Elsevier

Acta Materialia

Volume 186, March 2020, Pages 475-486
Acta Materialia

Full length article
Interactions between basal dislocations and β1 precipitates in Mg–4Zn alloy: Mechanisms and strengthening

https://doi.org/10.1016/j.actamat.2020.01.028Get rights and content

Abstract

The mechanisms of dislocation/precipitate interaction as well as the critical resolved shear stress were determined as a function of temperature in an Mg–4 wt% Zn alloy by means of micropillar compression tests. It was found that the mechanical properties were independent of the micropillar size when the cross-section was > 3 × 3 µm2. Transmission electron microscopy showed that deformation involved a mixture of dislocation bowing around the precipitates and precipitate shearing. The initial yield strength was compatible with the predictions of the Orowan model for dislocation bowing around the precipitates. Nevertheless, precipitate shearing was dominant afterwards, leading to the formation of slip bands in which the rod precipitates were transformed into globular particles, limiting the strain hardening. The importance of precipitate shearing increased with temperature and was responsible for the reduction in the mechanical properties of the alloy from 23 °C to 100 °C.

Introduction

Low density, good castability, high specific strength and stiffness, and reasonable cost make Mg alloys attractive for different applications. Despite these advantages, poor formability and ductility at room-temperature as well as the limited yield strength restrict their use as a conventional engineering material for structural applications [1], [2], [3], [4], [5]. The origin of these limitations in Mg and Mg alloys is related to their hexagonal close-packed (hcp) crystal structure, which leads to large differences in the critical resolved shear stresses (CRSS) to activate different deformation modes at low temperature (below 100 °C). The CRSS for 〈a〉 basal slip in pure Mg is ≈ 0.5 MPa at room-temperature [6], [7], [8], much lower than the CRSS for 〈a〉 prismatic slip (≈ 18 MPa) and for 〈c+a〉 pyramidal slip (≈ 40 MPa) [9,10]. As a result, basal slip is always the dominant deformation mechanism in Mg alloys, limiting the strength and also the ductility because of the localization of deformation in clusters of grains suitably oriented for basal slip [11]. Moreover, basal slip cannot account for the deformation along the c axis of the Mg lattice which is normally accommodated by twinning across the {101¯2} planes. Twining is a polar mechanism which is only activated by stress states leading to an extension of the c axis and, thus, gives rise to a strong plastic anisotropy in textured Mg alloys that also reduces the formability and the ductility.

Obviously, the strength and ductility of Mg alloys can be improved by increasing the CRSS of basal slip through either solid solution and/or precipitate hardening. Nevertheless, accurate experimental data on the effect of solute atoms or precipitates on the CRSS for basal slip are scarce because they are difficult to obtain from mechanical tests in polycrystalline samples because of the superposition of different strengthening contributions (grain boundaries, latent hardening, texture) that cannot be easily isolated [12]. The most reliable data were obtained from mechanical tests on single crystals strengthened by solid solution [13], [14], [15] or precipitates [14], [15], [16] that can be tested along specific crystallographic directions to ensure that only basal slip is activated. Nevertheless, this technique is extremely time consuming and expensive because of the cost associated with the manufacturing of the single crystals.

Alternatively, micromechanical testing techniques based on compression of single crystal micropillars or nanopillars manufactured by focused ion beam milling from polycrystals have been applied in the last decade to explore the different deformation modes of Mg [17], [18], [19], [20], [21], [22] and Mg alloys [23], [24], [25], [26], [27]. More recently, micropillar compression tests have been used to determine the influence of solid solution elements on the CRSS for basal slip in Mg alloys [28]. The main limitation of these micromechanical testing techniques to obtain reliable values of the CRSS is found in the strong size effects that appear when the volume of the specimen tested is of the order of tenths of µm3 [20,[29], [30], [31], [32]]. The origin of this size effect has been thoroughly analyzed in the past [30] and comes about as a result of the interaction between the critical dimensions of the specimen (i.e. the diameter of the micropillar) and material length scale that controls the strength (i.e. the average distance between dislocations or precipitates). Size effects appear when the latter approaches the former and tend to be very strong in well-annealed fcc and hcp metals and alloys deformed along the soft modes. However, size effects during micropillar compression tests are negligible in precipitation hardened alloys when the precipitate spacing is much lower than the micropillar dimensions [33].

