Microstructure and mechanical properties of two tool steels with ultrahigh boron content

[ES] En el presente trabajo se han modificado dos aceros para herramientas convencionales por la presencia de un contenido de boro entre 0,5 y 1 % en masa. Ambos aceros se procesaron por la ruta pulvimetalúrgica, incluyendo atomización por argón y compactación isostática en caliente. El material compactado presentó una microestructura caracterizada por una distribución de partículas de borocarburos M23(C,B)6 fma y homogénea en una matriz de ferrita-martensita. Esta microestructura permanece prácticamente inalterada tras los ensayos de cambios en la velocidad de deformación durante el ensayo de compresión a temperaturas entre 750 y 1.000 °C. En el caso del acero Fe-lB-lC se obtuvo un valor para el exponente de la tensión de 4,5, que sugiere que la deformación plástica está controlada por un mecanismo de fluencia por movimiento de dislocaciones. Por otro lado, para el acero Fe-0,5B-l,5C, se obtuvieron valores para el exponente de la tensión comprendidos entre 2 y 3, que sugieren que la deformación plástica está controlada por un mecanismo de deslizamiento de fronteras de granos. En ambos casos, se encontró un valor de la energía de activación para la deformación plástica que se asoció con la energía de autodifusión del hierro a través de la red.[EN] In the present work, two selected tool steels have been modified by a boron addition of 0.5 and 1 mass %. Both steels were processed by powder metallurgy methods, including argón atomization and hot isostatic pressing. The Consolidated materials presented a microstructure consisting of a fine and homogeneous distribution borocarbides M23(C,B)6 in a ferrite-martensite matrix. No changes are observed in the microstructure after deformation by compression-strain-rate-change tests at temperatures ranging from 700 to 1,100 °C. For the Fe-lB-lC steel, a stress exponent of 4.5 was obtained, that suggests that slip creep is the controlling deformation mechanism. On the other hand, a stress exponent between 2 and 3 was obtained for the Fe-0.5B~1.5C steel that suggests that grain boundary sliding is the controlling deformation mechanism. In both cases, the activation energy for creep was related to the activation energy for iron self-diffusion.Peer reviewe

Superplastic Behavior of a Fine Grained AZ61 Alloy Processed by Large Strain Hot Rolling

A processing route based on conventional hot rolling has been recently developed to induce significant grain refinement in Mg alloys. The simplicity and rapidity of the processing route as well as the fact that conventional rolling is used, may allow it to be put into practice successfully in industry. This method consists of only two to three rolling passes, each producing a large thickness reduction, and intermediate annealings of 5 minutes duration. The resulting microstructure is mainly formed by very fine grains. Tensile tests at different temperatures and strain rates were performed in order to analyse the mechanical behaviour of the processed AZ61 alloy under different testing conditions. Microstructure and texture evolution during deformation were examined by optical microscopy (OM) and X-ray diffraction, respectively. It is shown that the microstructures developed by large strain hot rolling are capable of exhibiting significant superplastic elongations at moderate to low temperatures. Stress exponents close to 2 were measured during deformation under optimum superplastic conditions. Additionally, grains remained equiaxed and a significant decrease in the texture intensity is observed. This is consistent with the predominance of grain boundary sliding as the main deformation mechanism responsible for superplasticityCICYT grant MAT2000-1313 is appreciated. MTP acknowledges a Ramón y Cajal 2001 contract, awarded by the Spanish Ministry of Science and Technology. JAV is thankful to CONICETPeer reviewe

Damage tolerance assessment by bend and shear tests of two multilayer composites: Glass fibre reinforced metal laminate and aluminium roll-bonded laminate

