Twin-induced plasticity of an ECAP-processed TWIP steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.Postprint (published version

Stress-strain response and microstructural evolution of a FeMnCAl TWIP steel during tension-compression tests

© 2016. The stress-strain response of a Fe-17.5Mn-0.7C-2Al TWIP steel during cyclic loading has been investigated by means of tension-compression tests within the strain limits of ±2%, ±5% and ±10%. In addition, the microstructural evolution during the ±5% cyclic test has also been studied. The difference between the forward and reverse stress for each pre-strain has been analyzed at 0.2% offset strain and at the strains in which forward and reverse curves were parallel in order to study the Bauschinger effect (BE) and permanent softening, respectively. The evolution of the BE with pre-strain for this steel is similar to other FeMnC TWIP steels, that is, increasing values of BE are obtained as the pre-strain increases. However, its absolute values are half those reported in the literature on other FeMnC steels. This diminution of the BE is related to the lower activity of mechanical twinning in FeMnCAl TWIP steels at the pre-strains herein investigated, which promotes less polarized stresses in the matrix due to the lower dislocation storage capacity.Regarding permanent softening, the evolution is similar to that of the BE and the same analysis can be applied. During reverse compression, a slight increase of twin thickness and twin spacing with respect to the first tensile stage took place. This fact might be linked to the lower flow stress observed in the permanent softening period during reverse straining.Peer ReviewedPostprint (author's final draft

3D printing of Inconel 718 by MEAM (Metal Extrusion Additive Manufacturing)

In this study the production of 3D-printed pieces made from Inconel 718 using Metal Extrusion Additive Manufacturing of highly-filled polymers has been investigated. The printer used was a Prusa I3 MK3 printer modified with a gravity fed-pellets screw extruder. The feedstock was pellets composed of metal powder and polymeric binders. Different types of geometries and sizes were printed during the study with the aim to determine both the limits of the technique and the final mechanical properties of the pieces after the whole process (Fig 1). After printing, debinding, and sintering some of the samples were cut and observed by SEM to determine the presence of remaining porosity inside the pieces. The samples for tensile tests were subjected to a final precipitation heat-treatment to check the mechanical properties. As a general conclusion of the study, better results were obtained in small pieces with a small thickness. As thickness increases porosity and the amount of large defects created during the printing or debinding steps rises. On the other hand, pieces with regular geometries show less warping. This warping comes from the large shrinkage experimented by the samples during sintering. The hardness and yield strength of the tensile samples are within the expected range for this material, but ductility is easily affected by the presence of defects

Studying the Hall-Petch effect regarding sub-micrometer steel (0.6% C)

Este trabajo describe la obtención y caracterización mecánica de una aleación de acero 0,6% C con estructura de tamaño de grano inferior a 1 μm. El proceso para la obtención de piezas masivas se inicia sometiendo el polvo a severa deformación plástica en un molino planetario de bolas y a continuación se realiza la consolidación a alta presión y temperaturas entre 350 y 500 °C. El estudio de la evolución del tamaño de grano muestra que los consolidados sin tratamiento térmico posterior conservan su estructura en el rango nanométrico. En muestras con tratamiento térmico se observa un crecimiento controlado debido a los numerosos puntos de nucleación y la presencia de precipitados de cementita. Los resultados de dureza y tamaño de grano obtenidos cumplen la relación de Hall -Petch. Finalmente se analiza la influencia de las técnicas de obtención y de caracterización mecánica empleadas en este trabajo frente a diferentes fuentes bibliográficas.Postprint (published version

Study of the nanometric grain size distribution in iron compacts obtained by mechanical milling

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Equal channel angular pressing of a TWIP steel: microstructure and mechanical response