In this investigation, the effect of precipitates on the CRSS for basal slip at 23 °C and 100 °C was analyzed by means of micropillar compression tests in an Mg–4 wt% Zn alloy which was aged at different temperatures to produce different precipitate distributions. This alloy was selected because the β1 precipitates, which form as elongated rods along the c axis of the Mg matrix, lead to one of the strongest age hardening responses among Mg alloys [34,35]. Although the mechanical properties of the Mg-Zn alloy are well-established [36], the CRSS for basal dislocations in this system has only been studied in [24,37]. Chung and Byrne [37] measured the CRSS in Mg-5.1 wt% Zn alloy from −269 °C to 27 °C using tensile tests of single crystals in alloys aged at 200 °C for 4 and 28 h. The presence of β1 precipitates was ascertained by transmission electron microscopy but no information was provided regarding the precipitates (diameter and spacing) and the dislocation/precipitate interactions. Wang and Stanford [24] measured the CRSS by compression of circular micropillars of 2 µm in diameter at 23 °C of a Mg- 5 wt% alloy aged at 150 °C during 8 days. They reported the actual values of the precipitate diameter and spacing but they did not check the effect of the micropillar diameter on the CRSS. They showed one low magnification transmission electron micrograph (Fig. 8b [24]), which was compatible with precipitate shearing, but the details of the dislocation/precipitate interaction mechanisms were not studied. In this paper, the effect of the micropillar size on the mechanical response was carefully analyzed to obtain results of the CRSS that were independent of the micropillar size and could be compared with the precipitation hardening models available in the literature. Moreover, the details of the dislocation/precipitate interactions were carefully examined as a function of temperature and provided new insights in the limited efficiency of precipitate strengthening in Mg alloys, as compared with other metallic alloys.

Section snippets

Processing

A Mg–4 wt% Zn alloy was prepared from high purity Mg (99.90 wt%) and Zn (99.99 wt%) pellets. They were melted using a graphite crucible in an induction furnace (VSG 002 DS, PVA TePla) under a protective Ar atmosphere to avoid oxidation. The melt was held at 750 °C for about 10 min to provide a homogeneous composition and was poured into a copper die, installed inside the furnace chamber, using a tilt-casting system to minimize casting defects and the melt turbulence. The cast rods, with

Morphology of the precipitates

High magnification TEM micrographs of the material after aging at 149 °C for 100 h and at 204 °C for 16 h are presented in Fig. 1, showing the general morphology of the precipitates. The scanning transmission electron microscopy (STEM) tomography movie of the precipitates in the material aged at 149 °C for 100 h is also shown in the supplementary material (Fig. S1). The rod-shape precipitates grew along the c-axis of matrix (Figs. 1a, 1b, and S1), and presented a more or less equiaxed cross

Interaction mechanisms of basal dislocations with β1 precipitates

The mechanical behavior of a Mg-5.1 wt% Zn alloy aged at 200 °C was studied as a function of test temperature (from 4.2 to 300 K) by Chun and Byrne [37] by means of mechanical tests of single crystals whose orientations were closer to the center of the inverse pole figure. A significant reduction in the CRSS with temperature was found for the peak-aged and overaged conditions. Moreover, the effect of the strain rate on the CRSS in the peak-aged material was negligible at 4.2 K and increased

Conclusions

The effect of β1 MgZn2 precipitates on the critical resolved shear stress for basal slip and on the dislocation/precipitate interactions was analyzed in an Mg-4 wt% Zn alloy at 23 °C and 100 °C by means of micropillar compression tests. The main conclusions of this investigation are the following:

  • It was found that the initial CRSS and the CRSS at 4% strain were independent of the micropillar dimensions for micropillars with a cross-section equal to or larger than 5 × 5 µm2 because these

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This investigation was supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Advanced Grant VIRMETAL, grant agreement No. 669141. R. Alizadeh also acknowledges the support from the Spanish Ministry of Science through the Juan de la Cierva program (FJCI-2016-29660). The authors acknowledge the support of Dr. Jingya Wang, Dr. Miguel Monclus and Dr. Miguel Castillo for their helps to carry out the high temperature micropillar

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