Non-basal Slip Research Articles

Insights into High‐Temperature Uniaxial Compression Deformation Behavior of Ti3AlC2

We report on strain‐rate‐dependent compression deformation behavior of Ti3AlC2 at 1000°C–1200°C. At 1000°C and high strain rate (10−2 or 10−3 s−1), Ti3AlC2 deforms in a nonplastic manner. Upon increasing temperature and reducing strain rate, Ti3AlC2 exhibits a limited plasticity. For instance, the true plastic strain at 1200°C and 10−4 s−1 is only 3%, beyond which strain softening following a short hardening regime occurs. The softening results from the formation of localized microvoids and microcracks. Decreasing the strain rate further to 10−5 s−1 at 1200°C, strain hardening instead of softening is identified. Under such conditions, the plastic strain remarkably increases, reaching a value as high as 27%. Postdeformation microstructural analyses of the dislocation configurations explicitly evidence the dislocation reactions, formation of hexagonal dislocation networks and dislocation entanglements. These account for the strain hardening. The extraordinary plasticity at 1200°C and 10−5 s−1 benefits from the initiation of nonbasal slip systems. Finally, a complete high‐temperature deformation scenario for nanolaminated Ti3AlC2 is elaborated.

Cyclic behavior of AZ31B Mg: Experiments and non-isothermal forming simulations

Magnesium (Mg) alloys have unique room temperature mechanical properties such as yielding asymmetry, anisotropy, and unusual hardening response under strain path change. Mg alloy sheets often represent inferior formability at room temperature due to their limited active slip systems induced from specific microstructure and texture. This low formability is known to be mitigated as temperature increases, which is the result of active non-basal slip systems. Considering these unique properties, Mg alloy sheets has been often formed at 200 °C or higher. For optimizing the forming process using Mg alloy sheets, accurate constitutive models describing the unique behavior of the materials under non-proportional loading and non-isothermal temperature conditions are vital. In this paper, the mechanical behavior of AZ31B Mg alloy sheets under in-plane tension–compression (or compression-tension) cyclic loading was experimentally measured for various pre-strains and temperatures. Then, a practical hardening model modified from existing advanced hardening law was applied to calculate the stress–strain responses and formability in the cross shape die forming under non-isothermal condition.

Role of yttrium in activation of 〈c + a〉 slip in magnesium: An atomistic approach

The effect of Y addition on the slip behavior of an edge dislocation on basal, prismatic and second-order pyramidal slip planes of Mg has been investigated using a molecular dynamics simulation. It is found that Y increases the critical resolved shear stress of basal slip more than that of non-basal slip and eventually reduces the difference in the CRSS between different slip systems, which is proposed as the reason for the experimentally reported activation of 〈c+a〉 slip in Mg–Y alloys.

Optimization of the hot workability for an extrude AZ80 Mg alloy using the processing map and Kriging meta-model

Mg alloy has a hexagonal close-packed(HCP) lattice crystal structure, which has fewer slip systems than other structures, and plastic deformation is difficult to achieve at room temperature. To improve its workability, the non-basal plane slip must be activated by increasing the deformation temperature. In this study, the hot deformation behavior of the AZ80 Mg alloy was examined within the temperature range of 250–400°C and the strain rate range of 0.001–10/s, based on the dynamic materials model. The hot deformation characteristic value was determined using the dynamic materials model (DMM), and was approximated using Kriging meta-model. Based on the results, deformation processing maps were derived, and the entire strains were examined to present the processing maps considering the strain during forming. In addition, the processing maps were established considering the hardness after forming, and the process window was presented considering both the formability and the strength of the parts.

Generalized-stacking-fault energy and twin-boundary energy of hexagonal close-packed Au: A first-principles calculation.

