Non-basal Slip Research Articles

Realization of high strength and high ductility for AZ61 magnesium alloy by severe warm working

Extruded Mg–6%Al–1%Zn (AZ61) alloy bar was subjected to 4-pass Equal Channel Angular Extrusion (ECAE) processing at 448–573 K. At the processing temperature of 448 K, extremely fine grains with the average grain size of 0.5 mm are formed as a result of dynamic recrystallization originated by fine Mg17Al12 (b) phase particles having 50–100 nm diameter dynamically-precipitated during ECAE processing. The sizes of both α matrix and β phase decrease with decreasing processing temperatures. In tensile test at room temperature under the strain rate of 1×10—3 s—1, tensile strength increases with decreasing ECAE processing temperatures due to fine grains, fine precipitates and residual strain hardening. Especially, highest strength of 351 MPa was achieved in the specimen ECAE-processed at 448 K. In addition to such high strength, elongation reaches 33% in that specimen. This specimen exhibits clear strain rate dependencies of both flow stress and elongation even at room temperature. As a result, higher elongation of 67% is obtained under low strain rate of 1×10—5 s—1.In such specimen, non-basal slip and grain boundary sliding occur in addition to basal slip. Furthermore, there are grains with no dislocations, suggesting the occurrence of dynamic recovery. The contribution of all the deformation mechanisms would cause high ductility in fine-grained AZ61 alloy specimen with high strength.

Dislocation Structure in Rapidly Solidified Mg<SUB>97</SUB>Zn<SUB>1</SUB>Y<SUB>2</SUB> Alloy with Long Period Stacking Order Phase

Dislocation structure in rapidly solidified Mg97Zn1Y2 ribbon with the long period stacking order (LPSO) phase is investigated by conventional transmission electron microscopy. In the grain with the LPSO phase there are no a dislocations lying on the basal plane, and a number of c+a dislocations are visible. On the other hand, in the grain without the LPSO phase there are many straight a dislocations lying on the basal plane. These facts indicate that the critical resolved shear stress of the basal plane increases by the formation of the LPSO phase and the non-basal slip is activated by the prevention of the basal slip. In other words, the former directly contributes to the strengthening and the latter relates to the improvement of ductility with the increment of the number of the slip system. Therefore, it is concluded that the LPSO phase plays a unique role which overcomes the conflicting properties of strength and ductility in the mechanical property of the present alloy.

Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B

Mechanistic explanations for the plastic behavior of a wrought magnesium alloy are developed using a combination of experimental and simulation techniques. Parameters affecting the practical sheet formability, such as strain hardening rate, strain rate sensitivity, the degree of anisotropy, and the stresses and strains at fracture, are examined systematically by conducting tensile tests of variously oriented samples at a range of temperatures (room temperature to 250 °C) and strain rates (10 −5–0.1 s −1). Polycrystal plasticity simulations are used to model the observed anisotropy and texture evolution. Strong in-plane anisotropy observed at low temperatures is attributed to the initial texture and the greater than anticipated non-basal cross-slip of dislocations with 〈 a〉 type Burgers vectors. The agreement between the measured and simulated anisotropy and texture is further validated by direct observations of the dislocation microstructures using transmission electron microscopy. The increase in the ductility with temperature is accompanied by a decrease in the flow stress, an increase in the strain rate sensitivity, and a decrease in the normal anisotropy. Polycrystal simulations indicate that an increased activity of non-basal, 〈 c + a〉, dislocations provides a self-consistent explanation for the observed changes in the anisotropy with increasing temperature.

비탄성 변형 이론을 바탕으로 한 Mg-Al 합금의 슬립기구 천이 현상 해석

Although magnesium has high potential for structural material due to the lightweight and high specific strength, the structural application has been limited by the low ductility at room temperature. The reason of the poor ductility is few activated slip systems of magnesium (HCP structure) during deformation. As temperature increases, however, additional non-basal slip systems are incorporated to exhibit higher ductility comparable to aluminum. In the present study, a series of tensile tests of Mg-Al alloy has been carried out to study deformation behavior with temperature variation. Analysis of load relaxation test results based on internal variable approach gave information about relationship between the micromechanical character and corresponding deformation behavior of magnesium. Especially, the material parameter, p representing dislocation permeability through barriers was altered from 0.1 to 0.15 as the non-basal slip systems were activated at high temperature.

Tensile properties of hot rolled AZ31 Mg alloy sheets at elevated temperatures

Mechanical properties of commercial wrought AZ31B magnesium alloy sheets were investigated by tensile tests at various temperatures between room temperature and 400 °C at a strain rate of 1.3×10 −4 s −1. The yield stress and the maximum stress decrease with increasing temperature. The ductility increases very rapidly with increasing temperature. The results are explained by the dislocation glide in the basal and non-basal slip systems and by deformation twinning.

