Texture evolution in two-phase Zr/Nb lamellar composites during accumulative roll bonding
Introduction
Multi-layered (lamellar) two-phase metals have been shown to exhibit a number of extraordinary properties, such as high strength, thermal stability, and hardness, which are not characteristics of their constituents (Anderson et al., 2003, Beyerlein et al., 2011a, Chu and Barnett, 1995, Embury and Hirth, 1994, Ham and Zhang, 2011, Mara et al., 2008, Misra and Kung, 2001, Was and Foecke, 1996). A majority of these bimetallic systems investigated recently were fabricated by near-equilibrium, thermodynamic techniques, such as solidification (Beyerlein et al., 2011a, Inoue et al., 2001), magnetron sputtering (Broussard et al., 1984, Fu et al., 2008, Ham and Zhang, 2011, Schweitz et al., 2001) and electro or chemical deposition (Huang and Spaepen, 2000, Zhai et al., 1997), that yield small volumes of material.
As a possible method for scaling up these two-phase metals to sizes suitable for structural applications, bulk severe plastic deformation (SPD) techniques such as equal channel angular pressing or extrusion (ECAE) or accumulative roll bonding (ARB) are being considered. These SPD techniques have been applied to single-phase metals, such as Cu (Li et al., 2005, Wang et al., 2003), Nb (Sandim et al., 2007, Zhu et al., 2013a, Zhu et al., 2013b), Ta (Wei et al., 2003), Ni (Zhang et al., 2011), Al (Saito et al., 1999), Ag (Beyerlein et al., 2007), Mg (Ma et al., 2009, Pérez-Prado et al., 2004), Zr (Jiang et al., 2008, Yapici et al., 2009), steel (Li et al., 2006) and Be (Beyerlein et al., 2010). The number of studies on SPD processing of two-phase metals is rising and include studies of co-deformation of Cu/Ag (Han et al., 1999, Ohsaki et al., 2007), Cu/Nb (Carpenter et al., 2012a, Demkowicz and Thilly, 2011, Lee et al., 2012, Segal et al., 1997, Thilly et al., 2001, Zheng et al., 2013), Al/Zn (Dehsorkhi et al., 2011), Cu/Ni (Liu et al., 2011), Ag/Fe (Yasuna et al., 2000), and even hard/soft systems like Cu/Cr or Ni/W (Embury and Sinclair, 2001, Sinclair et al., 1999).
Scaling up from the epitaxial growth of films to the SPD processing of bulk metals is not straightforward due to several differences in the final microstructure at several material length scales. One of the fundamental differences is that SPD causes substantial texture evolution (Beyerlein and Tóth, 2009, Jiang et al., 2008, Knezevic et al., 2013b, Yapici et al., 2009). The final textures depend on the specific SPD deformation path by which the material was processed (Beyerlein and Tóth, 2009). It is not expected that the texture resulting from the SPD technique will be similar to that from electro or chemical deposition. The plastic deformation behavior of the final material will depend on many microstructural aspects, one of them being texture evolution. Thus, understanding the mechanical properties of two-phase metals fabricated by SPD requires knowledge of their crystallographic texture.
Most studies of texture evolution of metals in SPD have largely focused on single-phase metals. Crystal plasticity models typically chosen for calculating texture evolution in these cases are mean-field homogenization schemes, such as the Taylor (Fast et al., 2008, Kalidindi et al., 1992, Kalidindi et al., 2006, Kalidindi et al., 2009, Knezevic et al., 2008, Knezevic et al., 2009, Knezevic and Kalidindi, 2007, Shaffer et al., 2010, Taylor, 1938, Wu et al., 2007) model or self-consistent modeling (Lebensohn and Tomé, 1993, Lebensohn et al., 2007), since they are capable of treating very large-strain deformation. Crystal plastic finite element models (CPFE) have also been used for moderate strain deformation (Kalidindi et al., 1992, Knezevic et al., 2010, Roters et al., 2010) with the advantage that, unlike the aforementioned schemes, grain–grain interactions can be captured.