Although solid Au is usually most stable as a face-centered cubic (fcc) structure, pure hexagonal close-packed (hcp) Au has been successfully fabricated recently. However, the phase stability and mechanical property of this new material are unclear, which may restrict its further applications. Here we present the evidence that hcp → fcc phase transformation can proceed easily in Au by first-principles calculations. The extremely low generalized-stacking-fault (GSF) energy in the basal slip system implies a great tendency to form basal stacking faults, which opens the door to phase transformation from hcp to fcc. Moreover, the Au lattice extends slightly within the superficial layers due to the self-assembly of alkanethiolate species on hcp Au (0001) surface, which may also contribute to the hcp → fcc phase transformation. Compared with hcp Mg, the GSF energies for non-basal slip systems and the twin-boundary (TB) energies for and twins are larger in hcp Au, which indicates the more difficulty in generating non-basal stacking faults and twins. The findings provide new insights for understanding the nature of the hcp → fcc phase transformation and guide the experiments of fabricating and developing materials with new structures.

Study on the assemblage of Y and Gd on microstructure and mechanical properties of hot extruded Mg–Al–Zn alloy

The microstructure and the mechanical properties of hot extruded Mg–Al–Zn alloy with the addition of Y and Gd were thoroughly investigated under two different conditions with and without aging treatments. The results showed that dynamic precipitation of Mg17Al12 phase occurred during hot deformation, and only Al2(Y,Gd) phase (no Al8Mn5 phase which was a common phase in Mg–Al–Zn alloy) was observed in the alloy. In addition, the basal texture intensity of Mg–Al–Zn alloy was weakened, which probably originates from both the promotion of nonbasal slip due to the addition of Y and Gd and the well-known particle-stimulated nucleation (PSN). Within the aged sample, there are less lamellar discontinuous precipitations (DPs), more globular DPs, and thinner continuous precipitation (CP) laths after Y and Gd addition. A new crystallographic orientation relationship (OR) between the β-Mg17Al12 precipitate and the α-Mg matrix was identified as (0001)m//(110)p and [011¯0]m//[1¯13]p. Finally, both tensile and compressive strengths are improved by the addition of Y and Gd, with the tension–compression yield asymmetry being reduced.

Effect of Li content on the indentation creep characteristics of cast Mg–Li–Zn alloys

Creep behavior of the hcp Mg–4Li–1Zn (LZ41), hcp/bcc Mg–8Li–1Zn (LZ81), and bcc Mg–12Li–1Zn (LZ121) cast alloys was studied by long-term Vickers indentation testing in the temperature range of 423–498 K and under constant load of 5 N. It was established that increasing Li content decreased the creep resistance at all test temperatures. This was stemmed from the effect of Li on the crystal structure of the alloys, the increment of which increased the volume fraction of the bcc β-phase. This phase activated non-basal slip and facilitated the slip-assisted deformation, and thus, resulted in lower strength and creep resistance. The average stress exponents of about 7.0, 4.5, and 4.2 and activation energies of 90.0, 91.7 and 98.2 kJ/mol were obtained for the LZ41, LZ81, and LZ121 alloys, respectively. It is suggested that the operative creep mechanism is dislocation pipe diffusion in the LZ41 alloy, and pipe-diffusion-controlled dislocation viscous glide in the LZ81 and LZ121 alloys.

Preparation of sound ribbons with submicrometer-grained microstructure on a Mg–Zn alloy

In this paper, a viable way to fabricate Mg alloy sound ribbons with ultra-fine-grained microstructure was presented. The hot-rolled and annealed Mg–0.4Zn (at%) alloy exhibited excellent rollability to form sound ribbons with submicrometer grains when subjected to one-pass cold rolling process. The more balanced multi-mode dislocation slips originated from the significant decrease of critical resolved shear stress for non-basal slip with the addition of solute Zn and the favorable crystallographic orientation were suggested to be responsible for the excellent cold rollability. The formation of ultra-fine-grained microstructure was attributed to low-temperature dynamic recrystallization occurring during the cold rolling process with large strain.

