Cyclic Plastic Deformation, Fatigue, and the Associated Micro-Mechanisms in Magnesium: from Single Crystal to Polycrystal

Magnesium and its alloys have received substantial interests as the government initiatives on energy saving and environment protection demand an increasing use of lightweight materials in structural parts, especially in transportation industries. A good understanding of fatigue behavior in magnesium is critical to ensure the reliability and durability of the magnesium components. Unlike the body centered cubic and face centered cubic metals, fundamental knowledge concerning the cyclic deformation and fatigue in hexagonal close packed magnesium is limited. The current research aims at a better understanding of the micro-mechanisms associated with the cyclic deformation and fatigue of magnesium. Magnesium single crystal was chosen to study the fundamental twinning/detwinning process while extruded polycrystalline pure magnesium was studied for the fatigue damage mechanisms. Cyclic deformation and the corresponding morphology evolution of {1 0 -1 2} twinning-detwinning-retwinning are, for the first time, characterized in magnesium single crystal under fully reserved strain-controlled tension-compression utilizing in situ optical microscopy. As loading cycles are increased, the activity of twinning-detwinning-retwinning gradually decreases. Microscopy after fatigue shows that the matrix region having experienced repeated twinning-detwinning cannot be completely detwinned to its original crystal orientation. Fragmented secondary tension twins are found to result from twin-twin interactions. Various twin-twin interaction structures exist in fatigued magnesium single crystal: quilted-looking twin structure, "apparent crossing" twin structure, and double tension twin structure. According to the crystallography of magnesium, twin-twin interactions are classified into Type I for two twin variants sharing the same zone axis and Type II for two twins with different zone axes. For Type I twin-twin interactions, one twin does not transmit across the twin boundary and into the other twin. For Type II twin-twin interactions, one twin can transmit into the other only under some special loading conditions. In most cases, twin transmission does not occur but, instead, twin-twin boundaries form that contain boundary dislocations. The formation mechanism of the twin-twin boundary is proposed based on the reaction of twinning dislocations. Twin-twin boundary is a low-angle tilt boundary for Type I co-zone twin-twin interaction whereas it adopts a high-index crystallographic plane for Type II twin-twin interaction according to a geometry analysis. Twin-twin boundary dislocations can be inferred by reactions of twinning dislocations associated with the two twin variants. An "apparent crossing" twin structure is a consequence of twin-twin boundary formation. Under reversed loading subsequent to twinning, detwinning is retarded and secondary twinning can be activated at the twin-twin boundary for Type II twin-twin interaction. The fatigue damage mechanisms in magnesium were studied in extruded coarse-grained polycrystalline pure magnesium through fully-reversed strain controlled tension-compression along its extrusion direction. Twinning/detwinning dominates the cyclic deformation at high strain amplitudes while dislocation slips are responsible for the cyclic deformation at low strain amplitudes. Microcrack initiation and early-stage crack growth strongly depend on cyclic loading magnitude. During most of fatigue life at twinning-dominated strain amplitudes, microcracks are incessantly initiated with limited propagation on both grain boundary and twin boundary. At slip-dominated strain amplitudes, microcracks are initiated predominantly by grain boundary cracking. Both intergranular and transgranular modes are observed for early-stage crack propagation. Early-stage transgranular propagation is dominated by cracking at twin boundaries at twinning-dominated strain amplitudes. At dislocation slip-dominated strain amplitudes, early-stage transgranular propagation is found to occur on the crystal planes, which is driven by alternative slip mechanism on two sets of second-order pyramidal slip system

