Observation of deformation modes in Mg-3Al-1Zn

Since magnesium alloys are the lightest metallic materials, they are very attractive for automotive and aerospace industries. The main problem of these alloys is limited ductility due to a shortage of independent slip systems. In order to improve the formability in these alloys, an understanding of the deformation modes is required. In the present work, different slip systems were investigated in rolled Mg-3Al-IZn by means of in situ tensile tests in the SEM. These permitted electron backscatter diffraction (EBSD) and electron backscatter diffraction imaging (QBSD) to be carried out during the test. The results show that non-basal slip systems are active at room temperature.

Texture, twinning and uniform elongation of wrought magnesium

A small number of crystal plasticity simulations and tensile tests are carried out with the aim of demonstrating that control of twinning can improve the uniform elongation of magnesium based alloys, It is suggested that this can be accomplished through texture manipulation because texture influences both the fraction of grains that undergo twinning and the strain required for the twinning reaction to go to completion.

Ferritic nitrocarburising of tool steels

Four different tool steel materials, P20, H13, M2 and D2, were nitrocarburised at 570°C in a fluidised bed furnace. The reactive diffusion of nitrogen and carbon into the various substrate microstructures is compared and related to the different alloy carbide distributions. The effect of carbon bearing gas (carbon dioxide, natural gas) on carbon absorption is reported, as well as its influence on compound layer growth and porosity. Partial reduction of Fe3O4 at the surface resulted in the formation of a complex, epsi-nitride containing oxide layer. In H13, carbon was deeply absorbed throughout the entire diffusion zone, affecting the growth of grain boundary cementite, nitrogen diffusivity and the sharpness of the compound layer: diffusion zone interface. When natural gas was used, carbon became highly concentrated in the compound layer, while surface decarburisation occurred with carbon dioxide. These microstructural effects are discussed in relation to hardness profiles, and compound layer hardness and ductility. The surfaces were characterised using glow discharge optical emission spectroscopy, optical and scanning electron microscopy and X-ray diffraction.

Superplasticity and cavitation of recycled AZ31 magnesium alloy fabricated by solid recycling process

Superplastic behaviour of Mg-alloy AZ31 was investigated to clarify the possibility of its use for superplastic forming (SPF) and to accurately evaluate material characteristics under a biaxial stress by utilizing a multi-dome test. The material characteristics were evaluated under three different superplastic temperatures , 643, 673, and 703 K in order to determine the most suitable superplastic temperature. Finite Element Method (FEM) simulation of rectangular pan forming was carried out to predict the formability of the material into a complex shape. The superplastic material properties are used for the simulation of a rectangular pan. Finally, the simulation results are compared with the experimental results to determine the accuracy of the superplastic material characteristics. The experimental results revealed that the m values are greater than 0.3 under the three superplastic temperatures, which is indicative of superplasticity. The optimum superplastic temperature is 673 K, at which a maximum m value and no grain growth were observed. The results of the FEM simulation revealed that certain localized thinning occurred at the die entrance of the deformed rectangular pan due to the insufficient ductility of the material. The simulation results also showed that the optimum superplastic temperature of AZ31 is 673 K.

Deformation behavior of nanostructured aluminum alloy processed by severe plastic deformation

Microstructure and deformation behavior of the commercial aluminum-based Al7.5%Zn–2.7%Mg–2.3%Cu–0.15%Zr alloy subjected to high pressure torsion (HPT) were studied in the present work. A small grain size less than 100 nm, high level of internal stresses and presence of second phase nanoparticles were revealed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nanostructured alloy processed by HPT exhibits tensile strength of 800 MPa and ductility of 20% at optimal temperature-strain rate conditions. Unusual influence of a short pre-annealing on tensile strength and ductility of as-processed alloy is discussed.

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited to 5-10 μm using conventional industrial approaches. Nowadays, though, several thermomechanical processes have been developed to produce ferrite grain sizes of 1-3 μm or less, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. The present paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. This requires deformation within the Ae3 to Ar3 temperature range for a given alloy. Here, the formation of ultrafine ferrite (UFF) involves the dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation (DSIT) arises from the introduction of extensive intragranular nucleation sites that are not present in conventional controlled rolling. The DSIT route has the potential to be adjusted to suit current industrial infrastructure. However, there are a number of significant issues that have been raised, both as gaps in our understanding and as obstacles to industrial implementation. One of the critical issues is that it appears that very large strains are required. Combined with this concern is the issue of whether a combination of dynamic and static transformation can be used to achieve an adequate level of refinement. Another issue that has also become apparent is that grain sizes of 1 μm can lead to low levels of ductility and hence many workers are attempting to obtain 2-3 μm grains, or to introduce a second phase to provide the required ductility. There are also a number of areas of disagreement between authors including the role of dynamic recrystallisation of ferrite in the production of UFF by DSIT, the reasons for the low coarsening rate of UFF grains, the role of microalloying elements and the effects of austenite grain size and strain rate. The present review discusses these areas of controversy and highlights cases where experimental results do not agree.

