Hall-Petch relationship in Mg alloys: A review Yu, Huihui; Xin, Yunchang; Wang, Maoyin ...
Journal of materials science & technology,
February 2018, 2018-02-00, Letnik:
34, Številka:
2
Journal Article
Recenzirano
Grain refinement could effectively enhance yield strength of Mg alloys according to the well-known Hall-Petch theory. For decades, many studies have been devoted to the factors influencing the ...Hall-Petch slope (k) in Mg alloys. Understanding the factors influencing k and their mechanisms could offer guidance to design and produce high-strength Mg alloys through effective grain refinement hardening. A review and comments of the past work on the factors influencing k in Mg alloys are presented. Results of these previous investigations demonstrate that the value of k in Mg alloys varies with texture, grain size, temperature and stain. The influence of texture and grain size on k is found to be an essential result of the variation of deformation mode on k value. Without the variation of deformation modes, it is revealed that texture could also impose a significant effect on k and this is also summarized and discussed in this paper. The reason for texture effect on k is analyzed based on the mechanism of Hall-Petch relationship. In addition, it is found in face-centered cubic (fcc) or body-centered cubic (bcc) metals that boundary parameters (boundary coherence, boundary energy and boundary diffusivity) could strengthen twinning or slips to a different extent. Therefore, the role of boundary parameters is also extended into the k values in Mg alloys and discussed in this paper. In the end, we discuss the future research perspective of Hall-Petch relationship in Mg alloys.
Abstract
Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain ...boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production.
A Hall-Petch slope (k) that is highly changeable with texture, as extensively reported in Mg alloys, is ultimately related to the variation of deformation modes. In this paper, the influence of ...different (0002) distributions on k for twinning and slip was systematically studied using an AZ31 rolled plate (0002//ND) and extruded rod (0002⊥ED together with a random distribution around the ED). The ND and ED refer to the normal direction of the plate and extrusion direction of the rod, respectively. A high dependency of k on the (0002) distribution is found, namely, a much lower k for {101¯2} twinning in the plate (219 MPa μm1/2) than that in the rod (435 MPa μm1/2), but a much higher k for slip in the plate (437 MPa μm1/2) than that in the rod (235 MPa μm1/2). Compound use of the difference in Schmid factor (ΔSF) and geometric compatibility factor (m′) quantitatively explains this orientation effect on k. ΔSF relates to the extra stress needed for the activation of slip/twinning in a neighboring grain, and m′ reflects the efficiency of the stress concentration at the onset of slip/twinning in an adjacent grain. The lower m′ for twinning in the rod versus the plate primarily accounts for the higher k for twinning in the rod. A much larger inclination of basal poles away from the ideal texture exists in the plate than in the rod, which induces a higher activity of basal slip during tension. The resultant high fraction of slip transfer from basal slip in one grain to prismatic slip in the neighboring grain largely amplifies ΔSF and reduces m′, both of which yield a higher k for slip in the plate than in the rod. The relationship between the crystallographic orientation and m′ was also calculated for different types of deformation transfer, and the main factor that determines m′ was revealed.
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AZ31 Mg alloy was processed by accumulative roll-bonding (ARB) and hot rolling (HR), respectively, followed by annealing. Layered bimodal structures characterized by an alternative distribution of ...fine-grained layers and coarse-grained layers were obtained in the ARB samples, while mixed bimodal structures were achieved in the HR samples. The ARB samples have superior combinations of high strength and good elongation compared to the HR samples, indicating a clear effect of layered bimodal structures on mechanical properties of the alloy. The strength of the ARB samples is related to the grain size; while the ductility is attributed to the activity of non-basal slip and the strong backstress.
Light-weight magnesium alloys with high strength are especially desirable for the applications in transportation, aerospace, electronic components, and implants owing to their high stiffness, ...abundant raw materials, and environmental friendliness. Unfortunately, conventional strengthening methods mainly involve the formation of internal defects, in which particles and grain boundaries prohibit dislocation motion as well as compromise ductility invariably. Herein, we report a novel strategy for simultaneously achieving high specific yield strength (∼160 kN m kg–1) and good elongation (∼23.6%) in a duplex magnesium alloy containing 8 wt % lithium at room temperature, based on the introduction of densely hierarchical {101̅1}–{101̅1} double contraction nanotwins (DCTWs) and full-coherent hexagonal close-packed (hcp) particles in twin boundaries by ultrahigh pressure technique. These hierarchical nanoscaled DCTWs with stable interface characteristics not only bestow a large fraction of twin interface but also form interlaced continuous grids, hindering possible dislocation motions. Meanwhile, orderly aggregated particles offer supplemental pinning effect for overcoming latent softening roles of twin interface movement and detwinning process. The processes lead to a concomitant but unusual situation where double contraction twinning strengthens rather than weakens magnesium alloys. Those cutting-edge results provide underlying insights toward designing alternative and more innovative hcp-type structural materials with superior mechanical properties.