Twin migration in Fe-based bcc crystals: Theory and experiments

We establish an overall energy expression to determine the twin migration stress in bcc metals. Twin migration succeeds twin nucleation often after a load drop, and a model to establish twin migration stress is of paramount importance. We compute the planar fault energy barriers and determine the elastic energies of twinning dislocations including the role of residual dislocations (br) and twin intersection types such as 1 1 0, 1 1 3 and 2 1 0. The energy expression derived provides the twin migration stress in relation to the twin nucleation stress with a ratio of 0.5-0.8 depending on the resultant residual burgers vector and the intersection types. Utilizing digital image correlation, it was possible to differentiate the twin nucleation and twin advancement events experimentally, and transmission electron microscopy observations provided further support to the modelling efforts. Overall, the methodology developed provides an enhanced understanding of twin progression in bcc metals, and most importantly the proposed model does not rely on empirical constants. We utilize Fe-50at.%Cr in our experiments, and subsequently predict the twin migration stress for pure Fe, and Fe-3at.%V from the literature showing excellent agreement with experiments. © 2014 Taylor & Francis

Investigation of the cryogenic deformation behaviour and subsequent recrystallization in magnesium alloy AZ31

Magnesium alloys attract great interest from industry and academia, as they are the lightest structural metals, which can be applied to achieve a weight reduction of car bodies to reduce energy consumption and air pollution. However, their ductility and strength are generally poor at room temperature, mainly because of their coarse grains and strong texture. This study aims to develop a novel method to refine grains and weaken the texture in magnesium (Mg) alloy AZ31, and investigate its underlying deformation and recrystallization mechanisms. This can contribute to improving the applications of Mg alloys. For this purpose, the hot deformation with pre-deformation at cryogenic temperature (CT) and room temperature (RT) was conducted and compared to the direct-hot-deformed Mg alloy. Recrystallization mainly occurred near deformation bands in RT-hot-deformed and direct-hot-deformed samples, while active recrystallization occurred near twin-twin interactions in the CT-hot-deformed sample. This resulted in finer grains and a weaker texture in the CT-hot-deformed sample, and its ductility and strength improved significantly. The pre-deformation behaviour at CT and RT was investigated with a quasi-in-situ Electron Backscatter Diffraction (EBSD). More twin-twin interactions were observed at CT than RT, which contributed to forming a weaker texture and finer twins, and higher hardening rates at CT. To further investigate how twinning and deformation bands evolve, rolling tests at CT and RT were conducted. With increasing strain, abundant deformation bands, evolving from {101 ̅1}-{101 ̅2} double twins, were observed in RT-rolled samples, while abundant twins, starting from {101 ̅2} tension twins to twin-twin interactions and twinning sequence, were observed in CT-rolled samples. During the subsequent annealing process, recrystallization occurred around deformation bands in the annealed RT-rolled sample, resulting in a strong basal texture. However, recrystallization occurred around twin-twin interactions in the annealed CT-rolled sample, forming the weak texture and fine grains.Open Acces