Precipitate characterisation of an advanced high-strength low-alloy (HSLA) steel using atom probe tomography

Increased fuel economy, combined with the need for the improved safety has generated the development of new hot-rolled high-strength low-alloy (HSLA) and multiphase steels such as dual-phase or transformation-induced plasticity steels with improved ductility without sacrificing strength and crash resistance. However, the modern multiphase steels with good strength-ductility balance showed deteriorated stretch-flangeability due to the stress concentration region between the soft ferrite and hard martensite phases [1]. Ferritic, hot-rolled steels can provide good local elongation and, in turn, good stretch-flangeability [2]. However, conventional HSLA ferritic steels only have a tensile strength of not, vert, similar600 MPa, while steels for the automotive industry are now required to have a high tensile strength of not, vert, similar780 MPa, with excellent elongation and stretch-flangeability [1]. This level of strength and stretch-flangeability can only be achieved by precipitation hardening of the ferrite matrix with very fine precipitates and by ferrite grain refinement. It has been suggested that Mo [3] and Ti [4] should be added to form carbides and decrease the coiling temperature to 650 °C since only a low precipitation temperature can provide the precipitation refinement [4]. These particles appeared to be (Ti, Mo)C, with a cubic lattice and a parameter of 0.433 nm, and they were aligned in rows [4]. It was reported [4] that the formation of these very fine carbides led to an increase in strength of not, vert, similar300 MPa. However, the detailed analysis of these particles has not been performed to date due to their nanoscale size. The aim of this work was to carry out a detailed investigation using atom probe tomography (APT) of precipitates formed in hot-rolled low-carbon steel containing additions Ti and Mo.

The investigated low-carbon steel, containing Fe–0.1C–1.24Mn–0.03Si–0.11Cr–0.11Mo–0.09Ti–0.091Al at.%, was produced by hot rolling. The processing route has been described in detail elsewhere [5] European Patent Application, 1616970 A1, 18.01.2006.[5]. The microstructure was characterised by transmission electron microscopy (TEM) on a Philips CM 20, operated at 200 kV using thin foil and carbon replica techniques. Qualitative energy dispersive X-ray spectroscopy (EDXS) was used to analyse the chemical composition of particles. The atomic level of particle characterisation was performed at the University of Sydney using a local electrode atom probe [6]. APT was carried out using a pulse repetition rate of 200 kHz and a 20% pulse fraction on the sample with temperature of 80 K. The extent of solute-enriched regions (radius of gyration) and the local solute concentrations in these regions were estimated using the maximum separation envelope method with a grid spacing of 0.1 nm [7]. A maximum separation distance between the atoms of interest of dmax = 1 nm was used.

The microstructure of the steel consisted of two types of fine ferrite grains: (i) small recrystallised grains with an average grain size of 1.4 ± 0.2 μm; and (ii) grains with a high dislocation density (5.8 ± 1.4 × 1014 m−2) and an average grain size of 1.9 ± 0.1 μm in thickness and 2.7 ± 0.1 μm in length (Fig. 1a). Some grains with high dislocation density displayed an elongated shape with Widmanstätten side plates and also the formation of cells and subgrains (Fig. 1a). The volume fraction of recrystallised grains was 34 ± 8%.

Cement composites reinforced with surface modified coir fibers

An experimental investigation of coir mesh reinforced mortar (CMRM) is conducted using nonwoven coir mesh matting. The main parameters in this study are the fiber volume fraction (number of mesh layers) and fiber surface treatment with a wetting agent. The composites are subjected to the four-point bending test. The short-term mechanical properties of CMRM are discussed. Scanning electron micrograph analysis is used to observe the fiber—matrix interfacial characteristics. The results indicate that the addition of coir mesh to mortar significantly improves the composite post-cracking flexural stress, toughness, ductility, and toughness index, compared to plain mortar materials. The Albatex © FFC wetting agent (2-ethylhexanol) can effectively improve water absorption of coir fiber and enhance the fiber—matrix bonding strength. These coir mesh reinforced composites may be useful in civil engineering applications.

