Materials Science Forum Vols. 503-504

Abstract: The precipitation and evolution of microstructure in a spray-cast Al-7034 alloy and a commercial wrought Al-2024 alloy were studied after equal-channel angular pressing (ECAP) using transmission electron microscopy and differential scanning calorimetry (DSC). Microstructural examination showed the grain sizes of both alloys were reduced to the range of ~0.3–0.5 μm through ECAP. The DSC analysis identified the occurrence of thermal effects involving the formation, coarsening, dissolution and melting of the precipitate phases and concurrent recrystallization. The heating and ageing response of the alloys processed by ECAP was identified by micro-hardness testing of the samples after interrupted heating and ageing treatments.

Abstract: Accumulative Roll Bonding (ARB) does not require any special equipment and enables the production of large amounts of ultra-fine grained (UFG) materials. Grain refinement is thermally stable in materials with finely dispersed particles such as twin-roll cast (TRC) aluminium alloy sheets, favourable materials for manufacturing UFG sheets. The results of a study of the effect of ARB temperature on bonding quality, structure and properties of TRC AA8006 sheet are presented. Examinations by light and transmission electron microscopy, positron annihilation spectroscopy (PAS), hardness and tensile tests were used in the study. After two cycles at 200°C, mean grain size of 0.4 - 0.8 μm is achieved, but areas with extremely fine grains of 0.1 to 0.3 μm in diameter are also observed. Hardness increases significantly after two cycles and it rises a little in subsequent cycles. Processing at higher temperatures (up to 350°C) results in better bonding but it produces smaller increase in hardness. Significant increase of dislocation density is observed by PAS after the first cycle at 250°C but it does not continue during subsequent cycles. Partial recrystallization occurs in samples processed at temperatures above 250°C explaining the smaller increase in hardness. Softening level depends on both ARB temperature and number of cycles. The thermal stability of refined structures produced by ARB at 250°C is better than these formed at higher temperatures.

Abstract: Severe plastic deformation processes like equal channel angular extrusion (ECAE) have been widely investigated for their ability to produce nano/ ultra fine-grained microstructures. It is well known that submicron sized grains/ sub grains can be produced in most Al alloys using this technique. However, industrial applications of ECAE will depend heavily on the advantages conferred by this process when it is used as an intermediate processing step prior to conventional forming. In the current investigation, the influence of pre processing by ECAE on subsequent post processing by conventional cold extrusion has been investigated. ECAE extrusion was carried out on cylindrical specimens of AA 6101 using an ECAE die with a die angle of 120 degrees. Extrusion was carried out for three passes using two different processing routes. The ECA extruded specimens were subsequently subjected to conventional cold extrusion. The differences in extrusion pressures, which have a strong influence on industrial applications, were noted. Changes in microstructure and mechanical properties were also determined. The obtained results of mechanical properties and microstructure evaluation show that for high strains (strain ε ≈ 2.01), ECAE through some processing routes can be effectively used as an intermediate processing step prior to conventional cold extrusion to obtain a product with enhanced mechanical properties.

Abstract: A commercial FeCo ferritic steel with an initial grain size of 10 μm was subjected to Severe Plastic Deformation in a temperature range between 293 K (0.16Tm, Tm: melting temperature in K) and 723 K (0.4Tm) up to strain levels where a saturation of the microstructural refinement is observed. The microstructure of the severely deformed state is analyzed by Back Scattered Electrons micrographs captured in a SEM. The magnetic properties were characterized by means of SQUID-magnetometer providing information about the magnetization behavior of the material in the as processed state. Depending on the deformation temperature mean microstructural sizes in the steady state of 50 nm and 270 nm were observed after SPD at 293 K and 723 K, respectively. These small microstructural sizes influences significantly the magnetic properties of the material: it shifts the behavior from soft-magnetic in the initial coarse grained state towards a hard-magnetic with decreasing size of the crystallites. For sizes of the crystallites smaller than about 100 nm the magnetic properties become again more soft-magnetic.

Abstract: Deformation microstructures were studied in a two-phase (about 60% ferrite and 40% austenite) Fe – 27%Cr – 9%Ni stainless steel. Severe plastic working was carried out by rolling from 21.3×21.3 mm2 to 7.8×7.8 mm2 square bar followed by swaging from Ø7.0 to 0.6 mm rod at an ambient temperature, providing a total strain of 6.9. After a rapid increase in the hardness at an early deformation, the rate of the strain hardening gradually decreased to almost zero at large strains above 4. In other words, the hardness approached a saturation level, leading to an apparent steadystate deformation behaviour during cold working. The severe deformation resulted in the evolution of highly elongated (sub)grains aligned along the rolling/swaging axis with the final transverse (sub)grain size of about 0.1 μm and the fraction of high-angle (sub)boundaries above 60%. However, the kinetics of microstructure evolution in the two phases was different. In the ferrite phase, the transverse size of deformation (sub)grains gradually decreased during the processing and approached 0.1 μm at strains of about 6.0, while the transverse size of the austenite (sub)grains rapidly reduced to its final value of 0.1 μm after a relatively low strain about 1.0.

