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

Influence of thermomechanical loads on the energetics of precipitation in magnesium aluminum alloys

We use first principles calculations to study the influence of thermomechanical loads on the energetics of precipitation in magnesium-aluminum alloys. Using Density Functional Theory simulations, we present expressions of the energy of magnesium-aluminum binary solid solutions as a function of concentration, strain and temperature. Additionally, from these calculations, we observe an increase in equilibrium volume (and hence the equilibrium lattice constants) with the increase in concentration of magnesium. We also observe an increase in the cohesive energy of solutions with increase in concentration, and also present their dependence on strain. Calculations also show that the formation enthalpy of β phase solutions to be strongly influenced by hydrostatic stress, however the formation enthalpy of α phase solutions remain unaffected by hydrostatic stress. We present an expression of the free energy of any magnesium aluminum solid solution, that takes into account the contributions of strain and temperature. We note that these expressions can serve as input to process models of magnesium-aluminum alloys. We use these expressions to report the influence of strains and temperature on the solubility limits and equilibrium chemical potential in Mg-Al alloys. Finally, we report the influence of thermomechanical loads on the growth of precipitates, where we observe compressive strains along the c axis promotes growth of precipiates with a (0001)_α habit plane, whereas strains along the a and b directions do not influence the growth of precipitates

Thermomechanical Processing Of Mg-Li-Al Ultralight Alloys

Globally, magnesium (Mg), as the lightest metallic material, imparts a significant long term impact on the stipulation of lightweight structures in aerospace and automotive industries. However, the deformation behavior of magnesium at ambient (room) temperature is not acceptable for most of the structural applications because of its hexagonal closed pack (hcp) structure and limited active slip systems which result in an unacceptable level of brittleness (literally no formability at ambient temperature). Having said this, alloying Mg with an element with more active slip system in its crystalline structure (i.e. lithium with body center cubic crystalline structure) might be a solution to improve strength and ductility of the Mg. Addition of the lithium (Li) as the lightest element (density 0.54 g/cm3) in Mg (density ~1.74 g/cm3) results in enhanced plasticity producing ultra-light metallic alloys of Mg-Li with density of 1.35-1.65 g/cm3. The Mg-Li alloys are considered as the lightest metallic alloys which make them unique for many weight-saving applications. Aluminum reinforcement in Mg-Li matrix develops the strength and corrosion resistance without introducing any second phase in the alloy. Because of its high solubility in Mg, aluminum can improve the strength without deterioration of density of Mg-Li alloy by means of grain size refining, solid solution hardening, and compound reinforcements. Although the alloy (Mg-Li-Al) possesses acceptable elastic modulus to density ratio, specific strength, damping capabilities, and electromagnetic shielding capability, extensive applications of Mg-Li alloys are still limited due to relatively low strength, poor corrosion resistance, and limited thermal stability. In the present project a single phase Mg-3.5Li-Al (wt%) and a dual phase Mg-14Li-Al (wt%) are first thermomechanically processed through hot compression tests. Then microstructure, post thermomechanical properties, and hot compression stress-strain curves are studied. The main objective is to correlate the microstructure with the small scale properties and processing parameters (i.e. thermomechanical parameters like temperature and strain rate). The thermomechanical cycle consists of uniaxial compression at temperatures of 250°C, 350°C, and 450°C and strain rates of 1, 0.1, 0.01, and 0.001 /s using a Gleeble® 3500 thermal-mechanical simulation testing system. True stress-true strain curves plotted from the thermomechanical tests were used to assess the working behavior of the materials, to analyze and to understand the microstructure evolution which reflect intrinsic mechanical properties. Microstructural changes, hot compression stress-strain curves, and nanoindentation load/displacement response, to study post processing mechanical properties, for both alloys were discussed in detail in the present research

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

Correlating microstrain and activated slip systems with mechanical properties within rotary swaged WNiCo pseudoalloy

