Gonzalez-Ojeda, Roberto
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Gonzalez-Ojeda, Roberto
Official Name
Gonzalez Ojeda, Roberto
Alternative Name
robglez
González-Ojeda, R.
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ORCID
Scopus Author ID
56314856300
Researcher ID
DTL-2716-2022

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Now showing 1 - 10 of 36
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Structural Ultrafine Grained Steels Obtained by Advanced Controlled Rolling

2013 , Gonzalez-Ojeda, Roberto , García García, José Ovidio , Barbés Fernández, Miguel Ángel , Quintana, María José , Verdeja González, Luis Felipe , Verdeja González, José Ignacio

Steels with ultrafine grains (lower than 5 μpa), which usually known as ultrafine ferrite or ultrafine grained materials, are presently the object of intense research, because of the improvement in resistance and fracture toughness they may reach compared to conventional steels (with grain sizes above this value). It is shown that the forenamed steels designated in the Euronorm EN 10149-2, which are manufactured by advanced techniques of controlled rolling and mainly used in automotive industry, have an ultrafine grain size in the range of 2. 5 to 3. 5 μm, and with elastic yield stresses higher than 400 MPa. Based on the Morrison-Miller criterion, it is shown that values of the strain-hardening coefficient lower than 0. 08 would make the industrial application of these steels unfeasible.

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New binder phases for the consolidation of TiB2 hardmetals

1996 , Gonzalez-Ojeda, Roberto , Barandika, M.G. , Oña, D. , Sánchez, J.M. , Villellas, A. , Valea, A. , Castro, F.

New TiB2-based cermets have been consolidated by hot isostatic pressing of powder mixtures of TiB2 and metallic alloys based on Fe, Ni and Co additions. Relevant thermodynamic aspects of the systems B-Ti-Ni, B-Ti-Fe and B-Ti-Co, essential for avoiding the formation of extremely brittle secondary borides, especially the secondary boride M23B6, are discussed in this work. The role of contaminants like oxygen, carbon and nitrogen, which can also lead to the formation of these compounds is also explained. The hardness, toughness and cutting behaviour of the cermets produced with these new binder phases are comparable with the highest commercial WC-Co grades.

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Mechanical development, production and testing of a miniaturized high fidelity accelerometer for a Cubesat

2022 , Zarate-Villazon, Ángel M. , Aizpuru-Gutierrez, Lorenzo , Espinosa-Juarez, Erick , Martinez-Quintana, Eduardo , Sanchez-Zavala, Alejandro , Gonzalez-Ojeda, Roberto

There are numerous scientific missions that rely on a high fidelity accelerometer. The application of these devices ranges from fundamental physics and on-orbit thruster characterization, to modeling gravitational and aerodynamic perturbations in orbit. These high fidelity accelerometers have been used across different missions like the Gravity Probe, the LISA Pathfinder and Champ. Moreover, this technology has not yet been successfully miniaturized for its usage within the Cubesat form factor . While the specific scientific application of the accelerometer defines its feasibility for usage in a Cubesat, studies suggest there are possible missions such as atmospheric density characterization in this type of spacecraft. This paper presents the development and validation of the mechanism for a miniaturized high fidelity accelerometer. The high fidelity accelerometer here presented measures the relative displacement of a free flying test mass isolated from external perturbations to which the satellite is subjected to. The most important mechanical advancement from this proposal is a mechanism that constrains the test mass during the launch phase and releases it at the center of the test chamber once it is in orbit. Additionally, a convenient feature is to have the ability to enact this mechanism whenever the test mass needs to be recentered. Moreover, the selection of materials has to comply with the scientific requirements on how good the test mass shall be isolated from external forces while meeting the mass and wear requirements of the mechanism. All of these with a target volume of 1U of the Cubesat. The work presents the design and testing phases of the mechanical device associated with this high fidelity accelerometer. The design phase considers a variety of finite element analysis on different configurations of the mechanism. It also presents a study on the convenience of the materials selected using Ashby materials charts. The production phase identifies the appropriate manufacturing processes for a Cubesat’s scale. The testing campaign consists of functionality testing, a durability analysis of the mechanism and finally a comparative test once the mechanism has been subjected to vibration testing. Additionally, the tests must also determine the need and quality of lubrication on the mechanism as well as define operational requirements to be considered in the operations design of the mission.

