Alberto Moreira Jorge

Influence o the microstructure of duplex stainless steels on their failure characteristics during hot deformation

Two types of duplex stainless steels were deformed by torsion at a temperature range of 900 to 1200 °C and strain rate of 1.0 s-1 and their final microstructures were observed. The austenite volume fraction of steel A (26.5Cr - 4.9Ni - 1.6Mo) is approximately 25% at room temperature, after conventional annealing, while that of steel B (24Cr - 7.5Ni - 2.3Mo) is around 55%. Experimental data show that steel A is ductile at high temperatures and displays low ductility at low temperatures, while steel B has low ductility in the entire range of temperatures studied. At high temperatures, steel A is essentially ferritic and shows dynamic recrystallized grains after deformation. When steel A is strained at low temperatures and displays low austenite volume fraction, microstructural observations indicate that failure is triggered by grain boundary sliding due to the formation of an austenite net structure at the ferrite grain boundaries. At intermediate volume fraction, when austenite forms a dispersed second-phase in steels A and B, failure begins at the ferrite/ferrite boundaries since some of the new ferrite grains may become immobilized by the austenite particles. When steel B is strained at volume fraction of around 50% of austenite and both phases percolate the microstructure, failure occurs after low straining as a consequence of the different plastic behaviors of each of the phases. The failure characteristics of both steels are correlated not only with the volume fraction of austenite but also with its distribution within the ferrite matrix, which limits attainable strain without failure.

Evidence of strain-induced precipitation of Ti4C2S2 during hot deformation of Ti-only stabilized IF steel

Abstract Hot rolling simulations of interstitial free steel were carried out by torsion. In the second pass of the finishing stage, a sharp rise of the flow stress level was observed and TEM micrographs revealed the presence of large and small precipitates. The stabilization of carbon by the formation of free-standing particles along with in situ transformation is analyzed.

Ag ion decoration for surface modifications of multi-walled carbon nanotubes

The production of high performance metal matrix composites depends on a proper design of the surface of the reinforcing phase, ensuring a good contact with a metal phase. In the present work, two Ag decorating procedures to modify the surface of multi-walled carbon nanotubes (MWCNT) were evaluated for further production of aluminum matrix composites. The procedures consisted in a two steps route based on acid oxidation of carbon nanotubes (CNT) followed by suspension in an Ag ion solution; and a single step route, based on the effect of n-dimethylformamide (DMF) as an activation agent of CNT surface, in presence of Ag ions. Transmission and scanning-transmission electron microscopy, Raman and Fourier-transformed infrared spectroscopy were employed in order to characterize the results. The two steps route resulted in Ag nano-particles homogeneously deposited over the CNT surface. The mechanism for the deposition is based on carboxyl and probably hydroxyl functional groups formed in the first step, acting as nucleation sites for Ag precipitation in the second step. The single step route resulted in the formation of sub-micrometric Ag particles heterogeneously mixed to CNT bundles.

Hydrogen Activation Behavior of Commercial Magnesium Processed by Different Severe Plastic Deformation Routes

Severe plastic deformation (SPD) techniques are being considered as low cost processing routes for Mg alloys, aiming hydrogen storage applications. The main objective is to develop air-resistant materials, with lower specific surface area in comparison with ball-milled powders, but with still attractive H-sorption kinetics associated to the microstructural refinement. In this study, the effects of different SPD processing routes (high-pressure torsion, extensive cold rolling and cold forging) in the hydrogen activation behavior of Mg was evaluated. The results show that both microstructural and textural aspects should be controlled during SPD processing to obtain Mg alloys with good H-sorption properties and enhanced activation kinetics.

Plastic behavior of medium carbon vanadium microalloyed steel at temperatures near gamma alpha transformation

Dilatometric techniques were used to build the continuous cooling transformation (CCT) diagram for a medium carbon microalloyed steel; the microstructure and hardness were determined at different cooling rates. The mechanical behavior of the steel in the austenite field and at temperatures approaching austenite to ferrite transformation was measured by means of hot torsion tests under isothermal and continuous cooling conditions. The no recrystallization temperatures, Tnr, and start of phase transformation, Ar3, were determined under continuous cooling condition using mean flow stress vs. inverse of absolute temperature diagrams. Interruption of static recrystallization within the interpass time in the austenite field indicated that the start of vanadium carbonitride precipitation occurred under 860 °C. Austenite transformation was found to start at around 710 °C, a temperature similar to that measured by dilatometry, suggesting that interphase precipitation delays the transformation of deformed austenite. Pearlite was observed at temperatures ranging from 650 °C to 600 °C, with the flow curves taking on a particular shape, i.e., stress rose sharply as strain was increased, reaching peak stress at low deformation, around 0.2, followed by an extensive softening region after peak stress.

