Izabel Fernanda Machado

On Cr2N precipitation mechanisms in high-nitrogen austenite

Precipitation mechanisms in a high-nitrogen (0.87 wt%) fully austenitic 25Cr–5Ni (wt%) steel have been studied after ageing at 900°C and 960°C. Ageing induced discontinuous precipitation of Cr2N from grain boundaries, discontinuous precipitation at twin fronts, continuous precipitation throughout the matrix, continuous precipitation in association with ferrite plate formation and also grain boundary precipitation in the form of both discrete precipitates and films. The observed morphologies at the onset of discontinuous precipitation from grain boundaries suggest a Tu–Turnbull initiation mechanism. The orientation relation between Cr2N and γ in well-developed lamellae is not strictly (111)γ//(0001)Cr2N and γ// Cr2N. The fact that the process shows some orientation tolerance, together with the lamellae irregular and curved interfaces, indicates that minimization of the system interfacial energy does not play a key role during growth. In this precipitation mechanism, twinning inside growing γ lamellae is associated with abrupt local changes in the growth direction. In the case of cooperative growth at incoherent twin fronts, the irregular habit surfaces of Cr2N point to a Fournelle–Clark initiation mechanism. At early precipitation stages, the interlamellar spacing for cooperative growth at twin fronts is similar to the one observed for cooperative growth from grain boundaries (∼100 nm). It is proposed that the low growth efficiency observed at twin fronts results from lower diffusivity conditions at twin fronts as compared with grain boundaries.

Microstructure and microtexture changes during solution nitriding to produce austenitic case on ferritic–austenitic duplex stainless steel

AbstractThe present paper reports an investigation into the microstructural changes occurring in a ferritic–austenitic duplex stainless steel during solution nitriding. Both microstructure and microtexture modifications as a result of nitrogenation have been studied, in addition to the interaction between solution nitriding, grain growth, and texture. Three microstructural zones were identified: a duplex ferrite–austenite region overlayed by a transition zone overlayed again at the surface by an austenitic zone. The solution nitriding led to nitrogen enrichment at the specimen surface, which in turn enlarged the austenitic field in this region. The ferrite dissolution here triggered secondary recrystallisation, accompanied by a texture change. Hence, the texture of the austenite in this region was different from that in the duplex region.

Nanomaterials processed by spark plasma sintering

The spark plasma sintering (SPS), a variant of field-assisted sintering (FAST) or pulsed electric current sintering (PECS), is a novel pressure assisted pulsed electric current sintering process, which utilizes ON-OFF DC pulse energizing. Due to the repeated application of an ON-OFF DC pulse voltage and current flow between powder particles, the spark discharges and the Joule heating (local high temperature state) are therefore dispersed to the overall specimen.The SPS process is based on the electrical spark discharge phenomenon and is a high efficient, energy saving technique with a high heating rate and a short holding time. The problem of rapid grain growth of nanomaterials during conventional sintering can be inhibited to a larger extent by using the SPS technique. The SPS can be used for diverse novel bulk material applications, but it is particularly suitable for the processing of nanomaterials. Despite such anticipated advantages, the optimization of the process window (heating rate-temperature-time) in SPS process is a challenging task. More importantly, the underlying mechanisms for superfast densification still remains to be explored.

Analysis of Sintering Parameters by SPS on Cemented Carbide

The aim is to analyze the influence of two sintering process parameters concerning the compact densification of WC-Co. The input variables studied were uniaxial pressure (40 and 60 MPa) and holding time (1 and 3 minutes). The output variables monitored were relative density, microstructural features, besides online monitoring of dynamics of sintering. The samples were sintered in a Dr. Sinter-Syntex machine of Spark Plasma Sintering (SPS), which allows heating rate greater than conventional sintering techniques, with the advantage of lower temperature, time and energy consumption. The powder used was a compaction residue of grade K (with content between 3 and 12%Co). The results showed significant influence of pressure on the density and percentage of the porous area; however, the process parameters did not affect the grain size.

Caracterização da perovskita CaCu3Ti4O12 consolidada por sinterização assistida por campo elétrico

Polycrystalline CaCu3Ti4O12 ceramics were prepared by the field-assisted sintering technique. The effects of the dwell time and temperature on structural, microstructural and dielectric properties of CaCu3Ti4O12 ceramics have been investigated. Powder mixtures were calcined at 900 oC for 18 h before consolidation. The dwell temperatures and times range between 750 and 930 oC and 1 to 8 min, respectively. Sintered pellets were characterized by X-ray diffraction, scanning electron microscopy and impedance spectroscopy. X-ray diffraction patterns show evidences of a single-phase perovskite-type structure and the calculated lattice parameter is 7.40 A. The hydrostatic density increases slightly with increasing dwell time and temperature. Scanning electron microscopy observations revealed a heterogeneous microstructure for all samples. The dielectric loss remains constant over a wide temperature range. The obtained permittivity is approximately 103 at 1 kHz. The increase of the dwell temperature was found to produce a brittle ceramic.