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Grain Size Distribution on a Forged Ni-Cr-Mo Low-Alloy Steel

2018 , Quintana Hernández, María José , Ales, T. , Gonzalez-Ojeda, Roberto , Collins, P. , Kenney, M.

Microalloyed Ni-Cr-Mo steel (8620) combines good characteristics of formability during forging at 1100°C with the capacity to transform into martensite, while distributing residual stresses and allowing surface treatments and modifications in mechanical parts such as gears and cams. The work presents grain size measurements at different locations of a hot forged gear analyzing it by quantitative metallography techniques and simulating deformation by finite element modeling to estimate local strain and strain rate. Dynamic recrystallization mathematical models for the austenitic initial grain size before forging were applied to calculate theoretical final grain size and correlate the results with microstructural observations, taking also into account the role of precipitation of Cr and Mo carbides. Copyright © 2018 MS&T18®

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Effect of Mo on high entropy Ti-Nb-Zr-Ta alloy: Phase equilibria, microstructure and mechanical properties

2023 , Aranda, Víctor A. , Figueroa, I. A. , Amigó-Borrás, V. , Gonzalez-Ojeda, Roberto , Lozada Flores, Octavio , Vidilli, André Luiz , Barcelos Otani, Lucas , González, Gonzalo

Thermal and mechanical properties of Ti-Nb-Ta-Zr high entropy alloys are often influenced by element content and manufacturing routes, producing significant differences between mechanical properties and microstructure. This work presents a Ti-Nb-Ta-Zr alloy in which Mo is added by adjusting the composition with phase equilibria simulation, improving the mechanical properties based on a mixture of two chemically different solid solutions (BCC1 and BCC2). The materials were produced by arc-melting suction casting. Characterization of the dendritic and interdendritic zones was carried out by means of X-ray spectroscopy, indicating the segregation of Ta and Nb in BCC1 and Zr and Ti in BCC2 phases. The dislocation density increased preferentially in the interdendritic Zr-Ti rich zones. The mechanical properties results were related to the chemical differences between the BCC1 and BCC2 lattice parameters induced by the Mo addition. With Mo segregating between both BCC cells, the resulting microstructure increased the yield strength, being confirmed with the kernel average maps, which showed that, after compression tests, the interdendritic zone accumulated a high density of dislocations, resulting in the segregation of Ti and Zr, affecting the mechanical response of Mo containing alloy. © 2023 Elsevier B.V.

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Ultrafine-grain Steels: Mechanical Behavior

2016 , Quintana Hernández, María José , Gonzalez-Ojeda, Roberto , MARÍA JOSÉ QUINTANA HERNÁNDEZ;576914

In recent years, both the steelmaking industry and laboratories in different parts of the world, have shown an increasing interest in reaching an industrial-level production of ultrafine grained steels (also known as ultrafine ferrite), which have a grain size d lower than 5 μm, and enhanced mechanical resistance and fracture toughness. At room temperature, the capacity of the material to be deformed during bending or drawing operations (typical of requirements for automotive parts applications) depends on the interaction of a hard and a soft phase in the microstructure. On the other hand, at high temperatures, these steels may show superplastic behavior if deformed at a precise combination of temperature and strain rate.

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Ultrafine Grained HSLA Steels for Cold Forming

2010 , Gonzalez-Ojeda, Roberto , García García, José Ovidio , Barbés Fernández, Miguel Ángel , Quintana, María José , Verdeja González, Luis Felipe , Verdeja González, José Ignacio

The industrial level production of ultrafine grained (or ultrafine ferrite) ferrous alloys was investigated through three examples of steels that complied with the EN 10149-2 Euronorm and were produced by advanced controlled hot rolling techniques. The steel samples were tension tested and chemically analyzed, and the microstructure was evaluated through quantitative metallographic techniques to determine parameters such as yield stress, amount of microalloying elements, strain hardening coefficient, grain size, and grain size distribution. These steels were micro-alloyed with Ti, Nb, and Mn with ASTM grain sizes of approximately 13–15. The careful control of chemical composition and deformation during production, giving a specific attention to the deformation sequences, austenite non-recrystallization temperatures and allotropic transformations during cooling, are indispensable to obtain steels with an adequate strain hardening coefficient that allows cold working operations such as bending, stretching or drawing.

