Elsevier

Acta Materialia

Volume 72, 15 June 2014, Pages 137-147
Acta Materialia

Influence of slip and twinning on the crystallographic stability of bimetal interfaces in nanocomposites under deformation

https://doi.org/10.1016/j.actamat.2014.03.041Get rights and content

Abstract

In this work, we examine the microstructural development of a bimetal multilayered composite over a broad range of individual layer thicknesses h from microns to nanometers during deformation. We observe two microstructural transitions, one at the submicron scale and another at the nanoscale. Remarkably, each transition is associated with the development of a preferred interface character. We show that the characteristics of these prevailing interfaces are strongly influenced by whether the adjoining crystals are deforming by slip only or by slip and twinning. We present a generalized theory that suggests that, in spite of their different origins, the crystallographic stability of their interface character with respect to deformation depends on the same few basic variables.

Introduction

It is widely recognized that internal grain boundaries (homophase interfaces) and heterophase interfaces greatly affect the properties of polycrystalline metals [1], [2], [3], [4], [5], [6], [7]. Through the control and optimization of these internal interfaces, potentially superior metals with unprecedented strengths and robustness can be developed [4], [5], [8], [9]. For instance, it has been shown that changes in interface properties within nanostructured metals can be made via different processing methods [10], [11], [12], [13], [14], [15], [16]. Thermodynamic, near-equilibrium processes, such as solid-state phase transformation, epitaxial growth or solidification processing, can produce highly textured nanostructured single-phase and composite metals with interfaces that are structurally ordered at the atomic scale [17], [18], [19], [20], [21]. They adopt interfaces with nearly the same crystallographic character (narrowly distributed within 15°) throughout the material. By virtue of this near-structural perfection, these materials have shown outstanding thermal stability [22], radiation tolerance [23], [24] and strength [25], [26]. Far-from-equilibrium mechanical processing, such as severe plastic deformation (SPD) techniques [14], [15], [27], [28], [29], [30], can also produce ultra-fine-grained and nanostructural metals comprised of a high density of interfaces. In single-phase SPD metals, while a small fraction of ordered boundaries has been reported [31], most of the grain boundaries are structurally disordered, and are often referred to as “non-equilibrium” boundaries [14], [32], [33]. Likewise, in bimetal wires fabricated by wire-drawing and bundling, several types of interfaces, both ordered and disordered, can form [29], [34], [35]. Although imperfect in structure, these SPD nanostructures possess superior strength, and in some cases, ductility [15], [36], [37]. However, the disorder renders them microstructurally unstable with respect to heating [38], [39], [40], [41].

Recently, it was reported that a well-known SPD process, called accumulative roll bonding (ARB) [28], [42], [43], [44], [45], induced the formation of an ordered bimetal Cu–Nb interface that prevailed ubiquitously over the bulk two-phase Cu–Nb layered composite. After extreme strains and substantial layer refinement (99.96–99.99% rolling reduction and individual layer thicknesses of h = 200–700 nm), a predominant bimetal interface emerged with a crystallographic character that was narrowly distributed about {1 1 2}〈1 1 1〉Cu||{1 1 2}〈1 1 0〉 Nb [44] and with a highly ordered atomic structure with little to no detectable defects [44], [46], [47]. In light of prior reports of interface structures after SPD, the emergence of a highly oriented and atomically ordered predominant interface from ARB is unexpected. The present knowledge base for microstructural evolution in SPD is insufficient for explaining SPD-induced ordering of interface structure.