The mechanisms of strengthening and toughening of hot-rolled AZ31 magnesium alloy by {10−12} extension twins during strain path changed recompression are studied in the current paper. Under the ...favorable orientation, {10−12} extension twinning can occur in {10−12} twins generated by precompression along the rolling direction. Grain refinement by extension twins dramatically enhances both yield stress and peak stress of reloading along the transverse direction without any degradation in elongation, while the effect is closely related with the level of prestrain.
► Pr-rolling along TD initiates (10–12) twins which incline basal plane by 86°. ► Inclination of basal plane can effectively improve rolling capability at 300
°C. ► Non-basal slip of 〈a〉 dislocation ...in pre-rolled sample can accommodate strain in ND.
A method to weaken the strong basal texture of rolled magnesium alloy by twinning deformation is reported. Pre-rolling along the transverse direction at room temperature initiates extension twinning and reorients the basal plane. A dramatically increased maximum thickness reduction per pass during subsequent rolling at 300
°C is achieved in pre-deformed sample compared to that of the directly rolled one. The corresponding mechanisms are addressed and discussed.
A hybrid extension twin structure generated by a cross compression of the hot-rolled Mg alloy along the transverse direction (TD) and the rolling direction (RD) is employed to reassign the ...crystallographic orientation along RD, TD and normal direction (ND). The orientation redistribution leads to multi-deformation modes during both the compression and the tension deformation. The grain refinement by twin lamellas and the crystallographic orientation redistribution greatly reduces the tension–compression yield asymmetries along RD, TD and ND in the pre-strained sample. The corresponding mechanism are studied and discussed, too.
{10-12} Twinning is one of the main deformation mechanisms in Mg–3Al–1Zn at room temperature. Twin variant activation is reported to follow the Schmid law and be activated as a unidirectional slip ...system, but local stress states are said to favor other twin variants. In a material containing a first generation of {10-12} twins, subsequent deformation can activate re-twinning and detwinning. In the present paper, a multiple twin structure containing two generations of twins was generated by two successive in-plane compressions along two different directions in a rolled plate. The plate was subsequently loaded to activate detwinning. A grain by grain analysis was performed after each compression to identify the twin variants selection mechanisms. During compression and re-compression, twinning was observed to obey the Schmid law well, with preferential activation of the variants with the highest Schmid factor. Other variants were observed and predicted by constraint models based on the Schmid law and extremum principles. Only a few twins and secondary twins {10-12}–{10-12} were favored by autotwinning and were less dependent on the Schmid factor. The grain boundary misorientations and first generation twin boundaries did not affect the nucleation of new twins statistically. Consequently the twin variant selection and the deformed texture can be predicted by self-consistent crystal plasticity models regardless of the neighboring grains orientations. On the other hand the activation of detwinning after annealing cannot be predicted from the twin orientation.
In the present paper, the influence of a {101¯2}–{101¯2} secondary twin on detwinning of a {101¯2} primary twin is systematically investigated by compression along the normal direction (ND) of ...pre-strained samples at room temperature. Samples containing both a {101¯2} primary twin and a {101¯2}–{101¯2} secondary twin are prepared by pre-compression along the transverse direction (TD) and subsequent re-compression along the rolling direction (RD) of a hot-rolled AZ31 Mg alloy plate at room temperature. Our results show that the introduction of a {101¯2}–{101¯2} secondary twin into the primary twin can enhance the compression yield strength along the ND. During compression along the ND, the {101¯2}–{101¯2} secondary twin mainly deforms by {101¯2} twinning, forming a {101¯2}–{101¯2}–{101¯2} ternary twin, while detwinning of the {101−2} primary twin also takes place. When a {101¯2}–{101¯2} secondary twin happens to totally separate a {101¯2} primary twin from the matrix, it can serve as a barrier to impede detwinning of this {101¯2} primary twin. The {101¯2} twinning in a {101¯2}–{101¯2} secondary twin can induce a preferred distribution of prismatic planes in twinned regions, which is closely related to the activation of preferential twin variants.
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