H. El Kadiri

A review on the effect of rare-earth elements on texture evolution during processing of magnesium alloys

Magnesium, the lightest structural metal, is approximately four times lighter than steel—the most widely used metal in industrial applications. Currently available Mg alloys, however, are impractically expensive for use in automotive structural components, as severe ductility problems require forming operations at elevated temperatures and an exclusion from critical safety components. With a strong impetus in research having sprung up during the last two decades, addition of rare-earth elements in small quantities emerged as a potential solution for simultaneously delivering the ductility and weight requirements for automotive applications. These improvements are arguably achieved by virtue of texture weakening and enhancement of non-basal slip. However, ways by which rare-earth elements modify texture remain very elusive, and no consensus on the driving mechanisms has been reached in the literature as of yet. We take a look back at different paradigms held for the action of rare-earth additions, and examine key facts that may reconcile controversies. We attempt to identify critical gaps and suggest venues to overcome them. These gaps, once filled, may promote Mg alloys to become a stronghold for lightweighting, which will exceptionally benefit our environment and wellbeing.

Extended zonal dislocations mediating twinning in titanium

Using molecular dynamics (MD) simulations, the twinning mechanism in titanium (Ti) was studied by analyzing the interfacial structure at the twin boundary (TB). The simulation results reveal interesting twin growth controlled by interfacial dislocations at the TB. The elementary twinning dislocations (bT ) nucleate and glide in pairs but separately and sequentially on two neighboring planes, significantly different from conventional zonal dislocations, which spread over two or more twinning planes with each plane comprising one Burgers vector of an elementary twinning dislocation. The twin growth can be approximately described as These two separate elementary twinning dislocations amount to a net Burgers vector 2bT ≈ 0.16 nm along the twinning vector , with the components in the in-plane direction perpendicular to η 1 canceled out. These results support the classical twinning theory in which a homogeneous shear and local shuffling have to be involved. A mechanism taking into consideration local structure of the twinning plane for such extended zonal dislocations is discussed.

Pore Formation in Laser-Assisted Powder Deposition Process

Pore formation remains a concern in the area of rapid manufacturing by the laser engineered net shaping process. Results usually conflict on the origin of these pores; whether it should stem from an effect due to the physical/mechanical properties of the material or from an effect purely related to the processing parameters. We investigated this problem spanning a range of process parameters for deposition and using three different material powders, namely, an AISI 410 grade stainless steel, AISI 316L grade stainless steel, and AISI 4140 grade medium-carbon low alloy steel. The volume fraction, number density, and size distribution of pores were quantified using X-ray computed tomography and optical microscopy. Pores formed both at the interface between the adjacent layers and within the bulk of the layer. They were systematically sensitive to both the powder material composition and the process parameters.

Structural Origin of Reversible Twinning, Non-Schmid Effect, Incoherent Twin Boundaries and Texture in Hexagonal Close-Packed Metals

Notably the most dominant twinning mode in hexagonal close-packed metals, {1012} twinning presents abnormal properties such as reversible twinning and non-Schmid effect. The twin boundaries may significantly deviate from the {1012} twinning plane. HCP metals also present a strong propensity to develop texture during processing. Through electron backscatter diffraction and high resolution transmission electron microscopy observations, we show that these properties can be well understood from the perspective of the atomic shuffling that dominates in the twinning.

Limitation of current hardening models in predicting anisotropy by twinning in hcp metals: Application to a rod-textured AM30 magnesium alloy

When a strongly textured hexagonal close packed (HCP) metal is loaded under an orientation causing profuse twinning or detwinning, the stress-strain curve is sigmoidal in shape and inflects at some threshold. Authors have largely attributed the dramatic stress increase in the lower-bound vicinity of the inflection point to a combined effect of a Hall-Petch mechanism correlated to grain refinement by twinning, and twinning-induced reorientation requiring activation of hard slip modes. We experimentally and numerically demonstrate that these two mechanisms alone are unable to reproduce the stress-strain behaviors obtained under intermediate loading orientations correlated to in-between profuse twinning and nominal twinning. We argue based on adopting various mechanistic approaches in hardening model correlations from the literature. We used both a physics dislocation based model and a phenomenological Voce hardening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a � 10¯0� -fiber parallel to the extrusion direction.

On laser welding of thin steel sheets

Abstract This paper presents a process–structure–property relationship study of laser welds as a continuous consolidation method for joining thin monophased steel foils, thereby providing a more effective, less costly method to construct automotive catalytic converters. A body centred cubic (bcc) iron–chromium–aluminium alloy doped with Mischmetal was utilised in this study. Both pulsed and continuous wave modes were used to establish the limit welding diagrams for lap joint configuration. Actual laser welding parameters were selected using several testing conditions. The laser welds behaved substantially different from the base material under creep and high temperature oxidation. The difference was mainly attributed to the changes in grain morphology, precipitation of aluminium nitrides and carbides, and relocalisation of the reactive elements during liquid metal flow upon keyhole formation, solidification and cooling.

The Effect of \{ 10\bar{1}2\} Twin Boundary on the Evolution of Defect Substructure

Pure Mg single crystals were deformed at room temperature along two orientations in sequence, in order to activate a specific dislocation slip mode followed by \( \left\{ {10\bar{1}2} \right\} \) twinning. The defects in both the matrix and twin crystals were analyzed with a transmission electron microscope (TEM). This study reveals the collective evolution of the defect substructure when a dislocated crystal is “invaded” by a moving twin boundary. When primarily \( \left[ c \right] \)-containing defects in the matrix were incorporated by a moving twin boundary, including \( \langle c + a\rangle \), pure \( \left[ c \right] \) dislocations and \( I_{1} \) stacking faults, the twin contains homogeneously distributed \( I_{1} \) stacking faults, which in some instances appear to be connected on twin boundary to the faults in the matrix.

Anomalous Twin Bands in AZ31 Magnesium Sheet Bending

Three-point bending was performed on an AZ31 magnesium sheet with a grain size ∼8.0 µm at room temperature. In-situ electron backscatter diffraction and metallography examinations revealed an anomalous twinning pattern during bending. In the compression zone, the {1012} twins appear in an extremely localized fashion, consisting of alternating twin bands. Each band comprises a high density of twins. In between the twin bands, twins are absent. As the bending angle increases, the twin bands grow into the tension zone although the stress disfavors the extension twinning.

Nucleation Mechanism for Shuffling Dominated Twinning in Magnesium

We observed nucleation of {10–12} twinning under tensile loading in magnesium rectangular rod system using atomistic molecular dynamic simulation. The rod axis is normal to basal plane of Mg crystal. The tensile deformation in c-axis nucleates {10–12} twinning starting at the corner of square of cross section of the rod. The twin boundary is spherical at the beginning and become a linear boundary in {10–12} planes as time goes by. The twinning and shuffling processes are described. The nucleation mechanism of the shuffling dominated twinning is explained.

Twinning Multiplicity in an AM30 Magnesium Alloy under Uniaxial Compression

Twinning under compression along two perpendicular directions of an AM30 alloy with a {1010} fiber texture was investigated. The primary extension {10 1 2} twinning occurred for both compression normal to the fiber and compression parallel to the fiber. These primary extension twins took forms of fully residual twins parallel to the fiber and the “stopped elastic” twins normal to the fiber. Both the {1011}-{1012} and {1013}-{1012} double twinning occurred in the matrix grains at the last stage of compression normal or parallel to the fiber, but as the “combined two shears” mode of the double twinning. Another time-sequence type of {1011}-{1012} double twin also activated early normal to the fiber. The primary extension twinning and contraction twinning seem to obey the Schmid law according to the texture evolution.