Material design for magnesium alloys with high deformability

Magnesium alloys are very promising structural material, especially because of their low density, but their poor deformability at room temperature is making their commercial application impractical. Some alloying elements have been shown to improve the ductility dramatically, and a search for the yet better alloying elements is now a pressing task. In this study, we investigated the effects of several alloying elements on the mechanical and deformation behaviour of magnesium alloys using both first-principles calculations as well as experiments. The first-principles calculations indicate that the influential key factor for the activation of non-basal slips is the electronegativity and atomic radius. The experimental result proves that the alloys with a similar electronegativity as magnesium and a little larger atomic radius than that of magnesium, such as Ca, Sr and some rare earth elements, show superior ductility due to activation of non-basal dislocation slips. These results propose a promising design principle for the alloys with the improved deformability at room temperature ranges.

Influence of temperature and strain rate on flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys

The effects of temperature and strain rate on the flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys were investigated under uniaxial tension. At high temperatures, dynamic recovery, continuous dynamic recrystallization, grain boundary sliding and the activation of additional slip systems lead to an improvement of the ductility of the alloys. The elongation to failure is nearly independent of the strain rate between 473 and 523 K at 10−2 s−1 and 10−1 s−1, which is related to the strain rate dependence of the critical resolved shear stress (CRSS) for nonbasal slip. Despite the high temperature, twins are even observed at 573 K and 10−3 s−1 because they have a low CRSS.

Evaluation of Critical Resolved Shear Stresses for Various Plastic Deformation Mechanisms To Understand Mechanical Properties of Magnesium-Yttrium Alloys

ABSTRACTIn order to understand enhanced mechanical properties of magnesium-yttrium (Mg-Y) alloys, applied stresses which were required to operate independent plastic deformation mechanisms on various stress axes were evaluated. Moreover, for this analysis, mechanical tests including newly-established testing method “pure-shear test” were conducted to evaluate Critical Resolved Shear Stresses (CRSSes) for various plastic deformation mechanisms of Mg-Y solid solution alloy single crystals with various Y concentration. Relatively higher solid solution strengthening of dominant plastic deformation mechanisms such as basal slip and extension twin at room temperature, results in increase in the activation of non-basal slip system. By a simple analysis based on von-Mises criterion with experimental CRSS values, it is revealed that enhanced mechanical properties of Mg-Y alloys might be attributed to the decrease of difference in the activity of plastic deformation mechanisms by Y addition.

Tensile and compressive creep behavior of extruded Mg–10Gd–3Y–0.5Zr (wt.%) alloy

The tensile and compressive creep behavior of an extruded Mg–10Gd–3Y–0.5Zr (wt.%) alloy was investigated at temperatures ranging from 200°C to 300°C and under stresses ranging from 30MPa to 120MPa. There existed an asymmetry in the tensile and compressive creep properties. The minimum creep rate of the alloy was slightly greater in tension than in compression. The measured values of the transient strain and initial creep rate in compression were greater than those in tension. The creep stress exponent was approximately 2.5 at low temperatures (T<250°C) and 3.4 at higher temperatures both in tension and in compression. The compression creep activation energy at low temperatures and high temperatures was 83.4 and 184.3kJ/mol respectively, while one activation energy (184kJ/mol) represented the tensile–creep behavior over the temperature range examined. Dislocation creep was suggested to be the main mechanism in tensile creep and in the high-temperature regime in compressive creep, while grain boundary sliding was suggested to dominate in the low-temperature regime in compressive creep. Precipitate free zones were observed near grain boundaries perpendicular to the loading direction in tension and parallel to the loading direction in compression. Electron backscattered diffraction analysis revealed that the texture changed slightly during creep. Non-basal slip was suggested to contribute to the deformation after basal slip was introduced. In the tensile–creep ruptured specimens, intergranular cracks were mainly observed at general high-angle boundaries.