The damage tolerance of an aluminium roll-bonded laminate (ALH19) and a glass fibre reinforced laminate (GLARE) (both based in Al 2024-T3) has been studied. The composite laminates have been tested under 3-point bend and shear tests on the interfaces to analyze their fracture behaviour. During the bend tests different fracture mechanisms were activated for both laminates, which depend on the constituent materials and their interfaces. The high intrinsic toughness of the pure Al 1050 layers present in the aluminium roll-bonded laminate (ALH19), together with extrinsic toughening mechanisms such as crack bridging and interface delamination were responsible for the enhanced toughness of this composite laminate. On the other hand, crack deflection by debonding between the glass fibres and the plastic resin in GLARE, was the main extrinsic toughening mechanism present in this composite laminateFinancial support from CICYT (Project MAT2003-01172) is gratefully acknowledged. C.M. Cepeda-Jiménez thanks the Spanish Ministry of Education and Science for a Juan de la Cierva contract. Authors also thank Delft University of Technology for providing the GLARE.Peer reviewe

Influence of thermomechanical processing on the grain size, texture and mechanical properties of Mg-Al alloys

El título de la revista en eslovaco es "Kovové Materiály"The work carried out by the authors over the last decade on the processing, microstructural characterization and the mechanical behaviour of Mg alloys is reviewed. In particular, the potential for grain refinement and for the development of specific textures of large strain hot rolling (LSHR), equal channel angular pressing (ECAP) and accumulative roll bonding (ARB) is discussed. The recrystallization and the deformation mechanisms predominant in Mg alloys are analyzed as a function of the grain size and the texture in a wide range of stresses, strain rates and temperatures. Finally, the the feasibility of superplatic forming of Mg alloys was examined, taking into account the influence of factors such as grain size stability and microstructural heterogeneities.The authors acknowledge financial support from CICYT under program MAT 2003/1172Peer reviewe

Influence of thermomechanical processing on the grain size, texture and mechanical properties of Mg-Al alloys

El título de la revista en eslovaco es "Kovové Materiály"The work carried out by the authors over the last decade on the processing, microstructural characterization and the mechanical behaviour of Mg alloys is reviewed. In particular, the potential for grain refinement and for the development of specific textures of large strain hot rolling (LSHR), equal channel angular pressing (ECAP) and accumulative roll bonding (ARB) is discussed. The recrystallization and the deformation mechanisms predominant in Mg alloys are analyzed as a function of the grain size and the texture in a wide range of stresses, strain rates and temperatures. Finally, the the feasibility of superplatic forming of Mg alloys was examined, taking into account the influence of factors such as grain size stability and microstructural heterogeneities.The authors acknowledge financial support from CICYT under program MAT 2003/1172Peer reviewe

Effect of thermal treatment on the interfacial shear toughness of an aluminium composite laminate

The microstructure and mechanical properties in the interface region of a multilayer composite laminate based on Al-Zn (Al 7075) and Al-Cu (Al 2024) alloys have been mainly characterized by EBSD and shear tests. It is shown that varying solution heat treatments affect the microstructure of the constituent aluminium alloys in the bonding region and, as a consequence, the interfacial mechanical properties. The increase in the solution treatment time improves the interfacial toughness of the multilayer aluminium laminate due to higher intrinsic toughness of the constituent aluminium alloys.Financial support from CICYT (Project MAT2003-01172 and MAT2006-11202) is gratefully acknowledged. C.M. Cepeda-Jiménez thanks the Spanish National Research Council (CSIC) for a I3P contract. We also thank F.F. González-Rodríguez for assistance during hot rolling. Finally, an especial mention in memory of P.J. González-Aparicio for his help and assistance with electron microscopy during all these years is made.Peer reviewe

High Strain Rate Superplasticity of WE54 Mg Alloy after Severe Friction Stir Processing

