Materials Science Forum Vols. 584-586

Abstract: Industrial available FeSi, FeCo and FeNi alloys with an initial grain size of 20-50 m were subjected to Severe Plastic Deformation (SPD) up to strain levels where a saturation of the microstructural refinement is observed. For both SPD conditions, ambient temperature (293 K) and liquid nitrogen temperature (77 K), the microstructure of the severely deformed state is analysed by Back Scattered Electrons (BSE) micrographs captured in a SEM. Additionally, samples that were deformed at 77 K are examined in a Transmission Electron Microscope (TEM). The magnetic properties were characterised by means of SQUID-magnetography providing information about the magnetization behaviour of the material in the as-processed state. Depending on the SPD conditions the mean microstructural sizes in the steady state are about 100-150 nm and 30-80 nm at 293 K and 77 K, respectively. The small microstructural sizes influence significantly the magnetic properties of these ferritic alloys. The initial soft-magnetic behaviour of the coarse grained state shifts towards a hard-magnetic with decreasing crystallite size. For crystallite sizes smaller than ~80 nm the magnetic properties become again more soft-magnetic. The experiments show that very low coercitivity can be obtained by SPD if the grain size is smaller than ~50 nm.

Abstract: Equal channel angular pressing (ECAP) was conducted at room temperature to a high strain level of ~24 in high purity copper. Tensile testing, Transition Electron Microscopy (TEM) and Electron backscatter diffraction (EBSD) were used to characterize the microstructure and property evolution with the increase of ECAP strain. It was found that tensile yield strength and the stored energy increases upon ECAP processing until a peak reached at 8~12 passes of ECAP, and their saturation was observed at higher ECAP passes. Continuous recrystallization phenomenon in microstructure was observed, where dislocation free crystallites with large misorientation to their surrounding matrix and smaller than the nuclei for discontinuous recrystallization were observed embodied in the matrix of deformed structure with high dislocation density. A two-step process was observed for the formation of these small crystallites, first the condensing of dislocation tangles into a narrow boundary, mostly low angle boundary; And second local migration (in sub-micrometer range) of short grain boundaries, in strong contrast to the dramatic migration of long large angle grain boundaries during discontinuous recrystallization to swallow the deformed matrix, was observed leading to vanish of small subgrains.

Abstract: Severe plastic deformation, SPD, enables the grain refinement of bulk materials. However, at strains larger than a critical value, no further microstructural refinement can be observed. This regime is denoted as saturation region of the microstructural size. It will be shown that this regime can be divided into a thermal and an athermal part. The transition between these two regimes was examined in an Al-3wt.%Mg alloy. The single phase alloy was deformed by high pressure torsion (HPT) at various temperatures and different rotational speeds. During the HPTdeformation the flow stress was measured by a torque cell in a temperature range between -196°C (evaporation temperature of the liquid nitrogen) and 450°C. The temperature and the strain rate dependent behavior reveal a shift of the onset of the thermal activated regime towards higher temperatures by an increase of the strain rate.

Abstract: In the present study the microstructure evolution of the aluminum alloy 3103 subjected to ECAP up to eight passes applying route Bc was investigated after deformation and subsequent isothermal annealing. The deformed and annealed states were analyzed by SEM, EBSD, optical microscopy and microhardness tests. It will be demonstrated that this ECAP deformed material shows an increased stability against discontinuous recrystallization with growing number of passes.

Abstract: High-purity compounds R2Fe14B (R = Y, Gd, Tb, Dy, Ho and Er) were prepared by arc melting using rare-earth metals purified by vacuum distillation-sublimation. The compounds R2Fe14B are single-phase and have well-defined directional structure. Nanocrystalline structure was formed by severe plastic deformation of the samples by means of torsion for 5 turns on the Bridgeman anvil under the pressure of 4 GPa at room temperature. The performed investigations of magnetic properties of these compounds allowed us to obtain reliable quantitative data on the intrinsic magnetic parameters, such as saturation magnetization, Curie temperature, the remanent magnetization and coercive force. The enhancement of the remanent magnetization was observed for R2Fe14B in nanocrystalline states compared with the crystalline samples due to the intercrystalline exchange interactions.

Abstract: Fe86Ni14 powder was prepared by mechanical alloying of elemental powders in a highenergy planetary ball mill. X-ray diffraction was used to investigate structure and phase constitution of samples, and thermomagnetic measurements were used to study phase transformation temperatures. MA led to formation of bcc α-Fe and fcc γ-Fe based solid solutions. Significant reduction of martensitic points for MA alloys was observed that was attributed both to nanocrystalline structure formation and samples impurity at milling.

Abstract: Annealed oxygen-free and tough-pitch copper samples have been processed by equalchannel angular pressing (ECAP) by route BC. The samples included 8 x 8 mm section pieces and a 40 mm diameter bar. Thermal stability was assessed based on the changes in the standard mechanical properties (conventional yield strength, tensile strength, elongation, proportional elongation and contraction) after annealing at different temperatures for 1 hour. Thermal stability of the same grade of material has been found to be different for different batches and to depend on the structural conditions of deformed material. The zone of thermal stability for copper of the two grades of interest does not depend on the material’s chemical composition.

Abstract: Recrystallization and grain growth were studied in an austenitic stainless steel 316LVM processed by hydrostatic extrusion (HE) to a total true strain of 2. HE processing produces in this material the microstructure which consists of nanoscale twins on average 19 nm in width and 168 nm in length. The samples after HE were annealed at various temperatures for 1 hour. The structural changes were investigated using TEM. The heat induced changes in nanotwinned austenitic steel are significantly different when compared to the ones in a conventionally deformed material. Microstructural changes take place at lower annealing temperature. Annealing at 600°C brings about a partial a nanostructure reorganization into nanograin of average size 54 nm. An uniform microstructure with nanograins of 68 nm in equivalent diameter was obtained after annealing at 700°C whereas conventional 316LVM steel fully recrystallizes after annealing at 900°C for 1h. Annealing at higher temperatures results in grain growth.

Abstract: Al-Ti alloys, which have Al3Ti platelet particles in Al matrix, were deformed by ECAP with routes A and Bc. With increasing the number of ECAP passes, Al3Ti platelet particles are fragmented and their sizes decrease. The microstructure of ECAPed Al-Ti alloy specimens by route A has a strong alignment of the fragmented Al3Ti particles. On the other hand, ECAPed Al-Ti alloy specimens by route Bc have a relatively homogeneous distribution of Al3Ti particles comparing with the specimen deformed by route A. Based on these results, it was found that ECAPed Al-Ti alloy specimen by route A has highly anisotropic microstructure. However, both ECAPed specimens with routes A and Bc have no anisotropic wear property. That is because the wear property of the Al-Ti alloy specimen depends on the shape of the Al3Ti particle. From these results, it was found that SPD induced by ECAP is an effective processing method to make homogeneous wear property for the metallic material containing platelet solid-particles.

Abstract: Fe60Co40 and Fe72Al28 nano-alloys were synthesized from elemental powders via highenergy mechanical alloying. The prepared samples were characterized using X-ray diffraction, scanning electron microscopy and X-band waveguide to measure the reflection loss in a frequency range of 9-10 GHz. The XRD patterns show that disordered Fe60Co40 solid solution with a bodycentred cubic structure is formed for milling times longer than 12 h, and after 4h milling, the solid solution Fe72Al28 has been largely formed. Morphological studies indicate an average grain size of 10 to 15 nm. The microwave- absorbing characteristic reveal good performance for Fe60Co40 compared to Fe72Al28, the maximum reflection loss is about -12 dB for the absorber.