Formation, Growth, and Interactions of Tension Twins in Magnesium

In the world of structural metals, magnesium and its alloys are much lighter than other traditionally used metals having a density of ~1/5 that of iron and ~2/3 that of aluminum – two commonly used metals. Unlike these isotropic cubic metals, magnesium has a hexagonal closed-packed (hcp) crystal structure and is not commonly used. The hcp structure has less symmetry than the face centered and body centered cubic structures, resulting in an anisotropic mechanical response with fewer available slip systems. C-axis deformation is particularly difficult to achieve through dislocation slip at room temperature. To accommodate deformation, tension twinning is activated, which rotates the crystal by almost 90° to better allow the easily activated basal {0001} slip system. As the microstructure dynamically changes by twinning during plastic deformation, so too do the local deformation processes. The microstructural evolution directly affects the mechanical behavior of magnesium under both monotonic and cyclic loading. To better design, process, and utilize the magnesium alloys in engineering applications, a fundamental understanding of the process of twinning and twin-twin interactions is required. The current research aims to capture and explain the microstructural evolution due to twinning during tension, compression, torsion, and uniaxial strain path changes.The nucleation, growth, and interaction of tension twins were directly observed utilizing a hybrid in situ optical microscopy, ex situ electron backscatter diffraction procedure. Cross-grain twin pair formation was captured in extruded pure polycrystalline magnesium where the twin in one grain assists in the nucleation of a similar twin in the adjacent grain. Two assisted formation processes were observed: the commonly observed twin propagation-assisted and the newly recorded twin thickening-assisted mechanisms. A twin chain spanning seven grains was rapidly developed through the twin propagation-assisted mechanism of two smaller chains and eventual conjoining within a middle grain. The two smaller chains form across grains with small misorientations from grain to grain, allowing for easy-cross grain twin formation by a twin-propagation assisted process. In the middle grain where the chains connect, the same twin variant is formed from both chains on either side of the grain, where they both grow and coalesce forming the larger twin chain. The new twin thickening-assisted formation is observed for the first time where the paired twin is formed across a grain boundary by the other twin thickening. Applying the hybrid testing procedure to nearly c-axis tension of single-crystalline pure magnesium reveals that basal slip is activated prior to the nucleation of tension twins. As twinning increases with increasing strain, the initial basal slip bands are deflected within the twinned region relative to the activated variant and the twinning shear. By the final plastic strain of 3.83%, all six tension twin variants are identified within the observed area along with all three types of twin-twin interactions: Type I co-zone, Type II(a) non-co-zone, and Type II(b) non-co-zone. A needle-like Type I interaction is captured by in situ observation for the first time. The initial interaction results in the normally occurring impinging and acute angle twin-twin boundary. On the obtuse angle side, twinning dislocations are deposited near the impinging twin-twin boundary, leading to incoherent curving twin boundaries. The combination of the acute angle twin-twin boundary and incoherent boundaries on the obtuse angle side result in the penetrating structure. Partial penetrating structures are observed in the Type II interactions, along with secondary twinning. Detwinning is quantitatively measured during unloading. Twinning behavior during strain path change and torsional loading was examined using extruded pure polycrystalline magnesium. Compression parallel to the extrusion direction of the bar results in severe twinning up to 95% of its volume by exhaustion. Using companion specimens, pre-compression was applied to -7.3% and -12.8% strains to induce varying twin severity prior to re-loading in tension. The subsequent tension reveals a combination of detwinning and secondary tension twinning of the initial twins formed during compression. Detwinning is more significant than secondary twinning in the -7.3% pre-strain case while the opposite is observed in the -12.8% pre-strain case. After detwinning is exhausted, non-Schmid factor twins are observed along with the retained secondary twins. Twinning during free-end torsion about the extrusion direction cannot reach the same global twin severity as compression as the texture is not entirely favorable for twinning. Grains favorably oriented for tension twinning are severely twinned. Less favorably oriented grains show high variability in terms of twin severity, while the unfavorably oriented grains show very little to some twinning. All six variants are observed in multiple grains, and more significantly, some twinned regions are highly favorable for further twinning, so secondary twinning is very common. By torsional failure, favorable secondary twins can be found fully encompassing their primary variant

On the relationship between {1 1 2¯ 2} and {1 1 2¯ 6} conjugate twins and double extension twins in rolled pure Mg

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Philosophical Magazine on February 2017, available online at: http://www.tandfonline.com/10.1080/14786435.2017.1290846The paper presents a new type of twin-like objects observed in rolled pure magnesium. They have {11¯26} and {11¯22} habit planes and their misorientations to the matrix are close to 56° and 63° about ¿10¯10¿ axis, respectively. The ad hoc performed theoretical analysis and atomic simulations allow to interpret the objects as {10¯12}-{10¯12} double twins formed by the simultaneous action of two twinning shears with completely re-twinned volume of primary twin. The observed inclinations from the ideal misorientations for such double twins can be explained by the compliance of the strain invariant condition in the twin boundary. It seems plausible that, once the double twin is formed, its twin boundaries are hard to move by glide of twinning disconnections. If so, these twins represent obstacles for the motion of crystal dislocations increasing the hardness of the metal.Peer ReviewedPostprint (author's final draft

