http://hdl.handle.net/2117/2175 2013-05-23T19:55:33Z http://hdl.handle.net/2117/18718 Title: Microstructural, thermical and mechanical characterization of ZK60 alloy processed through ECAP Authors: Dumitru, Florina Diana; Ghiban, Brândusa; Cabrera Marrero, José M.; Higuera, Oscar Fabián; Ghiban, Nicolae; Gurau, Gheorghe H. 2013-04-09T09:11:41Z http://hdl.handle.net/2117/18717 Title: Hot deformation activation energy (QHW) of austenitic Fe-22Mn-1.5Al-1.5Si-0.4C TWIP steels microalloyed with Nb, V, and Ti Authors: Reyes Calderón, F.; Mejía Granados, Ignacio; Cabrera Marrero, José M. Abstract: The activation energy for hot deformation ( Q HW ) of high-Mn microalloyed TWIP steels was determined from experimental uniaxial hot compression curves. The presence of microalloying elements such as Nb, V, and Ti, increases the Q HW value from 366 in the non-microalloyed one to 446 kJ/mol in the V-microalloyed TWIP steel. This change represents an increase from 16% up to 22% of Q HW values. 2013-04-09T09:06:14Z http://hdl.handle.net/2117/18714 Title: Effect of microalloying elements (Nb, V and Ti) on the hot flow behavior of high-Mn austenitic twinning induced plasticity (TWIP) steel Authors: Reyes Calderón, F.; Mejía Granados, Ignacio; Boulaajaj, A.; Cabrera Marrero, José M. Abstract: This research work studies the effect of microalloying elements such as Nb, V and Ti on the hot flow behavior of high-Mn austenitic TWIP steel. For this purpose, isothermal uniaxial hot compression tests were carried out at three temperatures (900, 1000 and 1100 1 C) and four constant strain rates (10 1 , 10 2 ,10 3 and 10 4 s 1 ). Experimental results revealed that hot flow curves of microalloyed TWIP steels show single peak curves for all test conditions. Results are discussed in terms of the peak stress ( s p ) and peak strain ( e p ) and its dependence on the strain rate ( _ e ) and temperature. The addition of microalloying elements such as Nb, V and Ti in TWIP steels generates a slight increase in the s p value, and Ti microalloyed TWIP steel exhibits the highest s p value. Hot deformed microstructures were analyzed by the Electron Back-Scattering Diffraction Technique (EBSD). The most important results of the austenitic recrystallized grain refinement were obtained for V and Ti microalloyed TWIP steels. 2013-04-09T07:38:14Z http://hdl.handle.net/2117/18712 Title: Determination of the critical parameters for the onset of dynamic recrystallization (DRX) in advanced ultrahigh strength steels (A-UHSS) microalloyed with boron Authors: Altamirano Guerrero, Gerardo; Mejía Granados, Ignacio; Cabrera Marrero, José M. Abstract: In this research work, the double differentiation mathematical method was used to identify more accurately the criti- cal stress ሺ ߪ ௖ ) and critical strain ሺ ߝ ௖ ) associated with the onset of dynamic recrystallization (DRX), which is based on changes of the strain hardening rate )ߝ߲/ߪ߲ ൌ ߠሺ as a function of the flow stress (Poliak and Jonas method, simplified by Najafizadeh and Jonas). For this purpose, a low carbon advanced ultra-high strength steel (A-UHSS) microalloyed with different amounts of boron (14, 33, 82, 126 and 214 ppm) was deformed by uniaxial hot-compression tests at high tempe- ratures 1100 ݀݊ܽ ሺ950,1000,1050 ° )ܥ and constant true strain rates ሺ 10 ିଷ ,10 ିଶ 10 ݀݊ܽ ିଵ ݏ ିଵ ) . Results indicate that both ߪ ௖ and ߝ ௖ increase with decreasing deformation temperature and in creasing strain rate. On the other hand, these criti- cal parameters tend to decrease as boron content increases. Such a behavior is attributed to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect. 2013-04-09T07:33:06Z http://hdl.handle.net/2117/18698 Title: On the onset of dynamic recrystallization in steels Authors: Varela Castro, Gonzalo; Cabrera Marrero, José M.; Prado Pozuelo, José Manuel Abstract: The knowledge of the flow behavior of metallic all oys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanis ms available for the control of microstructure. This mechanism occurs during hot forming operations over a wide range of metals and alloys and it is known to be as a powerful tool which can be used to the control of the microstructure and properties of alloys. Therefore is important to kno w, particularly in low stacking fault energy (SFE) materials, the precise time for which DRX is availa ble to act. At constant strain rate such time is defined by a critical strain, . Unfortunately this critical value is not directly measurable on the flow curve; as a result different methods have been developed to derive it. Focused on steels, in the present work the state of art on the critical strai n for the initiation of DRX is summarized and a review of the different methods and expressions for determining is included. The collected data is suitable to feeding constitutive models. 