5 mm thickness. The rolling was done at 350 °C to ensure dynamic recovery/recrystallization in Al layers and precipitation hardening of Al(Sc) layers during the rolling. The sheet crystallographic texture was distributed along β fibre orientations. The recrystallization texture of aluminum layers after annealed at 250-350 °C was randomly distributed. The tensile property of this novel composite has achieved a small improvement over the commercially available grades and delamination between the layers was identified as the key issue to improve in this fabrication technique.

Abstract: Microstructural changes during equal channel angular pressing (ECAP) at the temperatures of 250 and 300°C to the strains ~4, ~8 and ~12 were studied in a coarse-grained Al-5.4%Mg-0.5%Mn-0.1%Zr alloy. At a strain of ~4, the microstructural evolution is mainly characterized by the development of well-defined subgrains within interiors of initial grains and the formation of fine grains along original boundaries. Further straining leads to increase in the average misorientation angle, the fraction of high-angle grain boundaries and the fraction of new grains. However, only at 300°C, the plastic deformation to a strain of ~12 leads to the formation of almost uniform submicrocrystalline (SMC) grained structure with an average crystallites size of ~ 0.5 m. At 250°C, the microstructure remains non-uniform and consists of subgrains and new recrystallized grains. The mechanism of new SMC structure formation after ECAP is discussed.

Abstract: The structural changes and the strengthening of a Cu-3%Ag alloy subjected to large strain drawing and subsequent annealing were studied. The cold working was carried out at an ambient temperature up to total strain above 8. The hardness increased from 600 MPa in the initial state to about 1800 MPa with increasing the total strain. The annealing treatment at 400°C resulted in increase in the hardness to about 2000 MPa for the samples cold worked to total strains above 2. On the other hand, the hardness change of the samples annealed at 450°C dependent significantly on the preceding cold strain. Namely, annealing softening took place in the samples processed to strains below 5, while the samples processed to larger strains were characterized by remarkable hardening after annealing. The value of annealing hardening increased with increasing the previous cold strain, leading the hardness to 2500 MPa in the sample strained to 7.4. The cold worked and annealed samples were characterized by the development of lamella-type microstructure consisting of highly elongated copper grains with uniform distribution of nano-scaled silver particles having a size of about 2 nm.

Abstract: The low energy high current pulse electron beam (LEHCPEB) irradiation induces ultra fast dynamic temperature fields in the surface of the material to which is associated dynamic stress fields that causes intense deformation at the material surface and sub-surface. Improved surface properties (hardness, corrosion resistance) can be obtained using the LEHCPEB treatment. Under the “Melting” mode, the top surface (few µm) which is melted and rapidly solidified (107 K/s), can solidify has nano-domains formed from the highly under-cooled melt. The thermal stress wave that propagates in the sub-surface imposes strain hardening and grain size refinement. This induces a sub-surface hardening that can extent over about 100 µm. The use of the “Heating” mode is less conventional. This mode can promote grain size refinement, hardening as well as texture modification without modification of the sample geometry.

Abstract: Friction stir welding (FSW) was used to join the submicrocrystalline (SMC) grained Al-Cu-Mg-Ag sheets produced by equal channel angular pressing (ECAP) followed by hot rolling (HR). The effect of SPD and FSW on the microstructure and mechanical properties in the zone of base metal, as well as in the stirred zone (SZ) were examined. In addition, effect of standard heat treatment on microstructure and mechanical properties in these zones was considered. A refined microstructure with an average grain size of ~ 0.6 m and a portion of high-angle grain boundaries (HAGBs) of ~0.67 was produced in sheets by ECAP followed by HR at 250°C. The microcrystalline grained structure with average grain size of ~2.3 mm was found in joint weld. The moderate mechanical properties were revealed in SMC sheets and joint welds. Heat treatment considerably increases strength of the base metal as well as the joint welds. The higher strength of the alloy after T6 temper is attributed to the dense precipitations of  dispersoids having plate-like shape which are uniformly distributed within aluminum matrix. It was observed that FSW can produce full strength weld both in the tempered and in the un-tempered conditions.