Recently, two variables were proposed to influence the crystallographic stability of this interface [48]. The first variable is plastic stability in co-deformation, which refers to the preservation of interface character during plastic deformation. Crystal plasticity finite-element (CPFE) simulations of a Cu–Nb bicrystal with the {1 1 2}〈1 1 1〉Cu||{1 1 2}〈1 1 0〉Nb interface under plane strain compression found that its character misoriented only a few degrees to a stable end state [49], [50]. Repeating the analysis for other bicrystals in single-crystalline or polycrystalline layers suggested that when an interface is composed of two stable rolling orientations, its character tends to be preserved in rolling [49], [50]. This result forecasts that many interface characters could be plastically stable in co-deformation, not only the {1 1 2}〈1 1 1〉Cu||{1 1 2}〈1 1 0〉Nb interface. The second variable is interface formation energy, which can be calculated using molecular dynamics (MD) simulation [47], [51]. As layers refine in the ARB process and the interface density increases, a lower formation energy interface would result in lower stored energy in the material. However, while the {1 1 2}〈1 1 1〉Cu||{1 1 2}〈1 1 0〉Nb interface does not have the highest formation energy (825 mJ m−2 < 1000 mJ m−2), it also does not correspond to the lowest one (Kurdjumov–Sachs or Nishiyama–Wasserman ∼576–586 mJ m−2). The conclusion was that both variables mattered and attaining an optimal combination of both presents a severe constraint that far fewer interfaces satisfy.

These ideas were developed assuming that both metals deform by slip only. In this work, we investigate the evolution of interfaces when the face centered cubic (fcc) Cu phase undergoes twinning in addition to slip. When h in the Cu–Nb composite is refined via ARB to nanoscale dimensions (<60 nm), the Cu phase is found to deform via slip and twinning. Because of this change in deformation mechanism, the {1 1 2}〈1 1 1〉Cu||{1 1 2}〈1 1 0〉Nb interface becomes unstable. Here, with microstructural characterization and crystal plasticity analyses, we reveal that another stable, predominant interface emerges after twinning, distinct from the one that develops when the crystals deform purely by slip, which remarkably also exhibits a regular atomic structure. We analyze its origin and relationship to deformation twinning. The results enable us to advance the set of stability criteria not only to explain the development of both stable interfaces but also to provide insight into creating other crystallographically stable interfaces via deformation processing.

Section snippets

ARB processing

ARB processing of the Cu–Nb layered composite begins with an alternating stack of 2 mm sheets of coarse-grained polycrystalline Cu and Nb in equal fractions [45], [52]. Prior to ARB processing, the as-received Cu (99.99% purity) sheets were rolled to 60% reduction to 2 mm and subsequently annealed at 450 °C for 1 h. The as-received Nb sheets (99.94% purity) were rolled to 30% reduction to 2 mm and annealed afterwards at 950 °C for 1 h. To prevent exposure of the Cu–Nb interfaces to air during ARB

Two key variables for crystallographic stability of an interface under SPD

Regarding the origin of the two prevailing interfaces, the slip-fcc/bcc and twin-fcc/bcc, we now turn to the question: Can crystallographically stable interfaces that form in rolled bimetal material systems that deform via different mechanisms depend on the same few basic variables? In spite of their difference in character, these two stable interfaces appear to form from processes that have several aspects in common: (1) both interfaces join two crystals of ideal or nearly ideal rolling

Summary

In ARB processing of a 50/50 Cu–Nb layered composite, when h is refined to the submicron range, 1 μm to 500 nm (8–9 strain), a crystallographically stable interface emerges predominantly over the entire material. The interface remains stable with respect to continued extreme straining to h = 60 nm (10.6 strain). The evolution of textures and interfaces over the corresponding strain regime involves primarily slip in both layers. Upon further straining to reductions in h below 60 nm, the Cu phase

Acknowledgements

The authors gratefully acknowledge support by the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026. R.J.M. and J.S.C. acknowledge support by a Laboratory Directed Research and Development (LDRD) program 20140348ER.

References (85)

  • Y. Mishin et al.

    Acta Mater

    (2010)
  • H. Gleiter

    Prog Mater Sci

    (1989)
  • M.A. Meyers et al.

    Prog Mater Sci

    (2006)
  • M. Dao et al.

    Acta Mater

    (2007)
  • V. Randle

    Acta Mater

    (2004)
  • H. Gleiter

    Acta Mater

    (2008)
  • R.Z. Valiev et al.

    Prog Mater Sci

    (2006)
  • A. Misra
  • J.B. Liu et al.