Friction stir processing (FSP) was used on coarse-grained WE54 magnesium alloy plates of as-received material. These were subjected to FSP under two different cooling conditions, refrigerated and non-refrigerated, and different severe processing conditions characterized by low rotation rate and high traverse speed. After FSP, ultrafine equiaxed grains and refinement of the coarse precipitates were observed. The processed materials exhibited high resistance at room temperature and excellent superplasticity at the high strain rate of 10−2 s−1 and temperatures between 300 and 400 °C. Maximum tensile superplastic elongation of 726% was achieved at 400 °C. Beyond 400 °C, a noticeable loss of superplastic response occurred due to a loss of thermal stability of the grain size. Grain boundary sliding is the operative deformation mechanism that can explain the high-temperature flow behavior of the ultrafine grained FSP-WE54 alloy, showing increasing superplasticity with increasing processing severity.Project MAT2015-68919-R MINECO/FEDER, Spain, and FPI fellowship number BES2013-063963 (MINECO/FEDER/ESF).Peer reviewe

Evolution of the microstructure, texture and creep properties of the 7075 aluminium alloy during hot accumulative roll bonding

The 7075 Al alloy was severely deformed at 350 °C by a 3:1 thickness reduction per pass accumulative roll bonding (ARB) process up to six passes. It was found that discontinuous recrystallisation occurs during the inter-pass annealing stages from the third pass on, attributable to the increment of the mean particle size during processing. As a consequence, the mean crystallite size did not decrease, but remained approximately constant at 440 nm along the present ARB process and the mean boundary misorientation angle reached a maximum of 30° for the 3-passes sample. However, since nucleation of new grains takes place at the pre-existing grain boundaries, discontinuous recrystallisation results in slight changes in texture throughout the processing, being the orientations in the ARBed samples predominantly located along the typical rolling β-fibre. Uniaxial tests conducted at 300 °C and 350 °C revealed that the operating deformation mechanism in the processed alloy at such temperatures was grain boundary sliding; the optimum superplastic strain rate being 3×10−3–10−2 s−1. Boundary misorientation and thermal stability are the two main factors that contribute to high elongations to failureFinancial support from the CICYT (Projects MAT2009-14452 and MAT2012-38962)Peer reviewe

Analysis of adiabatic heating and its influence on the Garofalo equation parameters of a high nitrogen steel

Torsion tests at high temperatures and high strain rates were conducted on a high nitrogen steel (HNS). Under these conditions, adiabatic heating influences its flow behavior. This work focus on a new algorithm for conducting the adiabatic heating correction of stress-strain curves. The algorithm obtains the stress-strain curves at quasiisothermal conditions from those at adiabatic conditions. The corrections in stress obtained can be higher than 15% and increase with increasing strain rates and decreasing temperatures. On the other hand, an upper bound for the temperature rise was found using a dynamic material behavior approach. Finally, the influence of adiabatic heating correction on the Garofalo equation parameters of HNS was analyzed. High values of activation energy and stress exponent were attributed to reinforcement by dispersed carbonitrides and the high amount of alloying elements.The work was carried out through the Projects PBC-05-010-1 from JCCM (Castilla- La Mancha, Spain) and MAT2006-13348 from CICYT. Thanks are given to Prof. E Evangelista for valuable assistance.Peer reviewe

Influence of the thermomechanical processing on the superplastic forming of Mg-Al alloys

The aim of this paper is to study the influence of the initial microstructure of several Mg–Al alloys on their superplastic formability and on their post-forming microstructure and mechanical properties. Various thermomechanical processing routes, such as annealing, conventional rolling, severe rolling and cross rolling, were used in order to fabricate AZ31 and AZ61 alloys with different grain sizes. These materials were then blow formed into a hat shaped die. It was found that the processing route has only a small effect in the formability of Mg–Al alloys or on the postforming microstructures and properties due to rapid dynamic grain growth taking place at the forming temperatures. Nevertheless, good formability is achieved as a result of the simultaneous operation of grain boundary sliding and crystallographic slip during forming.The authors are thankful to Comunidad de Madrid for funding this work under grant GR-MAT-0715-2004. FS is grateful to CSIC for a postgraduate grant. JAV acknowledges support from a Ramón y Cajal contract awarded by the Spanish Ministry of Education and Science.Peer reviewe