Due to their superb mechanical properties and high specific mass, tungsten heavy alloys are used in demanding applications, such as kinetic penetrators, gyroscope rotors, or radiation shielding. However, their structure, consisting of hard tungsten particles embedded in a soft matrix, makes the deformation processing a challenging task. This study focused on the characterization of deformation behavior during thermomechanical processing of a WNiCo tungsten heavy alloy (THA) via the method of rotary swaging at various temperatures. Emphasis is given to microstrain development and determination of the activated slip systems and dislocation density via neutron diffraction. The analyses showed that the grains of the NiCo2W matrix refined significantly after the deformation treatments. The microstrain was higher in the cold swaged sample (44.2 x 10(-4)). Both the samples swaged at 20 degrees C and 900 degrees C exhibited the activation of edge dislocations with {110} or {111} slip systems, and/or screw dislocations with slip system in the NiCo2W matrix. Dislocation densities were determined and the results were correlated with the final mechanical properties of the swaged bars.Web of Science131art. no. 20

A combined neuro fuzzy-cellular automata based material model for finite element simulation of plane strain compression

This paper presents a modelling strategy that combines Neuro-Fuzzy methods to define the material model with Cellular Automata representations of the microstructure, all embedded within a Finite Element solver that can deal with the large deformations of metal processing technology. We use the acronym nf-CAFE as a label for the method. The need for such an approach arises from the twin demands of computational speed for quick solutions for efficient material characterisation by incorporating metallurgical knowledge for material design models and subsequent process control. In this strategy, the cellular automata hold the microstructural features in terms of sub-grain size and dislocation density which are modelled by a neuro-fuzzy system that predicts the flow stress. The proposed methodology is validated on a two dimensional (2D) plane strain compression finite element simulation with Al-1% Mg alloy. Results from the simulations show the potential of the model for incorporating the effects of the underlying microstructure on the evolving flow stress fields. In doing this, the paper highlights the importance of understanding the local transition rules that affect the global behaviour during deformation

The effects of powder metallurgical processing and intermediate thermal mechanical treatment on the fatigue properties of high strength aluminum alloys

Issued as Progress reports no. [1-5], Technical report, Annual report, and Final reports no. [1-2], Project no. E-19-61

The Influence of Secondary Processing Conditions on the Mechanical Properties and Microstructure of a Particle Reinforced Aluminium Metal Matrix Composite

The influence of secondary processing conditions on an aluminium metal matrix composite, comprising of an AA2124 matrix and 3 Jlm particulate SiC reinforcement at 25 volume percent was investigated. The metal matrix composite (MMC) was extruded at three different temperatures, 350???????C, 450???????C and 550???????C, at a ratio of20:1 and at three different ratios, 5:1, 10:1 and 20:1, at a temperature of 450???????C. It was subsequently solution heat treated and naturally aged. A mechanical property assessment was carried out using standard tensile and rotating bend fatigue test methods to determine the properties of the material extruded under each condition. A novel technique using a Focussed Ion Beam (FIB) Microscope was developed to prepare polished specimens and microtextural analysis was performed by FIB imaging. Additionally, techniques were successfully established, through the use of FIB milling and polishing, to provide site-specific electron transparent films, permitting detailed examination ofthe microstructure with a transmission electron microscope. Material extruded at 550???????C exhibited a lower yield strength than material extruded at 350???????C and 450???????C, which was attributed to grain coarsening and recrystallisation. Evidence of recrystallisation was found during texture analysis by X-Ray diffraction, where there was a reduction in the intensity of the fibre texture in the extrusion direction. The phenomenon was also observed during irticrostructural analysis work, where recrystallised grains at grain boundaries were observed. Higher extrusion ratios offered a small improvement in tensile properties, due to an enhanced fibre texture within the microstructure. Microtextural examination gave evidence of the existence of both high angle grain and low angle grain boundaries for the material extruded at 350???????C. It is believed that a subgrain structure was partially transformed during extrusion, through subgrain rotation, leading to the formation of high angle grain boundaries. This'microstructure was found to offer the optimum mechanical properties.Imperial Users onl