Microstructure and mechanical properties of extruded Mg96Zn1Y3 alloy

Microstructure and mechanical properties of an extruded Mg96Zn1Y3 alloy have been investigated in the current study. Microstructure of as extruded alloy consists of α-Mg, long period stacking ordered (LPSO) phase and parallelepiped Y-riched phases, which exhibits a bimodal distribution of fine recrystallized and coarse elongated grains. Fine recrystallized grains with an average grain size of 1.5μm are mainly pinned by bulk LPSO-14H phase, inside which both LPSO-18R phase and LPSO-14H phase coexist. The extruded Mg96Zn1Y3 alloy exhibits a good combination of yield strength and elongation below 200°C, which is mainly attributed to LPSO phases, stronger texture and finer DRX grains. The main deformation is controlled by load transfer mechanism below 250°C, and deformation is controlled by non-basal dislocation movement between 300°C and 325°C. A good plasticity is attained at 350°C with an elongation of 140%. The flow stress behavior at 350°C is mainly controlled by non-basal slip accompanying with dynamic recrystallization and growth of grain.

Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature

Coarse-grained Mg in the as-cast condition and fine-grained Mg in the extruded condition were processed by high pressure torsion (HPT) at room temperature for up to 16 turns. Microstructure observation and texture analysis indicate that to fulfil the Von Mises criterion, the non-basal slip is activated in the as-cast Mg and tension twinning is activated in the as-extruded Mg. Although the deformation mechanism is different in the as-cast Mg and the as-extruded Mg during HPT, their hardening evolutions are similar, i.e. after 1/8 turn of HPT, microhardness of the as-cast Mg and the extruded Mg both show a significant increase and further HPT processing does not significantly further increase the microhardness. Texture strengthening can explain the rapid hardening. Hardness anisotropy and texture data results suggest that texture strengthening plays an important role for both types of samples. Texture strengthening weakens with decreasing grain size.

Stochastic size-dependent slip-twinning competition in hexagonal close packed single crystals

In this paper, we present an analytical model to investigate stochastic size effects on the critical stress for the activation of non-basal slip and contraction twinning (CT) in miniaturized hexagonal-close packed (HCP) single crystals. The analysis is based on the basic model developed by Parthasarathy et al (Scr. Mater. 56 313–6), but extends it to account for the mechanism-based description of pyramidal slip and CT in HCP metals. The results demonstrate that for a given initial dislocation density, a probabilistic competition exists among CT nucleation, CT growth and pyramidal slip. Based on the stochastic model, we construct a mechanism map that provides regimes of dominant deformation modes that govern the macroscopic yield stress in HCP single crystals.

Experimental and simulation analysis of texture formation and deformation mechanism of rolled AZ31B magnesium alloy under dynamic loading

Magnesium alloys components are potentially used under shock loading conditions. Considering the fact that deformation behavior is completely different under high strain rate conditions compared to quasi-static conditions, it is very important to evaluate the dynamic mechanical response and deformation mechanisms of magnesium alloys under impact loading. In this research, texture formation, microstructural changes and dynamic deformation behavior of rolled AZ31B in the tempered H24 stress-relieved conditions, shock-loaded under compressive dynamic loading, were investigated. Texture measurements showed that the as-received AZ31B alloy had a strong (00.2) basal texture. Four groups of cylindrical samples were cut in the rolling direction (RD), in 45 degrees to the RD, in the transverse direction (TD) and in the direction perpendicular to the RD-TD plane. Dynamic shock loading of the test samples were conducted using Split Hopkinson Pressure Bar at room temperature at strain rates ranging from 600 to 1100 s(-1), while loading direction was parallel to the longitudinal axis of the cylindrical samples. After high strain rate deformation, although the loading direction was different, a strong (00.2) basal texture was observed in all samples. It was inferred that increasing the strain rate led to an increase in strength and ductility, but to a decrease in twinning fraction, indicating the possible activation of non-basal slip systems. Besides, a high degree of mechanical anisotropy was found for all strain rates used such that the lowest strength was registered for the samples cut along the direction parallel to the rolling direction. A viscoplastic self-consistent model with a tangent approach was used to corroborate the experimental textures and possible deformation mechanisms by simulation.