Evidence of Strain-Induced Precipitation on a Nb- and N-Bearing Austenitic Stainless Steel Biomaterial

Pilot-scale plate rolling experiments and laboratory thermomechanical processing experiments were carried out to understand the mechanism of microstructural banding in low-carbon microalloyed steels. The microstructural banding originates with large elongated austenite grains, which are present at the roughing stage of rolling. The large austenite grains develop when conditions favour abnormal grain growth during reheat and/or strain induced grain boundary migration (SIBM) in the first few rolling passes. Microstructural banding is eliminated by designing TMP schedules to avoid abnormal grain growth and SIBM.

Microstructural evolution of Ti-6Al-7Nb alloy during high pressure torsion

Ti-6Al-7Nb alloys are being evaluated for biomedical applications, in substitution of the more conventional Ti-6Al-7V. Both types of alloys present a microstructure containing the α and the β phases, which result in good compromise for mechanical applications. In the present work Ti-6Al-7Nb alloys were processed by High Pressure Torsion (HPT), varying the number of revolutions and thus the total imposed strain. X-Ray Diffraction (XRD) results revealed the formation of different crystallographic textures in samples subjected to HPT. Microhardness distribution, across the diameters of the disks, is rather homogeneous for all samples, with higher values for those subjected to 03 and 05 turns. Transmission electron microscopy (TEM) micrographs have showed that an ultra-fine grained microstructure was obtained in all the samples.

Influence of deformation on the kinetics of phase transformation in a forging steel during warm working

Dilatometric techniques were used to determine the start and finish transformation heating temperatures for a carbon steel (0.30% C - 1.5% Mn). The mechanical behavior of the steel was measured by torsion testing in the temperature range of 700 to 820 °C with holding times ranging from 1 to 30 min. The flow stress curves presented different shapes and stress levels. These differences were attributed to the ferrite and pearlite, ferrite and austenite, and austenite strained structures. When ferrite and pearlite were deformed together, the flow stress presented a hump with little straining; when the austenitic structure was deformed the shape of the flow stress curve was typical of materials having low stacking fault energy. The microstructural evolution observed by optical and scanning electron microscopy revealed that the evolution of the phase transformation was dependent on the testing temperatures, holding times and amount of straining. Comparisons were made on the kinetics of phase transformation with and without the application of plastic deformation, and evidence of strain-induced dynamic transformation was investigated.

Hot Extrusion of Nanostructured Al Alloy Powder: Extrusion Ratio and Temperature Effect on the Microstructure and Mechanical Properties

A nanostructured aluminium alloy powder, prepared by rapid solidification via gas atomization, was consolidated into bulk material under various processing conditions via hot extrusion. The microstructure modifications and mechanical properties of the consolidated alloys as a function of the extrusion conditions were investigated. The increase in the extrusion-load with the increase of extrusion-rate and decrease of temperature are shown and discussed in association with the modification in the microstructures. The differences in mechanical properties measured by compressive tests are also discussed in association with the extrusion parameters. Furthermore, suggestions are given for rationalising the extrusion ratio and temperature conditions for the consolidation of nanostructured aluminium alloy powders via hot extrusion.

Weight reduction of amorphous alloy core electrical transformers for aircraft applications

Catalogues of manufactured transformers show that amorphous alloy core transformers applied to 50 or 60 Hz lines have about 70% core electrical losses reduction when compared to same power output transformers made with Grain Oriented (GO) Silicon Iron (FeSi) core, however amorphous alloy core transformers are also heavier than FeSi core transformers. Such weight fact makes apparently prohibitive the use of amorphous alloy core transformers onboard aircrafts, but more electrical aircrafts (MEA) require much more electrical power, which means more FeSi core losses, therefore ways of electrical losses reduction onboard must be studied. This paper shows by calculations that transformers with amorphous alloy core can be less heavy and have reduced losses than an equal calculated power output transformer with GO FeSi core if core temperature, safety limits for core induction value and cooper wire cross section area reduction are considered.