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Effect of pressure in the microstructure of die cast Al-8.5Si-3.5cu alloys

2014 , Barbés Fernández, Miguel Ángel , Verdeja González, José Ignacio , Quintana, María José , Verdeja González, Luis Felipe , Gonzalez-Ojeda, Roberto

The microstructure of Pressure Die Casting of an Al-8.5 Si-3.5 Cu alloy used for clean room tiles shows amount of constituents (eutectic and Si phases) that do not correspond to the ones indicated by the phase diagram. Furthermore, there are differences in amount of constituents between the core and the surface zones of parts produced by this process. The work presents quantitative microstructural analysis of this type of industrially produced part, and as a possible cause for these differences, simulations of the effect of pressure on the eutectic (temperature and chemical composition) in the Al-Si system. As the production rate of these parts is very high, and solidification is very fast, microstructural heterogeneity may be related to local entrapment of gasses unable to escape during casting.

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Microstructural changes of a construction steel caused by hot deformation

2013 , Verdeja González, José Ignacio , Quintana, María José , Gonzalez-Ojeda, Roberto , Verdeja González, Luis Felipe

A construction steel (shipbuilding strip) obtained by Advanced Thermomechanical Controlled Rolling Processes presents a room temperature banded ferrite-pearlite microstructure, and when superplastically deformed at 800°C with a strain rate of 5.85x10-5 s-1, the bands disappear as there is grain boundary sliding and grain cluster rotation. Nevertheless, the superplastic deformation does not imply a decrease in mechanical properties, as room temperature tests with strain rates of 1.46x10-3 s -1 with the steel previously deformed in superplastic conditions (until a 110% of straining) result in similar mechanical data. If the steel is deformed at 750°C with low strain rates, cooling results in a microstructure formed only by ferrite and carbides (the pearlitic phase disappears). This behavior may be explained, from a thermodynamical point of view, by the effect of negative hydrostatic pressure during the tensile test and the pronounced ferrite- and carbide-former capacity of Ti and Nb microalloying elements. The samples, tensile tested, in both the hot rolled raw state and superplastically deformed and then room temperature tested, show in the fracture surface SEM analysis almost identical features: decohesions surrounding MnS and (C,N)(Ti,Nb) precipitates and between ferrite and pearlite grains, as well as bedding.

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Superplastic HSLA Steels: Microstructure and Failure

2013 , Fernández, Sara , Quintana, María José , García García, José Ovidio , Verdeja González, Luis Felipe , Gonzalez-Ojeda, Roberto , Verdeja González, José Ignacio

Certain materials can show superplasticity when traction tested at temperatures higher than 50% of their melting point and with low strain rates ( < 10−2 s−1), showing very high elongations (>100%) without localized necking and mainly intergranular fractures. This behavior requires that the starting grain size is small (<10 μm) so the flow of matter can be non-homogeneous (sliding and rotating of the grain boundaries, accommodated by diffusion). This work presents the superplastic characteristic of shipbuilding steel deformed at 800 °C and a strain rate slower than 10−3 s−1. The fine grain size (5 μm) is obtained when using Nb as a microalloying element and manufactured by controlled rolling processes (three stages). After the superplastic deformation, the steel presents mixed fractures: by decohesion of the hard (pearlite and carbides) and ductile (ferrite) phases and by intergranular sliding of ferrite/ferrite and ferrite/pearlite, just as it happens in stage III of the creep behavior. This is confirmed through the Ashby–Verrall model, according to which the dislocation creep (power-law creep) and diffusion creep (linear-viscous creep) occur simultaneously.

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