Modeling twinning and detwinning behavior of Mg alloy ZK60A during monotonic and cyclic loading

A recently developed crystal plasticity model for describing twinning and detwinning behavior is employed to simulate the behavior of extruded Mg alloy, ZK60A. Notably, accounting for the initial texture and calibrating the model using tension and compression along one direction permits prediction of the strength anisotropy, strength asymmetry, and strain hardening behavior along other directions, for cases for which the contribution of twinning is large, small and intermediate. The model discriminates between the stress required to initiate twinning and that required to grow (thicken) previously existing twins. This enables the model to simulate the unusual stress–strain hysteresis behavior during twinning (e.g., sharp yielding behavior) as well as that of detwinning (characterized by quite gradual yielding). The strain hardening plateau which occurs during both twinning and detwinning are captured, as are the rapid hardening induced by the exhaustion of these mechanisms. Finally, the modeling is validated using previously published in-situ neutron diffraction data. The predicted diffracted intensity evolution, which is indicative of the volume fraction of twinning compares well with the experimental data. For the first time, the lattice strain evolutions during cyclic loading (involving twinning and detwinning) of an extruded magnesium alloy are predicted. Most features of the experimentally observed internal strain evolution are well-described. In particular, the inflections which may be associated with the initiation of particular deformation mechanisms: basal and non-basal slips, as well as deformation twinning are predicted. Careful analysis of the lattice strains reveals greater than expected load sharing by the precipitate phase.

Detwinning behavior of Mg–3Al–1Zn alloy at elevated temperatures

Abstract Detwinning behavior of a strongly textured magnesium alloy AZ31 at different temperatures was investigated, with a focus on the effect of temperature on twin boundaries (TBs) migration. A hot extruded Mg alloy AZ31 rod was subjected to a 3% pre-compression along the extrusion direction (ED) to generate { 10 1 ¯ 2 } twins. Subsequent tensions along the ED at temperatures of 30 °C, 100 °C, 150 °C, 200 °C and 250 °C, respectively, were carried out to initiate detwinning. Mechanical behavior and microstructure evolution during detwinning deformation were examined. Our results show that the activation stress for { 10 1 ¯ 2 } TBs migration decreases with the increasing of temperature from 30 °C to 250 °C. Even at 250 °C, detwinning rather than non-basal slips dominates tension along the ED of pre-strained specimens. All strain hardening curves of detwinning deformation at temperatures of 30–250 °C contain three stages that often appear in a { 10 1 ¯ 2 } twinning predominant deformation. The length of Stage II is predominantly related to { 10 1 ¯ 2 } twin volume fraction. It is found that the peak hardening rate in Stage II decreases with increased temperature.

A Dislocation-Based Crystal Plasticity Simulation on Kink Band Formation and Evolution in Polycrystalline Mg Alloy with LPSO Phase

A three-dimensional compression analysis is performed by finite element method using a dislocation-based crystal plasticity model to clarify the formation mechanism of kink band in a polycrystalline Mg alloy with a long-period stacking ordered structure (LPSO) phase. The crystalline structure of LPSO phase is regarded as a HCP for simplicity, however, any deformation twinning is not taken into account. In addition, the activities of non-basal systems are considerably limited in the LPSO phase setting the values of their critical resolved shear stresses to large ones. We analyze a simple polycrystalline specimen composed of two α-Mg matrix phases and a LPSO phase both having a rectangular shape and twist grain boundaries are introduced into the interface. The obtained result shows that the kink band formation in the alloy is accomplished by the basal slips with different variants and the non-basal slips are activated on the grain boundary to maintain the continuity of deformation.

Analysis of Crystallographic Orientation Changes during Deformation of Magnesium Single Crystals

Magnesium single crystals of three di erent orientations were deformed by tension at room temperature to investigate the geometrical criterion resulting from the Schmid law for activation of basal and non-basal slip systems. Changes of crystallographic orientations of investigated single crystals were systematically measured during deformation and the geometrical criterion was examined in respect to the changes of crystallographic orientations during deformation. The geometrical criterion itself failed to explain lack of activity of non-basal slip systems in the deformed magnesium single crystals. Therefore the ratio of activation volumes of soft to hard slip systems is considered to be introduced for a better understanding of the deformation in hcp metals.