Plastic deformation and damage induced by fatigue inTWIP steels

Twinning Induced Plasticity steels exhibit a high strain hardening rate which translates into a remarkable combination of ductility and strength. A thorough experimental approach was performed by advanced characterization techniques to study the deformation mechanisms developed under high cycle fatigue conditions. Results clearly lay out that the cumulative strain damage leads to strengthening but also induces micro-cracks at the intersection of twin boundaries which promote fracturePeer ReviewedPostprint (author's final draft

Twin-solute, twin-dislocation and twin-twin interactions in magnesium

Magnesium alloys have received considerable research interest due to their lightweight, high specific strength and excellent castability. However, their plastic deformation is more complicated compared to cubic materials, primarily because their low-symmetry hexagonal closepacked (hcp) crystal structure. Deformation twinning is a crucial plastic deformation mechanism in magnesium, and twins can affect the evolution of microstructure by interacting with other lattice defects, thereby affecting the mechanical properties. This paper provides a review of the interactions between deformation twins and lattice defects, such as solute atoms, dislocations and twins, in magnesium and its alloys. This review starts with interactions between twin boundaries and substitutional solutes like yttrium, zinc, silver, as well as interstitial solutes like hydrogen and oxygen. This is followed by twin-dislocation interactions, which mainly involve those between {10[]2} tension or {10[]1} compression twins and 〈 a 〉 , 〈 c 〉 or 〈 c +-a 〉 type dislocations. The following section examines twin-twin interactions, which occur either among the six variants of the same {10[]2} or {10[]1} twin, or between different types of twins. The resulting structures, including twin-twin junctions or boundaries, tension-tension double twin, and compression-tension double twin, are discussed in detail. Lastly, this review highlights the remaining research issues concerning the interactions between twins and lattice defects in magnesium, and provides suggestions for future work in this area

Grain boundary orientation effects on deformation of Ta bicrystal nanopillars under high strain-rate compression

We investigate grain boundary (GB) orientation effects on deformation of Ta bicrystal nanopillars under high strain-rate, uniaxial compression with molecular dynamics simulations. The GB is of the ⟨110⟩90° twist grain boundary type. We vary the angle between the GB normal and the loading direction (θ) in the range of 0°–90° while keeping the GB type unchanged. The GB orientation has strong effects on deformation mechanism, yield stress, failure strain, and dynamics, due to the combined effects of Schmid factors in constituent crystals and resolved shear stress on the GB plane. Single crystal plasticity and GB deformation are competing factors, and the GB-initiated deformation mechanisms (stacking faults vs. twinning, and GB sliding) depend on the local stress level around the GB. The large Schmid factors in constituent single crystals for θ=0° lead to twinning in the single crystals and the lowest yield stress; the ensuing GB deformation is achieved via stacking fault formation due to premature stress relaxation. However, nanopillar deformation in the cases of higher angles is dominated by GB deformation largely in the form of twinning, driven by enhanced stress buildup. GB-initiated deformation in the high Schmid factor nanocrystal precedes and may drive that in the low Schmid factor nanocrystal. The details of twin/stacking fault nucleation and growth/shrinking, twin-twin interaction, and twin-GB interaction are also discussed

Preverbal Language Abilities in Monozygotic and Dizygotic Twins

This study examined prelinguistic language development of twins by delineating: differences from developmental expectations; twin-twin communication and twin-mother communication differences; and monozygotic-dizygotic differences. Nine families with twins between seven and 16 months of age participated in the study. Five families had dizygotic twins, three of which were male/female pairs, and four families had monozygotic twins, only one of which was female. The primary caregiver completed a developmental history. Data consisted of Dares Primitive Speech Acts (PSA) coding during in-home video-taping (30 minute sessions) with a second coding from the video material, and maternal reporting of vocabulary and communication using the MacArthur-Bates Communicative Development Inventory (CDJ) for each twin. Variations were found between twin-twin and twin-mother use of PSA with all twins using more PSA with mothers. Dizygotic twins used more PSA overall and had better communication scores on the CDJ than did monozygotic twins. Overall, results suggest that twins may be at a greater risk for language delay than singletons and monozygotic twins even more so