A comparative study of indentation induced creep in pure magnesium and AZ61 alloy

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Abstract

Nanoindentation creep tests were performed on pure Mg and Mg–6 wt% Al–1 wt% Zn (AZ61) alloy at room temperature. Plastic flow and creep were more pronounced in Mg than in AZ61. Pure Mg was found to exhibit two distinct steady state stress regimes, with the values of stress exponent equal to 4.5 and 12. Twinning is most likely to dominate initially, in the high stress regime. Crystallographic re-orientation, caused by twinning renders the grains favorable for 〈c+a〉 slip in low stress regime. Grain boundary creep was also investigated, and appeared to be related to grain boundary sliding (n=2) in low stress regime. Creep response of AZ61 consisted of a single stress exponent regime (n~1) for both grain interior as well as grain boundary. A combination of extension and contraction twinning seems to be the dominant deformation mechanism. Chemical potential gradient induced diffusion is also likely to play an important role in creep deformation.

Introduction

Mg and its alloys are attractive candidates for structural applications owing to their light weight. However, due to poor formability and inferior creep resistance, their widespread applications are limited. The former is attributed to limited slip systems due to hcp structure, while the latter is related to thermal instability of microstructure. The problem further aggravates due to sensitivity of deformation mechanisms to microstructural variables such as grain size, secondary phase particles, solute atoms and texture [1], [2], [3], [4]. Currently, there is very limited understanding on how these variables affect the deformation mechanisms and hence on the resulting creep behavior of magnesium and its alloys.

It is very important to understand mechanical properties and deformation behavior of a material if it is to be employed for structural applications. Creep, or time-dependent plastic deformation, is an important mode of deformation in structural materials. Numerous studies have been conducted on creep behavior of Mg using conventional uniaxial tensile/-compressive technique [5], [6], [7], [8], [9]. Tegart [5] investigated creep in polycrystalline Mg in the temperature range of 180–280 °C, and suggested dislocation climb (in low temperature regime) and cross slip from basal to prismatic planes (in high temperature regime) to be the rate controlling mechanisms. In another study [6], creep behavior was found to be dependent on strain rate, with dislocation climb and cross-slip from basal to prismatic planes as dominant creep mechanisms at slow (<10−6 s−1) and fast strain rates (>10−6 s−1), respectively. Vagarali and Langdon [9] carried out comprehensive study of creep deformation in Mg for different regimes of temperature (over a range of 200–540 °C) and stress (~1–10 MPa), and proposed dislocation climb (lower temperature), cross-slip of dislocations from the basal to the prismatic planes (high temperature, high stress) and Nabarro-Herring diffusion creep (higher temperature, very low stress) as the dominant creep mechanisms. While there is a consensus on stress exponent value for Mg in the range of 4–5, stress exponent as high as ~10 has been reported at higher temperatures [6], [8].

There are numerous reports on creep behavior of Mg–Al alloys as well by conventional uniaxial and impression techniques (in which flat-ended cylindrical punches are impressed upon the material) [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Creep in AZ31 alloy has been reported to be stress dependent, with solute drag, viscous glide, dislocation climb and power law breakdown as dominant mechanisms for different conditions [14], [16], [19], [21]. In the temperature range of 320–420 °C, creep in AZ31 has been found to be dislocation-glide controlled, whereas it has been determined to be diffusion-driven for higher temperatures [12], [17]. Creep in AZ91 has been reported to be controlled by dislocation-climb [11], [18]. Wang and Huang [13] identified GBS and lattice diffusion as dominant creep mechanism in AZ61 for temperature range of 250–400 °C. However, in another study [22], creep in AZ61 was identified to be controlled by dislocation-glide (350–450 °C). Investigation by impression technique [20] in the temperature range of 150–200 °C revealed creep in the same alloy to be driven by dislocation-climb and power law breakdown. Somekawa et al. [15] presented a comprehensive study of creep behavior of AZ31, AZ61 and AZ91 (at 200–350 °C), and determined dislocation pipe diffusion (low temperature regime) and lattice diffusion (high temperature regime) as mechanisms for climb-controlled dislocation creep in these alloys. All these studies have been conducted at elevated temperature (150–400 °C). There is a dearth of reports on room temperature creep behavior of Mg alloy. There is one study by Miller [10] on room temperature tensile creep in AZ91 alloy, investigated for 60–180 MPa stress range. Stress exponent of 4.5 was calculated and dislocation-climb was established as the dominant mechanism. There is no report on room temperature creep behavior of AZ61.

The creep studies mentioned above have been carried out using either conventional tensile/compressive method, or impression testing technique. These studies conducted with bulk testing methods do not differentiate between micro-features in the material. There has been a recent upsurge in interest in indentation techniques to study creep because of their experimental simplicity and non-destructive nature [23], [24], [25], [26], [27], [28], [29], [30]. Indentation tests require minimal sample preparation, and can probe different volumes of material by using appropriate combination of indentation load and indenter geometry [24], [31]. Nanoindentation study would facilitate the examination of creep properties at micro-nanostructural scale. It can be used to probe localized features, like an individual grain and grain boundary, which is not possible by conventional creep tests. Wang et al. [32] investigated nanoindentation creep in pure Mg and compared it with uniaxial tensile results. The results obtained by the two techniques were in consonance with each other, and the interplay of dislocation slips and twinning was proposed as the dominant creep mechanism. Nanoindentation creep at grain boundary has also been studied in pure Mg [33], and grain boundary sliding (GBS) was proposed as dominant creep mechanism. However, these reports do not throw light on relationship between texture and deformation mechanisms. Also, these studies do not report features of indentation region and material flow in the vicinity of the indenter tip. There is a dearth of studies on nanoindentation creep of Mg alloys. There is only one report on nanoindentation creep of AC52 alloy [34]. Nanoindentation creep in AZ61 has hitherto not been studied.

