Alberto Moreira Jorge Junior

Prediction of steel flow stresses under hot working conditions

An austenitic stainless steel was deformed in torsion over a temperature range of 900-1200 °C using strain rates of 1, 5 and 10 s-1. The stress vs. strain curves determined were corrected for deformation heating and the flow stress was found to rise in the initial work-hardening regime, reaching a maximum before dropping to the steady state due to softening brought about by dynamic recrystallization. In order to determine the onset of dynamic recrystallization, diagrams of work-hardening rate vs. applied stress were drawn up for the hardening region of the flow stress curves. The flow stress curves were modeled by adjusting an evolution equation having one internal variable that describes the plastic behavior in the work-hardening regime to the experimental data. The flow stress after the onset of dynamic recrystallization was determined by incorporating the fractional softening into the evolution equation. Describing the effects of temperature and strain rate on the evolution equation through Zener-Hollomon parameters, a database was constructed for use in computer models to predict the roll force of rolling or forging loads under hot working conditions.

Ultra Grain Refinement During the Simulated Thermomechanical-processing of Low Carbon Steel

Grain re Þ nement is a useful method to improve the strength and toughness of steels without changing their chemical composition. In this study, critical temperatures for the thermomechanical treatment of niobium microalloyed steel were determined experimentally and through thermodynamic data. Simulations of conventional and controlled thermomechanical processing and a thermomechanical treatment to obtain ultraÞ ne-grained microstructures were conducted using torsion tests. The Þ nal microstructures displayed signiÞ cant grain size reÞ nement. Conventional processing produced grains with an average size of 12 .m, while controlled processing led to an average grain size of 4.9 .m and severe plastic deformation at warm temperatures resulted in a grain size of 1.3 .m. The ultra reÞ nement of ferrite grains was associated with strain-induced dynamic phase transformation and dynamic recrystallization of as-transformed ferrite.

Magnesium-Nickel alloy for hydrogen storage produced by melt spinning followed by cold rolling

Severe plastic deformation routes (SPD) have been shown to be attractive for short time preparation of magnesium alloys for hydrogen storage, generating refined microstructures and interesting hydrogen storage properties when compared to the same materials processed by high-energy ball milling (HEBM), but with the benefit of higher air resistance. In this study, we present results of a new processing route for Mg alloys for hydrogen storage: rapid solidification followed by cold work. A Mg

The formation of quasicrystal phase in Al-Cu-Fe system by mechanical alloying

In order to obtain quasicrystalline (QC) phase by mechanical alloying (MA) in the Al-Cu-Fe system, mixtures of elementary Al, Cu and Fe in the proportion of 65-20-15 (at. %) were produced by high energy ball milling (HEBM). A very high energy type mill (spex) and short milling times (up to 5 hours) were employed. The resulting powders were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). QC phase was not directly formed by milling under the conditions employed in this work. However, phase transformations identified by DSC analysis reveals that annealing after HEBM possibly results in the formation of the ψ QC phase.

Analytical modeling of the thermomechanical behavior of ASTM F-1586 high nitrogen austenitic stainless steel used as a biomaterial under multipass deformation

Precipitation-recrystallization interactions in ASTM F-1586 austenitic stainless steel were studied by means of hot torsion tests with multipass deformation under continuous cooling, simulating an industrial laminating process. Samples were deformed at 0.2 and 0.3 at a strain rate of 1.0s(-1), in a temperature range of 900 to 1200°C and interpass times varying from 5 to 80s. The tests indicate that the stress level depends on deformation temperature and the slope of the equivalent mean stress (EMS) vs. 1/T presents two distinct behaviors, with a transition at around 1100°C, the non-recrystallization temperature (Tnr). Below the Tnr, strain-induced precipitation of Z-phase (NbCrN) occurs in short interpass times (tpass<30s), inhibiting recrystallization and promoting stepwise stress build-up with strong recovery, which is responsible for increasing the Tnr. At interpass times longer than 30s, the coalescence and dissolution of precipitates promote a decrease in the Tnr and favor the formation of recrystallized grains. Based on this evidence, the physical simulation of controlled processing allows for a domain refined grain with better mechanical properties.

Hot Consolidation of Partially Amorphous Cu-Ti Based Alloy: a Comparison Between Hot Extrusion and Hot Compaction by Sintering

Consolidation of amorphous powders, which take advantage of the supercooled liquid region, is an alternative way to overcome the size limitation in marginal metallic glasses. Cu36Ti34Zr22Ni8based amorphous powders were obtained during high energy ball-milling. The analyses revealed that amorphous structures of powders and ribbons are quite different and this led to a different thermal behavior being the powders more thermally stable than the ribbons. Extrusion was initially proposed as a consolidation process; however, the decrease in viscosity in remainder amorphous matrix was not sufficient for that process, but certainly sufficient for sintering the sample during hot consolidation. An amorphous/nanocrystalline microstructure known for enhancing the mechanical properties of their fully amorphous counterparts was obtained. Evaluation of mechanical properties by microhardness revealed the relatively high hardness of HV 768. From these results, consolidation by sintering seems a promising route to produce bulk metallic glasses nanocomposites.

Efeito do tempo entre deformações na interação recristalização : precipitação de um aço inoxidável austenítico com alto teor de nióbio e nitrogênio

Neste trabalho foi avaliada a influencia do tempo entre passes de deformacao na interacao entre precipitacao e recristalizacao durante o processamento termomecânico do aco ISO 5832-9. A evolucao da microestrutura foi investigada por microscopia otica, eletronica de transmissao e EBSD. Observou-se que quanto menor o tempo entre passes de deformacao, maiores foram as tensoes necessarias para promover a deformacao e menores os tamanhos dos graos e precipitados obtidos. Apos o encharque, somente precipitados de TiNbN foram encontrados, entretanto NbCrN (fase ? ) e NbN precipitaram durante o processamento termomecânico. O material sofre recristalizacao dinâmica e particulas de 50 nm foram efetivas no ancoramento dos contornos de graos e discordâncias. Palavras-chave: Aco inoxidavel; MET; Interacao recristalizacao-precipitacao; Niobio

Consolidation of the Cu46Zr42Al7Y5 amorphous ribbons and powder alloy by hot extrusion

The amorphous Cu46Zr42Al7Y5 alloy presents large supercooled liquid region (ΔTX = 100 K), with a viscosity of about 106 N.s/m2 where the material can flow as a liquid, making it possible an easy deformation in this temperature region. The aim of this work was to analyze processing routes to produce bulks of metallic glasses. Two kinds of materials were used: amorphous powders and ribbons, both were consolidated by hot extrusion in temperatures inside the range between Tg and Tx, with a ram speed of 1 mm/min and extrusion ratio of 3 : 1. Analysis of X-Ray Diffratometry (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM), revealed that the proposed consolidation routes were effective to produce large bulks of amorphous materials, even with the strong decreasing of ΔTX observed after deformation by milling and during extrusion.