Deformation behaviour of Mg–Li–Al alloys

Two Mg–Li–Al alloys (LA43 and LA45) were deformed in compression at elevated temperatures. Stress relaxation tests were performed with the aim to reveal mechanism(s) of thermally activated process. Decrease of the c lattice parameter due to the Li addition results in the easier non-basal slip and reduces also the flow stress necessary for deformation of polycrystals and increases the ductility. Precipitates and solute atoms are expected to be the main obstacles for dislocation motion at lower temperatures. The dominant thermally activated mechanism at higher temperatures is very probably cross slip. The post relaxation effect (an increase of the stress after the stress relaxation test) was observed at room temperature for both alloys and also at 50 °C for LA45. A local maximum observed in the temperature dependencies of the yield as well as the maximum stress and post relaxation effect indicate dynamic strain ageing. The strain ageing is due to migration of dissolved Li and Al atoms towards dislocations.

Significance of stable and metastable phases in high temperature creep resistant magnesium–rare earth base alloys

The relationship between microstructure and creep properties is discussed for a number of Mg–rare earth cast alloys with reference to the minimum creep rate. Plate shaped precipitates, which form on the prismatic planes of the matrix in a dense triangular arrangement, provide not only most effective barriers to the motion of basal dislocations motion but are also very effective against creep deformation. Shear-resistant small discs or plates parallel to the basal planes of the α-matrix are the least effective barriers to the motion of basal dislocations but are strong obstacles to cross slip of basal dislocations and to non-basal slip. The influence of stacking fault energy decrease on the minimum creep rate is also mentioned.

Dislocation Plasticity and Complementary Deformation Mechanisms in Polycrystalline Mg Alloys

Deformation mechanisms of Mg-Al-Zn (AZ31) alloys were investigated by performing tensile test at room temperature. In fine grain Mg alloys deformed at room temperature, nonbasal slip systems were found to be active as well as basal slip systems because of grain-boundary compatibility effect. Slip-induced grain-boundary sliding occurred as a complementary deformation mechanism to give rise to c-axis component of strain. With increasing grain size, the activation of the nonbasal slip systems was limited near grain boundaries. Instead of grain-boundary sliding, twinning occurred as a complementary deformation mechanism in large grained samples. Orientation analysis of twins indicated that twinning is induced by stress concentration due to the pile up of basal dislocations. The grain-size dependence on deformation mechanism was found to affect yielding behavior both microscopically and macroscopically which can influence various mechanical properties such as fatigue and creep.

ＡＺ３１マグネシウム合金圧延板の引張特性に及ぼすＭｎ量の影響

Samples of Mg-3%Al-1%Zn(AZ31) alloy containing 0∼0.5 mass%Mn were prepared. They were then hot rolled and subsequently annealed. The effect of the amount of Mn content on their microstructures and deformation behavior during tensile tests was investigated. As Mn content increases, a large amount of Al-Mn compounds is crystallized in the alloys. The grain sizes of hot rolled specimens are significantly reduced but there is only a slight difference in the grain size of each alloy. After annealing for 1 h, significant grain growth occurs in 0%Mn alloy. On the other hand, the grain growth is sufficiently suppressed at 0.15%Mn content but it is hardly affected by further Mn addition. The tensile strength and 0.2% proof stress are improved by Mn addition of 0.05% or more, but those of the 0.1%∼0.5%Mn alloys are almost the same, because particles of Al-Mn compounds do not contribute to the strength of the alloys. In the annealed specimens, larger elongations are obtained in 0.15%Mn-containing alloy than in other alloys due to activation of non-basal slip and uniform elongation without deformation band.

Glide along non‐basal slip planes in InGaN epilayers

We have observed a systematic nucleation of misfit dislocations at the InGaN/GaN heterointerface. This occurs when InGaN films are grown on an epitaxially laterally overgrown GaN substrate with a reduced dislocation density. The misfit dislocations are aligned along 〈〉 directions forming a symmetric hexagonal array. Potential wurtzite slip systems were analysed by extending the Matthews-Blakeslee model to include Peierls forces. Due to an inactive basal plane in the c-growth direction, non-basal slip is necessary for plastic relaxation. The active slip system was identified to be {} 〈〉. The possibility of activation of other slip systems is also discussed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Imaging dislocations in ice.