2013-04-08T12:00:48Z http://hdl.handle.net/2117/18696 Title: Equal channel angular pressing of Cu-Al bimetallic rods Authors: Llorca Isern, Nuria; Escobar, Ana Maria; Roca, Antoni; Cabrera Marrero, José M. Abstract: Coextrusion and corolling are the major processes to produce bimetallic rods, tubes and wires, the objective being to perform clad metals, bimetallic joints or seals. The aim of the present work is to produce bimetallic rods showing an ultra fine grained microstructure with enhanced properties. Bimetallic Cu-Al rods were deformed by equal channel angular pressing (ECAP) in order to study their microstructure. ECAP is an int eresting process for producing bulk materials with refined microstructure and, consequently, chan ges in physical, chemical and mechanical properties can be observed. Higher shear strength a nd dimensional stability are among the advantages of this process. A comparative experimen tal study of pure commercial copper with cylindrical inner aluminium rods of different diame ters processed by one-pass equal channel angular pressing has been carried out. The ECAP die used in this research was a 90º 2-channels intersecting angle. Electron backscattered (EBSD) a nd X-ray diffraction techniques were used for microstructure characterization (deformation, grain fragmentation and microstrain evaluation) at the interfaces and away from them. It was found that th e microstructure in the ECAP deformed Cu-Al bimetallic rods was influenced by the dimensions of the aluminium inner rod. In fact, the microstructure appeared to be much more elongated a nd refined in the samples containing smaller diameter aluminium rods. 2013-04-08T11:39:28Z http://hdl.handle.net/2117/18693 Title: Comportamiento termomecánico de un acero microaleado al Nb-V Authors: Cartaya, E. A.; Varela, G.; Cabrera Marrero, José M.; Prado Pozuelo, José Manuel Abstract: Empleando ensayos de compresión simple en ca- liente y relajación de tensiones después de la de - formación en un dilatómetro modelo DIL805T, se obtuvieron las curvas de fluencia y relajación de dos aceros, el primero de ellos al C – Mn y otro HS- LA con idéntica composición química base, micro- aleado con Nb – V. Los ensayos de compresión simple fueron efectua- dos a temperaturas en el rango austenítico, desde 900 – 1.200 ºC y velocidades de deformación de 0.001, 0.01, 0.1, 1 y 10 s - 1 . Se obtuvieron las curvas de fluencia para las diferentes condiciones, permitien- do el estudio del efecto de los elementos microale- antes sobre la recristalización dinámica y la micro- estructura final; pudiendo comprobar mediante un análisis microestructural el efecto retardador en la cinética de recristalización que presenta el acero microaleado, el incremento en la deformación críti- ca y en la energía de activación. Se determina asi- mismo por medio del método de relajación de ten - siones la cinética de precipitación, elaborando los diagramas precipitación – temperatura – tiempo 2013-04-08T11:10:36Z http://hdl.handle.net/2117/16664 Title: The origin of microstructure inhomogeneity in Mg-3Al-1Zn processed by severe plastic deformation Authors: Fatemi Varzaneh, Mahmood; Zarei-Hanzaki, A.; Vaghar, R.; Cabrera Marrero, José M. Abstract: A Mg–3Al–1Zn alloy has been severely deformed by accumulative back extrusion to investigate the effects of processing parameters (temperature and deformation pass) on the microstructure homogeneity. The results indicate that the activation of different grain refinement mechanisms in magnesium may result in an inhomogeneous microstructure, i.e. a bimodal grain size distribution. However, a rapid rate of homogeneity development has been occurred through applying further subsequent passes. This has been justified considering the imposed strain reversal during accumulative back extrusion processing. 2012-10-08T09:47:10Z http://hdl.handle.net/2117/16599 Title: Effect of V on Hot Deformation Characteristics of TWIP Steels Authors: Reyes, Francisco; Calvo Muñoz, Jessica; Cabrera Marrero, José M.; Mejía, Ignacio Abstract: Twinning induced plasticity (TWIP) steels, which rely on high Mn contents to promote twinning as the deformation mechanism, exhibit high ultimate strengths together with outstanding combinations of ultimate strength and ductility. In terms of mechanical properties, one of the most important microstructural features is grain size. The knowledge of the kinetics of recrystallization mechanisms, i.e., dynamic recrystallization (DRX) and static recrystallization (SRX), can be used in order to control the grain size of the final product by a proper rolling schedule design. The focus of this work is the characterization of the DRX kinetics of two TWIP steels. The basic composition of the steels is Fe–21Mn–0.4C–1.5Al–1.5Si, and one of them is further alloyed with 0.12% V. With this objective, compression tests were carried out at 900, 1000, and 11008C and strain rates ranging from 1 10 1 s 1 to 1 10 4 s 1. Furthermore, metallographic observation by optical microscopy (OM) was done to assess the evolution of grain size for the different deformation conditions. According to the results, the existence of V in the composition does not affect the hot flow behavior of the steel, although recrystallization fraction and recrystallized grain size decrease for the V-containing steel. 