    J Alloys Compd

    (2008)
  • I.J. Beyerlein et al.

    Int J Plast

    (2011)
  • I.J. Beyerlein et al.

    Mater Today

    (2013)
  • H. Huang et al.

    Acta Mater

    (2000)
  • K. Han et al.

    Acta Mater

    (1998)
  • Y. Saito et al.

    Acta Mater

    (1999)
  • L. Thilly et al.

    Mater Sci Eng A

    (2001)
  • V.M. Segal et al.

    Mater Sci Eng A

    (1997)
  • A.A. Nazarov et al.

    Acta Metall Mater

    (1993)
  • Q. Xue et al.

    Acta Mater

    (2007)
  • X. Sauvage et al.

    Acta Mater

    (2001)
  • M.J. Demkowicz et al.

    Acta Mater

    (2011)
  • Y. Estrin et al.

    Acta Mater

    (2013)
  • H.G. Jiang et al.

    Mater Sci Eng A

    (2000)
  • J.S. Carpenter et al.

    Acta Mater

    (2012)
  • S.C.V. Lim et al.

    Mater Sci Eng A

    (2009)
  • S.B. Lee et al.

    Acta Mater

    (2012)
  • S.J. Zheng et al.

    Acta Mater

    (2012)
  • J.R. Mayeur et al.

    Int J Plast

    (2013)
  • C.W. Sinclair et al.

    Mater Sci Eng A

    (1999)
  • B.L. Hansen et al.

    Int J Plast

    (2013)
  • J. Hirsch et al.

    Acta Metall

    (1988)
  • D. Raabe et al.

    Scripta Metall Mater

    (1992)
  • J. Hirsch et al.

    Acta Metall

    (1988)
  • J.K. Chen et al.

    Philos Mag A

    (1997)
  • I.J. Beyerlein et al.

    Acta Mater

    (2013)
  • Y. Zhou et al.

    Acta Metall Mater

    (1992)
  • Z.Q. Wang et al.

    Int J Plast

    (2011)
  • U.F. Kocks et al.

    Prog Mater Sci

    (2003)
  • R.F. Zhang et al.

    Scripta Mater

    (2013)
  • R.Z. Valiev et al.

    Phys Status Solidi

    (1986)
  • B. Chalmers

    Proc Roy Soc

    (1948)
  • S.A. Barnett et al.

    Ann Rev Mater Sci

    (1994)
  • T. Chookajorn et al.

    Science

    (2012)
  • Cited by (42)

    • Heterostructured stainless steel: Properties, current trends, and future perspectives

      2022, Materials Science and Engineering R: Reports
      Citation Excerpt :

      The textures of the other Cu/Nb LS material shown in Fig. 15(a-b) were manufactured by an SPD technique, i.e., ARB, which involves repeated rolling, cutting, and stacking at room temperature [556]. The Cu and Nb phase textures in this SPD sheet are also strong, with the {112} poles aligned through the thickness of the sheet [557]. Fig. 15 compares their {111} pole figures to show that despite being composed of the same two materials, Cu and Nb, the textures are different from those of the PVD film by virtue of their distinct manufacturing processes.

    • Interface-mediated plasticity and fracture in nanoscale Cu/Nb multilayers as revealed by in situ clamped microbeam bending

      2021, Materials Science and Engineering: A
      Citation Excerpt :

      Results will be put in the context of previous reports in the literature for the same materials system [2,8,11,17,41,42], including our work [28,29,36,38]. The effects of individual layer thickness in nanoscale Cu/Nb ARB multi-layered materials have been reported using various experimental testing methodologies [42–47] indicating the generally observed “smaller is stronger” phenomena (in terms of yield strength, plastic anisotropy, number of discrete slip events, etc.) and unique interface-based plasticity such as deformation twinning on the interfaces [42,48], interface-induced texture evolution during the ARB processing [45], etc. In the present study, our in situ experiments allow investigation of a rather unique interface-based plasticity, which is the localized shear on the interfaces.

    View all citing articles on Scopus
    View full text