Three techniques have been used to study dislocations in ice: etch pitting-replication, transmission electron microscopy, and X-ray topography (XT). It is shown that, because ice is a weak absorber of X-rays and can be produced with a low dislocation density, allowing relatively thick specimens to be studied, the most useful technique is XT. The observations that have been made with conventional XT are briefly outlined. However, the introduction of high-intensity synchrotron radiation, with its concomitant short exposure times, showed that images obtained through conventional XT observations were of dislocations that had undergone recovery. The important dynamic observations and measurements that have been made using synchrotron X-ray topography are presented. Dynamic synchrotron X-ray topography observations of ice single crystals undergoing deformation in situ have shown that slip mainly occurs by the movement of screw and 60 degrees (1/3) [1120] dislocations on the basal plane, although non-basal slip by edge dislocations can also occur. The operation of Frank-Read and other dislocation multiplication sources have been clearly demonstrated and dislocation velocities have been measured. In contrast, in polycrystals, dislocation generation occurred at grain boundaries where there are stress concentrations before lattice dislocation generation mechanisms operate. Faulted dislocation loops have been determined to be mainly interstitial in both polycrystals and single crystals.

Effect of deformation temperature on Hall–Petch relationship registered for polycrystalline magnesium

Temperature dependence of the frictional stress derived from the Hall–Petch relation for pure magnesium polycrystals was compared with the critical resolved shear stress for single crystals with orientation preferable for basal or, alternatively, non-basal slip systems. The presented results suggest that the non-basal slip plays dominant role in the room temperature plastic deformation of polycrystalline magnesium, which is in excellent agreement with the recent output of computer simulation of mechanical behavior of Mg and its alloys using viscoplastic self-consistent model [Acta Mater. 49 (2001) 4277].

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Fine-grained alloys of Mg-3Al-1Zn-0.2Mn in wt.% (AZ31B) were obtained by an equal-channel angular extrusion technique and subsequent annealing at elevated temperatures. Tensile tests were performed at room temperature at a strain rate of 1x10 -3 s -1. The alloys exhibited an apparent steady-state deformation region and a large tensile elongation of 47%. The deformed microstructure at an elongation of 2% indicated substantial cross-slip to non-basal planes induced by plastic compatibility stress associated with grain boundaries. The non-basal segment of dislocations was found to consist of 40% of the total dislocation density at a yield anisotropy factor of only 1.1 instead of an expected value of 100 obtained from single-crystal experiments. The deformed microstructure at an elongation of 16% indicated recovered regions within twins as well as untwinned matrices. These results indicate that dynamic recovery can occur in Mg alloys at room temperature.

Strengthening effect of Zn in heat resistant Mg–Y–Zn solid solution alloys

The addition of zinc is effective in improving creep strength of Mg–Y solid solution alloys at temperatures 550–650 K because zinc suppresses the non-basal slip that operates predominantly at such temperatures. This suppression is caused by the formation of planar defects on the (0 0 0 1) plane and confinement of dislocations in the basal plane due to a decrease in stacking fault energy.

Anisotropy and Non-Uniformity in Plastic Behavior of AZ31 Magnesium Alloy Plates

On the AZ31-O magnesium alloy plates of 20 mm in thickness, basal plane texture was studied for the samples taken from different layers. Compression tests were carried out in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. The formability in deep drawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties. The severity of the basal plane texture is higher at the surface layer than the inner layers. In tensile tests at room temperature, proof stress is higher for the surface layer than the inner layers, whereas elongation is lower and r-value is higher at the surface layer. In compression tests at room temperature, yield stress in the rolling and width directions is appreciably lower than in the thickness direction. At 573 K, anisotropic and non-uniform deformation behavior disappeared. Thin sheet specimens taken from inner layers of the plates showed higher formability than those from the surface layer in deepdrawing and stretch forming. It is concluded that the formability of magnesium alloy sheets can be improved by decreasing the severity of the basal plane texture. In rolling of magnesium and its alloys, the basal plane of the hexagonal lattice becomes aligned parallel to the sheet plane so that a strong basal plane texture is formed. Such textures are maintained even after recrystallization by annealing. Thus, rolled sheets of magnesium and its alloys generally show a strong anisotropy in plastic deformation behavior at around room temperature. Deformation accom- panying with thickness reduction is always difficult, that is, they show low limits of plastic deformation in cold rolling and in sheet metal forming. This is the main reason why the rolled sheets of magnesium and its alloys are not widely used in practice in spite of the recent high demands for the light- weight materials. However, high plastic anisotropy of magnesium alloy sheets becomes disappeared at tempera- tures above 473 K due to contribution of non-basal plane slip systems. At these temperatures, sheet rolling of magnesium alloys becomes much easier and the formability in deep drawing, 1-3) stretch forming, 4) bending 5) and forming limit curves 6) is appreciably improved. In thick plates of magnesium alloys, basal plane texture is considered to be less developed than in thin sheets due to lower reduction and higher rolling temperature. Additionally, mechanical properties in the thickness direction can be evaluated in thick plates. The anisotropy and non-uniformity of the basal plane texture and mechanical properties can also be discussed by testing specimens from the different layers of the plates. Obtained results are considered to suggest the desirable textures and mechanical properties for higher formability at room temperature. It has been reported in plane strain compression tests of AZ31 magnesium alloy plates that the compressive yield stress in the rolling direction is appreciably lower than in the other directions, which can be attributed to the highly developed basal plane textures. 7) However, very little has been known on the plastic behavior of magnesium plates. It is the purpose of this work to elucidate the anisotropy and non-uniformity of plastic behavior of the magnesium plates. In this work, the basal plane texture was studied for the samples taken from different layers of AZ31-O magnesium alloy plates of 20 mm in thickness. Compression tests were done in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. Finally, formability in deepdrawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties.