Abstract: Refinement of grain size is one of the biggest challenges to produce steels with improved combination of strength and toughness. Ultrafine structures are being produced world-wide on various materials, including low carbon steel, using different types of processes. However, the majority of these processes also exhibit severe limitations because they are generally restricted to small samples and are difficult to be implemented on an industrial scale. A promising technique for industrial implementation is the Accumulative Roll Bonding (ARB), a process able to supply large samples, even in the laboratory scale. In this paper, warm intense straining (ε = 4) by ARB was applied to a plain low-C steel in order to develop ultrafine grains, aiming at sizes around 1-2 μm, suitable to maintain an adequate combination of strength and ductility. The effect of annealing conditions on the evolution of the work-hardened microstructure and the bonding behaviour after each pass were investigated. Orientation Imaging Microscopy was used to investigate the microstructure and give a quantitative assessment of high angle and low angle boundaries. It is showed that the frequency of high angle grain boundaries increases with the strain but the misorientation distribution remained far from that typical of a recrystallised material.

Abstract: Work hardening behavior and microstructure development during deformation by cold rolling were investigated in iron with different grain size. Grain refinement makes the introduction of dislocation easier. For instance, under the same deformation condition (5% reduction in thickness), dislocation density is the order of 1014m-2 in a coarse grained material (mean grain size; 20μm), while it reaches 7×1015m-2 in an ultrafine grained material (0.25μm). It is well known that the yield stress of metals is enlarged with an increase in dislocation density on the basis of the Bailey-Hirsch relationship. However, it should be noted that the ultrafine grained material never undergoes usual work hardening although the dislocation density is surely enhanced to around the order of 1016m-2: 0.2% proof stress is almost constant at 1.4 ~ 1.5GPa regardless of the amount of deformation. The dislocation density of 1016m-2 is thought to be the limit value which can be achieved by cold working of iron and the yield stress of iron with this dislocation density (ρ) is estimated at 1.1GPa from the Bailey-Hirsch relationship; σd [Pa] = 0.1×109 + 10 ρ1/2. On the other hand, yield stress of iron is enhanced by grain refinement on the basis of the Hall-Petch relationship; σgb [Pa] = 0.1×109 + 0.6×109 d-1/2 as to the grain size d [μm]. This equation indicates that the grain size of 0.35 μm gives the same yield stress as that estimated for the limit of dislocation strengthening (1.1GPa). As a result, it was concluded that work hardening can not take place in ultrafine grained iron with the grain size less than 0.35 μm because dislocation strengthening can not exceed the initial yield stress obtained by grain refinement strengthening.

Abstract: The annealing behaviour of an Fe – 22%Cr – 3%Ni ferritic stainless steel processed by bar rolling/swaging to total strain of 4.4 at an ambient temperature was studied in the temperature range of 400 ~ 700oC. The annealing behaviour was characterised by the development of continuous recrystallization involving recovery processes followed by a normal grain growth. The large strain deformation caused the very fast recovery resulting in the development of almost equiaxed polygonized microstructure in place of the highly elongated deformation (sub)grains. The polygonization development was accompanied by some increase in the transverse (sub)grain size and the formation of many low-angle subboundaries. The latter ones could be composed from the dislocations, which were emitted by the strain-induced deformation (sub)boundaries. In spite of relatively large fraction of low-angle subboundaries, such polygonized microstructure was essentially stable against a discontinuous grain coarsening. Upon further annealing, therefore, the microstructure evolution was considered as a normal grain growth.

Abstract: Ultrafine-grained structures formed dynamically through simple compression at warm deformation temperatures were investigated in a 0.15%C- 0.4%Si-1.5%Mn steel. The effects of strain, strain rate and deformation temperature on the microstructural evolution were examined using an isothermal plane strain compression technique with a pair of anvils. The maximum strain was 4, the deformation temperature was below the AC1 temperature, and the Zener-Hollomon parameter (Z) ranged between 1012 s-1 and 1016 s-1. Ultrafine ferrite grains surrounded by high angle boundaries are generated by simple compression when the strain exceeded a critical value. The number of newly generated ultrafine grains increased with the strain; however, the average sizes were found to be independent of strain. The grain size, `d`, was found to depend on Z parameter. An equation, d (μm) =102.07Z-0.16, was found to satisfy the experimentally obtained data. This study demonstrates the possibility of obtaining ultrafine ferrite through multi-pass caliber rolling as a high Z- large strain deformation technique for producing bulk engineering components. It was also noted that the empirical relation established based on single pass compression tests is valid for multi-pass caliber rolling.