In two-phase composites, the presence of another phase or the bi-phase interface can potentially affect texture evolution. While not extensively investigated, these effects have been studied in two-phase cubic systems, such as Cu/Nb (Beyerlein et al., 2013, Lee et al., 2012) and Cu/Ag (Beyerlein et al., 2011a). The Cu/Nb works collectively reveal the importance of individual layer thickness, h, on texture evolution (Carpenter et al., 2012a). Large layer thicknesses led to rolling textures that displayed no signs of interface effects, whereas small layer thicknesses, comprised of a single grain across the thickness, produced unusual deformation textures. Texture evolution in the former case was successfully simulated by crystal plasticity finite element method (Hansen et al., 2013). Recently, using the same technique, the phenomenon of shear banding in Cu/Nb and Cu/Ag laminates was studied (Jia et al., 2013). As an example of unusual textures, in a study of an ultra-fine Cu/Ag laminate, it was found that Ag promoted profuse twinning in Cu, which would have been uncommon if Cu were rolled alone (Beyerlein et al., 2011a). Studies on texture evolution in the SPD of composite material systems containing low symmetry metals, such as hcp/bcc or hcp/fcc interfaces are even more limited.
Here we fabricate a novel hcp Zr/bcc Nb layered composite and study texture evolution that develops in large strain roll bonding. We show that these composites can be successfully fabricated with controlled individual layer spacing, h, down to h = 4 μm. Achieving this fine scale required strains of ∼4. The resulting texture was similar to the rolling textures, found in literature, in the corresponding monolithic materials after large strains. As a means of confirming this result, we calculated the rolling textures of the individual phases using a visco-plastic self-consistent polycrystal plasticity code that integrates a hardening law based on dislocation densities (Beyerlein and Tomé, 2008). Doing so required adapting a dislocation density based hardening law for Nb. At large strains, the predicted textures agreed well with the measured deformation textures for each phase in the composite. Thus, from the experimental and simulation results, we conclude that the interface did not affect bulk texture development in the micron-scale Zr/Nb composite fabricated by ARB. The model indicated that Zr deformed by a combination of prismatic 〈a〉 slip, pyramidal 〈c + a〉 slip, and anomalously basal 〈a〉 slip. This result arises in spite of large strain development, fine lamellar layer thickness (4 μm), and close proximity of the interfaces to the grain boundaries.
Section snippets
Materials and experiments
This section describes the starting microstructure of Zr and Nb, the ARB fabrication process and measured texture evolution as a function of rolling passes in the Zr/Nb layered composites to a final mean layer thickness of ∼4 μm.
Single-crystal to polycrystal modeling
In the measurements, we have found no apparent Zr/Nb interface effects on texture evolution of the Zr and Nb phases. As another means of understanding the texture evolution observed here and the corresponding plastic deformation mechanisms, texture has been simulated in each phase individually to test how this independent prediction compares with the measurements.
To model texture evolution in each phase, we use the visco-plastic self-consistent scheme (VPSC) for the polycrystal model. This
Results of rolling texture simulations
Using the above model, we carried out simulations of rolling to high strains sufficient to achieve texture saturation for each phase. Since our measurements indicate that the interface did not affect bulk texture evolution, we consider separate homogeneous effective media i.e. one for Zr and another for Nb. Fig. 6 shows the predicted deformation textures for both phases. It appears that the model captures the main texture features consistent with experimental observation (see Fig. 3).
For a
Discussion
In this work, we explored texture developments in an hcp Zr/bcc Nb composite during ARB processing. The finest composite we made had 4 μm Zr and Nb layers and microstructural characterization found them to be polycrystalline in-plane and through-thickness. In these conditions, we successfully predict texture evolution and deformation modes in both phases using VPSC. Much recent experimental work suggests that even finer composites may exhibit unusual microstructural evolution and properties. For
Conclusions
In this work, we demonstrated that it is possible to fabricate a Zr/Nb composite via the accumulative roll bonding technique. With large strains (∼4), the individual layer thickness was refined to approximately 4 μm. We show that the deformation textures in Zr and Nb are consistent with those typically found when Zr and Nb are rolled alone. The measurements show the characteristic spread of the basal pole along the transverse direction and its concentration 30–50° from the normal direction in
Acknowledgments
MK and MA were supported by the University of New Hampshire faculty startup funds. IJB was supported by a Los Alamos National Laboratory LDRD program 20140348ER. TMP and NAM wish to acknowledge support by the UC Lab Fees Research Program # UCD-12-0045.15. TN was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program.
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