Grain size in Mg alloys: recrystallization and mechanical consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying additionon both of these reactions is briefly considered.

Effect of microalloying with rare-earth elements on the texture of extruded magnesium-based alloys

A series of alloys have been produced with microalloying additions of rare-earth (RE) elements in the range of 0.1–0.4 wt.%. The alloys have been extruded to produce grain sizes of 23 ± 5 μm. The texture of the extruded alloys was measured, and it was found that the extrusion texture was weakened by the addition of RE elements. The samples with weakened extrusion textures exhibited an increase in the tensile elongation.

Necking and failure at low strains in a coarse-grained wrought Mg alloy

Tensile testing of rolled AZ31 alloy with a mean grain size of 80 μm reveals localization and failure prior to diffuse necking. Optical microscopy reveals that failure is caused by voids that have formed within twins. A simple localization criterion is proposed that captures the role of grain size in the effect. Such early failure is only predicted for coarse grain sizes, in line with observation.

Analytic prediction of structural stress-strain relations of microstructured metal

Nanostructured and ultra-fine grained metals have higher strength but extremely limited ductility compared to coarse grained metals. However, their ductility can be greatly improved by introducing a specific range of grain sizes in the microstructures. In the paper, multiscale unit cell approach (UCA) is developed and applied to predict the averaged stress-strain relations of the multiscale microstructure metals. The unit cell models are three-phase structured at different scale lengths of 100 nm, 1 μm and 10 μm with different volume fractions and periodic boundary conditions. The contributions of multi-scale microstructures to the macroscopic structural properties of metals are also studied using a analytic approach—two-step mean-field method (TSMF), where three microstructural parameters are introduced and thus mechanical properties such as strength and ductility are presented as a function of these parameters. For verification of these proposed numerical and theoretical algorithms, the structural properties of the pure nickel with three-grain microstructures are studied and the results from FEA and the proposed theory have good agreement.

Characterization of hot-pressed films from superfine wool powder

In this study, superfine wool powder was plasticized with glycerol and hot-pressed into a film. Scanning electron microscopy photos showed that the superfine wool powder could be molded into a smooth film and that the wool powder was distributed evenly in the cross section of the film. Fourier transform infrared analysis revealed no substantial changes in the chemical structure of the wool powder after hot pressing, but the absorbing peaks of glycerol were found in the spectrum. X-ray diffraction analysis showed that the overall crystallinity increased after the wool powder was hot-pressed into film. Thermogravimetry (TG) analysis indicated that the thermal stability of the hot-pressed film decreased. A transition point appeared in the TG curve of the wool hot-pressed film as glycerol was added. The differential thermal analysis curve of the film showed sharp absorbing peaks similar to that of wool powder. With increasing glycerol content, the film showed increasing ductility and softness.

Tensile deformation of a ultrafine-grained aluminium alloy: micro shear banding and grain boundary sliding

This work investigates the relationship between the strain rate and the ductility and the underlying deformation mechanisms in an ultrafine-grained Al6082 alloy. At room temperature the uniform elongation of the material exhibits a marked increase with decreasing strain rate. This effect is related to the activation of micro shear banding, which is controlled by grain boundary sliding. The contribution of these mechanisms to uniform elongation is estimated. It is proposed that the grain boundary sliding suppresses the transformation of micro shear bands into macro shear bands. The activity of other deformation mechanisms during plastic deformation of the ultrafine-grained material is also discussed.

T-Shaped equi-channel angular pressing of Pb-Sn eutectic and its tensile properties

Equi-channel angular pressing (ECAP) of a Pb–Sn eutectic alloy up to six passes in a T-shaped die, rather than a conventional L-shaped die, was studied for grain refinement. The effect of ECAP on the hardness and tensile properties was studied. Microstructure predominately changed in the early part of the ECAP process and became equiaxed and uniformly distributed in both the longitudinal and the transverse sections after four passes. There occurred substantial softening over the first two passes—hardness of 10 Hv, yield strength of 14.2 MPa and tensile strength of 16.3 MPa in the as-cast condition decreased upon two passes to 6 Hv, 9.7 MPa and 13.0 MPa, respectively. The ductility (% elongation) increased drastically from <50% in the as-cast condition to 150% upon two passes, and further increased to 230% after four passes. Various tensile properties and concurrent microstructural evolution were used to develop a mutual relationship among them.