Materials Science Forum Vols. 654-656

Abstract: A composite wire combining steel and magnesium has been prepared co-extrusion. Both the microstructures and the mechanical properties were characterized showing the opportunity for the development of original lightweight multi-metallic composites with strength and ductility.

Abstract: Dynamic recrystallization (DRX) behavior in a Cu-0.65Sn-0.025P (mass%) alloy (Cu-Sn-P), which had been newly developed for high strength copper tubes, was systematically investigated. For this purpose, an orientation-controlled bicrystal ( =28o); a model samples of the as-casted billet having coarse columnar grains, was hot deformed in compression at 1073 K at true strain rates from 2 x 10-3 s-1 to 2 x 10-1 s-1 in vacuum. Appearance of peak stress, where DRX sets in, was much delayed in Cu-Sn-P alloy compared with that in Cu and the other copper alloys. While nucleation of new grains preferentially took place at grain boundary, this tendency became more significant with decreasing strain rate. Almost all the new grains were annealing twins (3) formed behind the migrating grain boundary. The more preferential nucleation at grain boundary with decreasing strain rate could be, therefore, reasonably understood by easier and more extensive occurrence of grain boundary migration at lower strain rate.

Abstract: Dynamic recrystallization (DRX) behaviour in a newly developed Cu-Sn-P alloy for heat exchangers and tubes was systematically investigated. For this purpose, Cu-Sn-P alloys with different content of Sn were deformed in compression at temperatures between 1073 K and 1213 K and at various strain rates from 2 x 10-4 s-1 to 2 x 10-1 s-1. The onset of DRX was more advanced with increasing temperature and with decreasing strain rate. Full DRX was not achieved at the testing conditions of lower temperature and higher strain rate even after straining to ε = 1.0. This tendency was more significant in the alloy with higher Sn content. With increasing Sn content, the flow stress and the obtained grains size became higher and finer, respectively. These experimental results indicate the important role of Sn for strengthening and microstructual control.

Abstract: Dynamic recrystallization behavior of Co-29Cr-6Mo-0.16N alloy was analyzed in details. Compression tests were carried out in a computer aided Thermecmaster- Z hot forging simulator. The results showed that uniformly distributed superfine grain size could be obtained by continuous dynamic recrystallization (DRX) process; Texture-free microstructure with uniformly distributed equiaxed fine grains was obtained. The formation of profuse stacking faults and their subsequent intersections are considered to be the principle mechanisms of DRX.

Abstract: The texture and substructure development during post-dynamic annealing of an austenitic Ni-30%Fe model alloy after complete dynamic recrystallization was investigated using electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). A novel mechanism of metadynamic softening is proposed based on the experimental investigation of the grain structure, crystallographic texture and dislocation substructure evolution. The initial softening stage involved rapid growth of the dynamically formed nuclei and migration of the mobile boundaries. The subboundaries within DRX grains progressively disintegrated through dislocation climb and dislocation annihilation, which ultimately led to the formation of dislocation-free grains, while the grain boundary migration gradually became slower. As a result, the DRX texture was largely preserved throughout the annealing process.

Abstract: Plastic slip deformations of tricrystals with simplified geometries are numerically analyzed by a FEA-based crystal plasticity code. Accumulation of geometrically necessary (GN) dislocations, distributions of the total slip, plastic work density and GN dislocations on slip systems, as well as some indices for the intensity of slip multiplication are evaluated. Results show that coexistence of GN dislocations on different slip systems is prominent at triple junctions of grain boundaries.

Abstract: Cu-2.2wt%Ni-0.5wt%Si alloy single crystals were grown by the Bridgman method and aged at 723 K for 10 h to form Ni2Si precipitates. Fully reversed tension-compression fatigue tests were conducted on the aged single crystals with a single slip orientation under constant plastic-strain amplitudes at room temperature. Cyclic softening occurred at plastic-strain amplitudes between 2.5x10-4 and 2.5x10-2. Using the maximum stress amplitude in each cyclic hardening/softening curve, a pseudo cyclic stress-strain curve (CSSC) was obtained. The CSSC was found to exhibit a plateau region with a stress level of about 167 MPa. Transmission electron microscopic observation revealed the formation of persistent slip bands (PSBs) in the plateau regime. It was found that the Ni2Si precipitate particles were intensively sheared by glide dislocations within the PSBs and were eventually re-dissolved into the Cu matrix. The macroscopic cyclic softening can be attributed to the local softening induced by the re-dissolution of the Ni2Si particles in the PSBs.

Abstract: The behavior of different precipitates during hot deformation of a low carbon, low manganese, and titanium added steel has been studied. Microstructural and inclusion analyses from the centerline of as-cast slabs revealed the presence of a wide variety of simple and/or complex precipitates with different size and morphologies. These precipitates were mostly deformed along the deformation direction and affected the recrystallization as well as the mechanical properties of the material. However, those effects altered by changing the re-heating temperature and/or deformation rate.

Abstract: The friction between the die and workpiece plays an important role in determining the quality of the finished products during the hot deformation. Based on the experimental results, a rigid-viscoplastic finite element model was made to study the effects of friction conditions on the strain distribution and microstructural evolution in high strength low alloy steel during hot upsetting process. Also, the effects of friction conditions on the forging loadings and shape geometry of the deformed specimen were also investigated. The results show that: (1) the deformation of specimen is inhomogeneous, and the degree of deformation inhomogeneity decreases with the increase of friction coefficient; (2) the dynamic recrystallization volume fraction decreases with the increase of friction coefficient; (3) the distribution of the average grain size is inhomogeneous in the deformed specimen, and the average grain size increase as the friction coefficient is increased; (5) the effects of friction conditions on the peak forging loading and shape geometry of the deformed specimen are significant.

Abstract: The brittle-to-ductile transition (BDT) in Czochralski (CZ) grown silicon single crystals and floating-zone (FZ) grown silicon single crystals was investigated by three-point bending. The temperature dependence of the apparent fracture toughness was measured in three different cross-head speeds. It was found that the BDT temperature in the CZ silicon crystal was higher than that in FZ silicon crystal, suggesting that the solute oxygen decreases dislocation mobility. However, the activation energies obtained from the strain rate dependence of the BDT temperatures were nearly the same in both the CZ and FZ silicon crystals, indicating that the dislocation mobility is not influenced by the solute oxygen. In this paper, the origin of the difference in the BDT temperature is discussed, focusing the role of the solute oxygen on the dislocation glide.