Abstract: The repeated plastic working deformation(RPW) process can reduce grain size of a Mg alloy from 50-100 um to 10-500 nm, but the mechanism responsible for it has not been clear up to now. In the present paper, the effect of RPW deformation process on the grain size of Mg-5Gd-1Er alloy has been studied, and a series microstructural evaluations were performed to investigate the possible mechanism for RPW deformation by using transmission electron microscopy. Although there are no twinning or fibrous microstructures, the dynamic recrystallization, which usually occurs in high stacking fault energy metals, has been found in the alloy deformed by RPW process. The results indicated that the vacancy diffusion played an important role in this kind of dynamic recrystallization. According to the evolution of microstructure, a mechanism was proposed to explain the dynamic recrystallization for RPW deformation process.

Abstract: Structural and phase transformations in V95 (type of A7075) alloy upon high pressure torsion (HPT) up to high strain and upon dynamic channel angular pressing (DCAP) were studied using electron microscopy (TEM, SEM) and x-rays diffraction. It was established that the nanostructure with average grain size of 80-55 nm and hardness value of 2.5 GPa is formed by HPT at strain of e=5.5-6.4. It was shown the hardening metastable phase MgZn2 precipitates from supersaturated α-solid solution during dynamic deformation aging; its quantity grows with increasing of strain. Ultra fine grained structure with high- angle boundaries is formed in V95 alloy after already 2 cycles of DCAP. The average grain size is 200 nm in DCAP samples.

Abstract: Pure nickel sheets are severe plastically deformed by constrained groove pressing technique at room temperature up to three passes. A total strain magnitude of 3.48 is imparted to the sheets and further processing is limited by initiation of surface microcracks. The grain size evolution studied by optical microscopy reveals significant grain refinement at the end of third pass evidently illustrating the effectiveness of groove pressing technique for grain refinement in sheet materials. Vickers microhardness measured during different stages of groove pressing process clearly describes the deformation behaviour in different segments of slant and flat regions. The change in mechanical properties of constrained groove pressed sheet is evaluated by room temperature tensile and microhardness tests. Results showed considerable increase in strength and hardness during initial passes followed by slight drop during third pass.

Abstract: Plastic deformation leads to a structural refinement by introducing low angle dislocation boundaries and high angle boundaries in the initial coarse grains. To understand the mechanisms for the structural refinement and to establish the structure-strength relationship requires a precise characterization of key structural parameters, namely the boundary spacing and boundary misorientation angle. This study gives the results of such a characterization of pure Ni subjected to high pressure torsion (HPT) up to a strain of 300. The structural analysis was carried out by transmission electron microscopy in the longitudinal sample section in which the detailed structural features can be resolved. It is found that the microstructure in the HPT Ni samples is dominated by a lamellar structure. The spacing of the lamellar boundaries decreases and their misorientation angle increases with strain following a power law up to strain of 12, above which saturation is reached at a strain of about 34. The distribution of lamellar spacings normalized by their respective average values at each strain show an identical form. This scaling behavior is discussed also with reference to other metals and processing routes.

Abstract: The main aim of this paper is to show how ECAP influences the porosity distribution of PM aluminium alloys. The dimensional and morphological characteristics of investigated materials were measured individually for each pore. A commercial ready-to-press aluminium based powder (ECKA Alumix) was used as material to be investigated. After applying different compacting pressures (400 - 700 MPa), specimens were debinded in a ventilated furnace (Nabertherm) at 400 °C for 60 min. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. When severe plastic deformation is applied, the stress distribution in deformed specimens causes the powder particles to squeeze together to such an extent that the initially interconnected pores transform to small isolated pores, determining a given value of the dimensional characteristics. Consequently, ECAP influences the porosity distribution in terms of the involved severe shear deformation and therefore influences the pore morphology.