Intergranular strain accumulation in a near-alpha titanium alloy during plastic deformation

Neutron diffractometry has been used to characterise the evolution of intergranular lattice strains during the tensile loading of the near-α titanium alloy Ti-834. The measurements were made on tensile testpieces in directions both parallel and perpendicular to the applied load. To rationalise the observations, an elastic–plastic self-consistent model for the α phase is used, in which the three basal and two non-basal slip systems observed in titanium alloys are invoked; it is found that it is possible to reproduce successfully the observed trends and magnitudes of the microstrains in both the longitudinal and transverse directions. Unfortunately, no reflection was found for which intergranular strain accumulation is small, i.e. less than 1×10 -4. This situation should be recognised when making measurements of residual strains and stresses in large-scale engineering components, since intergranular strains must be distinguished from the macro-scale engineering strains. To circumvent this difficulty a novel method is proposed whereby the results of two or more peaks are combined to form a synthetic ‘actor’ which accumulates near-zero intergranular strain in both the parallel and perpendicular directions.

High temperature deformation behavior of permanent casting AZ91 alloy with and without Sb addition

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10−4s−1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg3Sb2 precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The \((01\overline 1 2)\) deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process.

Non-basal slip systems in HCP metals and alloys: source mechanisms

A possible source mechanism for non-basal 〈 c+ a〉 slip dislocations is proposed based on the formation of an attractive junction between glissile 〈 a〉 and sessile c dislocations from the prism plane into a pyramidal plane. The driving force for the junction formation, which comes from the long-range elastic interaction between c and 〈 a〉 dislocations, is relatively large in most hexagonal close-packed (hcp) metals. Beryllium, which has an unusually low Poisson's ratio, is an exception to this rule. The cross-slip process is energetically unfavorable in Mg and Ti, from a viewpoint of the change in anisotropic elastic line tension, but it becomes favorable in Ti at elevated temperatures above 300°C. Discussion is given on intrinsic stacking fault energies and kinetic aspects of the cross slip.

Creep deformation behavior and dislocation substructures of Mg–Y binary alloys

Compressive creep behavior and deformation substructures of binary Mg–Y alloys containing 0.2–2.4 mol% Y were investigated at 550–650 K under 4–200 MPa. The addition of yttrium improves creep strength of magnesium more efficiently than aluminum and manganese. This efficiency decreases with increasing temperature. The apparent activation energy for creep is substantially greater than that for self-diffusion in magnesium. Transmission electron microscopy revealed that the non-basal slip systems are activated at 650 K although the basal slip systems mainly operate at 550 K. Both the high activation energy and the cross-slip of dislocations from the basal to the non-basal planes can be explained consistently by the cross-slip mechanism.

Microstructural Evolution in ZK60 Magnesium Alloy during Severe Plastic Deformation

A ZK60 alloy was subjected to severe plastic deformation by torsion straining under high pressure at ambient temperature. The data of microhardness measurements, X-ray analysis and TEM observation showed that intense plastic straining resulted in formation of nanometer-scale structure characterized by presence of high internal stress fields. The effect of phase composition of the magnesium alloy on the formation of ultrafine grain structure was examined. It was shown that prior aging promoted the formation of fully grained structure during severe plastic deformation. At the same time, the size of new grains formed was found to be smaller in the quenched state of the magnesium alloy than in the aged state. Grain formation during intense plastic straining was interpreted in terms of low temperature dynamic recrystallization. The mechanisms of ultrafine grain structure formation in hcp material during severe plastic deformation and the role of non-basal slip in this process are discussed.