Numerical Simulation of Residual Stresses in Quenched Steel Bodies Using Subroutines to Represent TTT and CCT Diagrams

The objective of this work is to analyze residual strains and stresses and volumetric expansion due to phase transformations that occur during quenching of a steel body. Three different models are proposed, based on the finite element software ABAQUS ® and on the use of FORTRAN subroutines. The time-tempreature-transformation (TTT) or continuous-cooling-transformation (CCT) diagrams of SAE 4140 steel are represented differently in each model, depending on the transformed phases and correspondent volumetric expansion. In the first model, diagrams are intentionally simplified in order to consider only the austenite-martensite transformation. In the second model, the thermomechanical-phase transformation coupling is represented through the incorporation of the austenite-pearlite transformation into the TTT diagram used in the first model and considering that this transformation occurs for cooling rates lower than the limit for martensitic transformation. The third model is based on a subroutine that calculates all the microstructures resulting from quenching (ferrite, pearlite, bainite, and martensite), depending on cooling rate. This subroutine includes information from all the TTT and CCT diagrams of SAE 4140 into a FORTRAN code. Model testing was conducted based on the analysis of the quenching of a cylinder with diameter of 45 mm and based on the comparison with results presented in the literature. Significant differences are observed in the numerical results provided by each model, which, in some cases, also provided data with significant differences from literature.

Development of WC-Fe3Al Composites by Spark Plasma Sintering Process

Tungsten carbide (WC) based composites are usually produced with cobalt, but this binder has the inconvenience of shortage, unstable price and potential carcinogenicity. The objective of this study was to develop WC composite with intermetallic Fe3Al matrix. Powders of WC, iron and aluminum, with composition WC-10 wt% Fe3Al, and 0.5 wt% zinc stearate were milled in a vibration mill for 6 h and sintered in a SPS (spark plasma sintering) furnace at 1150 °C for 8 min under pressure of 30 MPa. Measured density and microstructure analysis showed that the composite had significant densification during the (low-temperature, short time) sintering, and X-ray diffraction analysis showed the formation of intermetallic Fe3Al. Analysis by Vickers indentation resulted in hardness of 11.2 GPa and fracture toughness of 24.6 MPa.m1/2, showing the feasibility of producing dense WC-Fe3Al composite with high mechanical properties using the SPS technique.

Effects of Milling Time on Microstructure and Mechanical Properties of Composite WC-(Fe3Al-B) Consolidated by Spark Plasma Sintering

Cobalt is widely used to produce WC-Co hard metals, but this binder has problems of shortage and unstable price. In this work, cobalt was replaced by an iron aluminide intermetallic binder. WC-10%(Fe3Al-3%B) composite was prepared by vibration milling of WC, Fe, Fe-B, and Al powders and sintered by spark plasma sintering (SPS) at 1150 °C for 8 min under 30 MPa. The milling time was 0.17, 12, 25 and 50 h. The SPS was efficient to consolidate the composite resulting in relative density of ~98% or higher. With increasing milling time, Vickers hardness (HV30) of composite increased from 12 to 14 GPa due to the enhanced homogeneity of microstructure, while the fracture toughness, KIc, determined by an indention fracture method using Shetty equation, remained constant at around 9.1 MPa.m1/2.

The use of the Hilbert-Huang transform in the analysis of machining vibrations in machine tools

This paper presents a research on the effects produced in a induction motor torque while it is driving a machine tool working in a metal cutting process. The machine tool used in the investigation was a lathe. The research is based on two data acquisition processes, the first using a Kistler stationary dynamometer and the second using an induction motor torque estimator. Both datasets acquired were processed with use of the FFT and the Hilbert-Huang transform algorithms. The FFT algorithm has been used for a long time in the analysis of vibrations processes and its stationary characteristic is very well known. The Hilbert-Huang transform (HHT) is an adaptive method that includes the properties to analyze non-linear signals and properly performs the supplying of the lacks of FFT method. The HHT method has an empirical basis that is the Empirical Mode of Decomposition (EMD) process, but is a powerful tool to perform vibration analysis. The main target of the project was verify if the monitoring of the machining process by observation of the torque estimated by measuring the electrical quantities of the mains against the dominant cutting force measured by a specialized equipment is good enough to justify the use of the method in industrial machine tools.

NdFeB Type Magnets Produced by Spark Plasma Sintering

The shortage of dysprosium as an alloying element has directed the research on the grain size refining of NdFeB, since higher coercivities can be obtained by decreasing the grain size, without Dy addition. The Spark Plasma Sintering (SPS) is a consolidation process which allows densification at lower temperatures and shorter dwell times of sintering, thus avoiding the grain growth. Therefore, the typical temperature of sintering of NdFeB magnets can be decreased from 1050°C to around 800°C, as it was evidenced by means of SPS shrinkage curves and the high densified microestructure obtained in this work.