Anisotropic deformation induced by spherical indentation of pure Mg single crystals
Introduction
Due to their light weight and excellent strength-to-weight ratio, magnesium (Mg) and Mg alloys are very attractive candidate materials for use in lightweight technologies. Energy efficiency issues indicate that their application will expand into the fields of structural materials for transport equipment such as automobiles, trains and airplanes. With a hexagonal close packed (hcp) structure, Mg has many slip and twin systems, and its deformation mode thus depends on crystal orientation. This often leads to problems in various plastic forming processes at room temperature. To expand the applicability of Mg alloys, deformation mechanisms, slips and twinning of Mg polycrystals have been much studied [1], [2], [3].
Single crystals are suitable for clarification of crystal orientation dependence on deformation behavior. Mg single crystals have undergone uniaxial deformation tests such as tension and compression, and the active deformation mechanism including basal slip, non-basal slip and twinning and their critical resolved shear stresses (CRSSs) have been reported or proposed [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. This mechanical testing applies a uniaxial load to entire Mg single-crystal specimens. However, because grains with different crystal orientations are all bound together except at sample surfaces, stress or strain actually ranges within an individual grain in polycrystalline materials under deformation.
The indentation technique locally deforms surfaces of metallic materials, allowing anisotropic deformation to occur beneath and around indentations. Partridge and Roberts [19] employed a pyramidal indenter and reported on movements of twin boundaries using Mg single crystals and annealed Zn polycrystals. In their study, a pyramidal indenter was impressed within a large twin induced by bending, and subsequent formation and behavior of incoherent twin boundaries were investigated. Shin et al. [20] impressed a Berkovich type nano-indenter on two low index planes in small-scale Mg single crystals, finding that, beneath indentations, twinning depends on the position of stress state, and also that twinning plays a role in accommodating strain induced with the nano indenter. Such an anisotropic indenter impressed on surfaces of Mg single crystals with anisotropic deformation behavior would result in complex stress or strain distribution. However, such studies on the deformation mechanism of Mg single crystals using indenters remain limited. In addition, quantitative microstructural evaluations on such deformation mechanisms in Mg single crystals have yet to be numerically simulated.
In this study, a steel ball used as an isotropic indenter is impressed on low index planes of Mg single crystals. Deformation structures such as the morphology of indentations, slip lines and twins both around and beneath each indentation are investigated. Finally, mechanisms of indentation development and resulting twin regions are discussed in terms of the activated deformation modes resulting from a crystal plasticity finite element analysis.
Section snippets
Mg single crystals and indentation testing
Mg single crystals were prepared by Bridgman technique using polycrystalline Mg (99.99% purity). Each crystal was analyzed using the Laue method and was cut into cubic specimens with surfaces of low index planes: , and . The surfaces of the specimens were chemically polished using a 200 ml C2H5OH + 70 ml H2O2 + 50 ml NHO3 solution. The polished specimens were then annealed in a thermal cyclic to remove the strain induced from cutting and polishing. For each cycle, the polished
Experimental results
Fig. 2 shows an optical micrograph of an indentation on . The indentation shows circular morphology because atomic arrangement is isotropic on basal planes. In addition, no slip lines are observed around the indentation. Though some small twins are observed within the indentation, they would not affect formation of the indentation due to their limited size; also, EBSD analysis detected no twins beneath the indentation. Therefore, the indentation on can only have been formed by slip
Discussion
Mechanisms of orientation-dependent deformation and twin region development are discussed in this section based on the experimental and numerical results in previous sections.
Conclusions
- 1.
A ball indenter was orthogonally impressed on , and planes of pure Mg single crystals. Qualitative trends of twin regions around and beneath these indentations were successfully simulated by using the crystal plasticity finite element analysis method with simple constitutive equations.
- 2.
When indented on and , indentations show elliptic morphology that greatly differs from isotropic indentations on . Crystal plasticity analysis suggests that the isotropic
Acknowledgments
A part of the present study was financially supported by MEXT KAKENHI Grant Numbers 24760081, 26109717 and 26420020 and JSPS Core-to-Core Program B. The authors are very grateful for this support.
References (34)
- et al.
Int J Plast
(2005) Mater Sci Eng, A
(2007)Mater Sci Eng A
(2007)- et al.
Acta Metall
(1973) - et al.
Scr Mater
(2010) - et al.
Acta Mater
(2011) - et al.
Acta Metall
(1964) - et al.
Scr Mater
(2013) - et al.
Comput Mater Sci
(2009) - et al.
Int J Plast
(2011)
Acta Metall
Int J Plast
Int J Plast
Compu Mater Sci
J Mech Phys Solids
Acta Metall
Int J Solids Struct
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