2012-10-02T11:13:46Z http://hdl.handle.net/2117/16185 Title: Characterization of precipitation kinetics of Inconel 718 superalloy by the stress relaxation technique Authors: Calvo Muñoz, Jessica; Cabrera Marrero, José M.; Shu, Sheyu Abstract: Inconel 718 is a nickel-chromium-iron superalloy which presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These characteristics can be optimized with an appropriate control of microstructural features such as grain size and precipitation. Precipitates of different nature can form in these alloys, i.e. γ’’ (a metastable metallic compound Ni3Nb), γ’ (Ni3(Ti, Al), carbides and/or δ phase (intermetallic Ni3Nb). Aging treatments are usually designed to obtain the precipitation required in order to optimize mechanical properties. However, precipitation can also appear induced by deformation and therefore interfere with hot forming operations, such as forging. Under these conditions, precipitation may lead to an increase of the loads required to carry out the process. The aim of the work was the characterization of precipitation kinetics for Inconel 718. With this purpose, stress relaxation tests were carried out at temperatures ranging from 950°C to 800°C. Moreover, different amounts of deformation were applied to the samples, prior to stress relaxation, to evaluate the effect of this variable on inducing precipitation. Some samples were quenched at different relaxation times for metallographic evaluation. The results obtained through mechanical testing, together with a proper characterization of precipitation by Scanning Electron Microscopy, were the basis for obtaining precipitation-timetemperature (PTT) diagrams after different deformation conditions. 2012-07-05T10:01:19Z http://hdl.handle.net/2117/16183 Title: Comportamiento a fractura de aceros metaestables Authors: Mateo García, Antonio Manuel; Gutierrez, D.; Lara, A.; Zapata Dederle, Ana Cristina; Rodríguez Calvillo, Pablo; Fargas Ribas, Gemma; Calvo Muñoz, Jessica; Casellas Padró, Daniel 2012-07-05T09:39:11Z http://hdl.handle.net/2117/16169 Title: Effect of the testing conditions on the hot ductility and fracture mechanisms of a C-Mn steel Authors: Calvo Muñoz, Jessica; Cabrera Marrero, José M.; Rezaeian, A.; Yue, S. 2012-07-04T11:21:28Z http://hdl.handle.net/2117/15681 Title: EBSD study of a hot deformed austenitic stainless steel Authors: Mirzadeh, H.; Cabrera Marrero, José M.; Najafizadeh, A.; Rodríguez Calvillo, Pablo Abstract: The microstructural evolution of a 304 H austenitic stainless steel subjected to hot compression was studied by the electron backscattered diffraction (EBSD) technique. Detailed data about the boundaries, coincidence site lattice (CSL) relationships and grain size were acquired from the orientation imaging microscopy (OIM) maps. It was found that twins play an important role in the nucleation and growth of dynamic recrystallization (DRX) during hot deformation. Moreover, the conventional discontinuous DRX (DDRX) was found to be in charge of grain refinement reached under the testing conditions studied. Furthermore, the recrystallized fraction (X) was determined from the grain average misorientation (GAM) distribution based on the threshold value of 1.55°. The frequency of high angle boundaries showed a direct relationship with X. A time exponent of 1.11 was determined from Avrami analysis, which was related to the observed single-peak behavior in the stress–strain flow curves. 2012-03-28T11:17:09Z http://hdl.handle.net/2117/15240 Title: OptiLam: design of optimised rolling schedules Authors: Peña, B.; Arribas, M.; Carrillo, A.R.; Barbero, J. I.; Calvo Muñoz, Jessica; Song, Yue Abstract: Nowadays il is known that the Ihennomcchanical schedules applied during hot rolling of fIal products provide the steel with improvcd mechanical properties. In Ihis work an oplimisation tool, OptiLam (OptiLam v.l), bascd on él predictive software alld capabIe oC generating optimised rolling schedulcs to obtain Ihe dcsired mechanical properties in the fiual product is described. OptiLam includes somc well-knowll Illctallurgical modcls which predict microstmctural evolution duril1g hot rolling and the trnnsConnation austenite/ferrite during the cooling. Furthennore, an oplimisation algorithlll, which is bascd 011 the gradient mclhod, has been added, in order to design thermoI11cchanical sequences whell a spccific final grain size is desired. OptiLam has been used to optimise rolling parameters, such as slmin and tempcrature. Here, some of the results of Ihe software validation performed by means of hol torsion tests are presented, showing also the fUllctionality of the tool. Finally, the application of classical optimisatiol1 1U0dels, based 011 Ihe gradient melhod, lo hot rolling operations, is al so discussed. 2012-02-20T10:20:55Z http://hdl.handle.net/2117/14944 Title: Studying the Hall-Petch effect regarding sub-micrometer steel (0.6% C) Authors: Rodríguez Baracaldo, R.; Cabrera Marrero, José M.; Benito Páramo, José Antonio Abstract: 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. 2012-02-06T09:32:21Z