The objective of the present work is to investigate nanoindentation creep behavior of pure Mg and solution-treated Mg−6 wt% Al−1 wt% Zn alloy (AZ61) at room temperature. Strongly textured specimens were studied to establish relationship between texture and deformation mechanisms. The alloy was studied in solution-treated condition, and its creep response was compared with pure Mg to determine the effect of solute atoms on creep behavior. Creep behavior of grain boundary has also been probed, and compared with creep inside the grain. Indentation region has been imaged by employing atomic force microscopy (AFM). Creep behavior in this study has been reported for indentation along the extrusion direction.

Section snippets

Material

The materials used in this study were extruded rods of pure Mg and AZ61 (Mg–6 wt% Al–1 wt% Zn) alloy received from Good Fellow, UK. It is known that extruded rods of Mg and Mg alloys exhibit very strong texture, such that 〈c〉 axis of the grain are aligned nearly perpendicular to the extrusion direction [35], as shown in the schematic diagram (Fig. 1). Small cuboidal specimens of dimensions 10 mm×8 mm×7 mm were cut from the extruded rods, employing wire electric discharge machining (EDM) technique.

Microstructure of pure Mg and AZ61

The optical micrographs of pure Mg and AZ61 samples are shown in Fig. 2. From Fig. 2(b), it is clear that the grains are free of any precipitates. The average grain size for pure Mg was approximately 100 μm, whereas that of AZ61 was found to be 30 μm. Grain size of both pure Mg and AZ61 is large enough to ensure that grain boundaries do not have significant impact on nanoindentation deformation inside the grains. It has been observed that grain boundary affects deformation behavior up to a

Nanoindentation creep mechanisms in pure Mg

In nanoindentation, stress state in the vicinity of the indenter is multi-axial [42]. A schematic representation of stress state under the indenter in pure Mg is shown in Fig. 8(a). Just underneath the indenter, the stress is acting perpendicular to c-axis of the grains. Regions around the indenter are also under stress, such that c-axis is under compression. None of the two orientations is favorable for basal slip, because the resolved shear stress component on basal plane is nearly zero for

Conclusions

Nanoindentation creep behavior of pure Mg and AZ61 alloy was compared to assess the impact of alloying on room temperature creep behavior. Grain boundary creep response was also recorded and analyzed. The following conclusions can be drawn from this study:

  • (1)

    Addition of Al and Zn solutes enhances creep resistance of Mg at room temperature.

  • (2)

    Pure Mg exhibits two distinct steady state stress exponent regimes, with n equal to 4.5 and 12 inside the grain. Twinning is most likely the dominant creep

Acknowledgment

The authors would like to thank Mrs. Shanta Mohapatra (IIT Delhi) for her assistance in experimentation. Jayant Jain would like to thank DST-SERB, India (Project No.: RP02797) for providing the financial support in the project.

References (52)

  • S.R. Agnew et al.

    Int. J. Plast.

    (2005)
  • J. Koike et al.

    Acta Mater.

    (2003)
  • W.J. McG Tegart

    Acta Metall.

    (1961)
  • R.B. Jones et al.
    (1963)
  • S.S. Vagarali et al.

    Acta Metall.

    (1981)
  • S. Spigarelli et al.

    Mater. Sci. Eng. A

    (2000)
  • H. Watanabe et al.

    Int. J. Plast.

    (2001)
  • H. Somekawa et al.

    Mater. Sci. Eng. A

    (2005)
  • S. Ansary et al.

    Mater. Sci. Eng. A

    (2012)
  • E. Mohammadi Mazraeshahi et al.

    Mater. Des.

    (2013)
  • M.L. Olguín-González et al.

    Mater. Sci. Eng. A

    (2014)
  • R. Goodall et al.

    Acta Mater.

    (2006)
  • C.L. Wang et al.

    Scr. Mater.

    (2010)
  • L. Shen et al.

    Mater. Sci. Eng. A

    (2012)
  • A. Gouldstone et al.

    Acta Mater.

    (2007)
  • L. Han et al.

    Mater. Sci. Eng. A

    (2009)
  • D. Duly et al.

    Acta Metall. Mater.

    (1995)
  • H. Abrams

    Metallography

    (1971)
  • M.J. Mayo et al.

    Acta Metall.

    (1988)
  • J.-H. Shin et al.

    Scr. Mater.

    (2013)
  • W.B. Li et al.

    Acta Metall. Mater.

    (1993)
  • S. Hwang et al.

    Scr. Mater.

    (2001)
  • J. Jain et al.

    Mater. Sci. Eng. A

    (2012)
  • N. Stanford et al.

    Int. J. Plast.

    (2013)
  • A. Akhtar et al.

    Trans. Jpn. Inst. Met.

    (1968)
  • M.A. Gharghouri et al.

    Phil. Mag. A

    (1999)
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      The stresses are marginally higher at 176 °C than at room temperature, because of precipitate hardening of the alloy. The slopes of the terminal portion of these plots (steady-state region) are computed by linear fitting to obtain the stress exponents (based on Eq. (7)) [34–36]. The values of n at different indentation temperatures are marked in Fig. 4b. Stress exponent is an indicator of creep mechanisms in the material [37].

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