A Fe–20.1Mn–1.23Si–1.72Al–0.5C TWIP steel with ultrafine grain structure was successfully processed through equal channel angular pressing (ECAP) at warm temperature up to four passes following the BC route. The microstructure evolution was characterized by electron backscattered diffraction to obtain the grain maps, which revealed an obvious reduction in grain size, as well as a decrease in the twin fraction, with increasing number of ECAP passes. The texture evolution during ECAP was analyzed by orientation distribution function. The results show that the annealed material presents brass (B) as dominant component. After ECAP, the one pass sample presents A1* and A2* as the strongest components, while the two passes and four passes samples change gradually toward B/B¯B/B¯ components. TEM analysis shows that all samples present twins. The twin thickness is reduced with increasing the number of ECAP passes. Nano-twins, as a result of secondary twinning, are also observed in the one and two passes samples. In the four passes sample, the microstructure is extensively refined by the joint action of ultrafine subgrains, grains and twins. The mechanical behavior was studied by tensile samples, and it was found that the yield strength and the ultimate tensile strength are significantly enhanced at increasing number of ECAP passes. Although the ductility and strain hardening capability are reduced with ECAP process, the present TWIP steel shows significant uniform deformation periods with positive work hardening rates.Postprint (author's final draft

Analysis of the decrease of the apparent young's modulus of advanced high strength steels and its effect in bending simulations

In this paper the evolution of the Young’s Modulus (E) during unloading with plastic deformation has been studied for different Dual-Phase AHSS from DP780 to DP1400. During unloading, all the DP steels studied showed the presence of microplasticity so an Apparent Young’s Modulus (EA) has been defined. Although that in all cases EA decreased with a non-linear behavior as the plastic strain was increased, it has been observed that the final percentage of decrease seems to be related to the microstructure of DP steels. As the ferrite content increased as in the lower strength DP steels, the reduction of EA is larger, reaching a 21%. The introduction of the variation of the elastic response during unloading in the simulation of a bending operation has allowed obtaining an improvement of the accuracy in springback prediction in all the DP steels studied. For the low strength DP steels the final shape obtained by simulation is in fact the same than the real one. As the strength of steel is increased, the accuracy is less, especially in the DP 1400 steel, in which differences in bending angle higher than a 15% are still found.Postprint (published version

Process planning and metallurgical issues for laser asisted spin froming of dual phase automotive steel

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Study of the nanometric grain size distribution in iron compacts obtained by mechanical milling

A study has been carried out on the grain size distribution of cylindrical compacts obtained by consolidation of iron powder severely deformed by mechanical milling. Consolidation has been performed in two consecutive steps: cold and hot conditions. The hot one was done at two temperatures, namely 425 and 475ºC. After milling, the iron powder has a grain size of 8 nm (± 4 nm) with an average hardness of 800 HV. After hot compaction the grain size increases up to 50 nm, especially at 475ºC where a small fraction of grains reach larger values than the average. The grain size was evaluated by two different techniques, X-Ray Diffraction and Transmission Electron Microscopy. Results showed some differences between both methods. The advantage of using TEM is that grain size distribution, and not only the average size, can be obtained. Small discs were also obtained from the compacted specimen in order to fracture them on a “ball on three balls” equipment. The fracture behaviour of the samples was then studied by SEM.Postprint (published version

Activation process of air stable nanoscale zero-valent iron particles

Nanoscale Zero Valent Iron (nZVI) represents a promising material for subsurface water remediation technology. However, dry, bare nZVI particles are highly reactive, being pyrophoric when they are in contact with air. The current trends of nZVI manufacturing lead to the surface passivation of dry nZVI particles with a thin oxide layer, which entails a decrease in their reactivity. In this work an activation procedure to recover the reactivity of air-stable nZVI particles is presented. The method consists of exposing nZVI to water for 36 h just before the reaction with the pollutants. To assess the increase in nZVI reactivity based on the activation procedure, three types of nZVI particles with different oxide shell thicknesses have been tested for Cr(VI) removal. The two types of air-stable nZVI particles with an oxide shell thickness of around 3.4 and 6.5 nm increased their reactivity by a factor of 4.7 and 3.4 after activation, respectively. However, the pyrophoric nZVI particles displayed no significant improvement in reactivity. The improvement in reactivity is related mainly to the degradation of the oxide shell, which enhances electron transfer and leads secondarily to an increase in the specific surface area of the nZVI after the activation process. In order to validate the activation process, additional tests with selected chlorinated compounds demonstrated an increase in the degradation rate by activated nZVI particles.Postprint (author's final draft