Mechanical behaviour and microstructural evolution of constrained groove pressed nickel sheets
Introduction
Emphasis on processing of ultra-fine grains (UFG) and nanostructures in bulk materials by severe plastic deformation (SPD) techniques have created greater interests among researchers in the last decade due to the superior mechanical and physical properties exhibited by fine grained materials (Azushima et al., 2008). SPD techniques involve intense plastic straining of materials without concomitant changes in cross-sectional dimensions resulting in fine grained structures and enhanced mechanical properties. Among the SPD methods, equal channel angular pressing (ECAP) proposed by Segal (1995) have attracted greater attention and the potentiality of ECAP technique to process fine grained materials is reviewed in detail by Valiev et al. (2000). Of late, ECAP technique have also been found to be successful in processing commercial aluminium alloys like 1100, 2024, 3004, 5083, 6061 and 7075 (Horita et al., 2001) and Al–Mn alloy (Luis Pérez et al., 2003). Apart from ECAP, high pressure torsion (HPT) technique capable of processing fine grained disc samples (Valiev et al., 1991), multi-axial forging (MAF) employed for processing bulk sub-microcrystalline titanium alloys (Zherebtsov et al., 2004) and iron based alloys (Han and Xu, 2008) have also drawn greater interest among researchers. However these processes (ECAP, HPT, MAF) are mainly intended for processing materials in the form of square/cylindrical billets or discs. Subsequently the possibility of processing sheet materials by ECAP technique explored by Segal (2008) have been successfully employed for processing fine grained aluminium alloy sheets (Lapovok et al., 2008). Meanwhile various SPD techniques exclusively meant for processing ultra-fine grained sheet materials have been put forth. Accumulative roll bonding (ARB) (Saito et al., 1998), constrained groove pressing (CGP) (Shin et al., 2002), repetitive corrugation and straightening (RCS) (Huang et al., 2004) are some of the SPD techniques proposed for processing bulk materials in sheet forms. However, the tendency of likely degradation of ARB processed sheets due to imperfect bonding (Shirdel et al., 2010) and relatively weak interface layers of roll-bonded sheets (Kazeminezhad and Hosseini, 2010) along with pre-requisite stringent surface preparation (Saito et al., 1998) makes the process less attractive for processing fine-grained sheet materials. Whereas in RCS process, inhomogeneous strain imparted by elongation of sheets and continuously changing dimension limits the efficiency of the process (Hosseini et al., 2009). The absence of aforesaid problems in the CGP process proposed by Shin et al. (2002) makes the process more attractive and feasible method for processing ultrafine grained sheet materials.
The principle of constrained groove pressing is subjecting sheet material to repetitive shear deformation under plane strain deformation condition by alternately pressing the sheets between asymmetrically grooved dies and flat dies (Shin et al., 2002). The asymmetric grooved die (Fig. 1) consists of equally spaced groove and a groove angle (θ) of 45° is maintained to obtain maximum uniform shear strain during CGP processing. The width and depth of the grooves are maintained similar to the thickness of sheet (Fig. 1) and pressing is carried out such that the gap between the upper die and lower die is equal to the sample thickness. During pressing, the inclined region of the sample parallel to X–Y plane is subjected to pure shear deformation under plane strain deformation conditions (Zrnik et al., 2009). The theoretical strain imparted during CGP processing is computed using analytical equations (Shirdel et al., 2010) and the schematic of stage-wise strain evolution at various zones during different stages of CGP processing illustrated in Fig. 2 clearly indicates that during every stage of pressing an effective plastic strain of 0.58 is imparted in the shear regions of sheets, whereas the flat regions are left undeformed. Plain strain deformation condition is ensured by constraining both end of sheet and the detailed processing procedures are explained elsewhere (Shin et al., 2002). An effective plastic strain of 1.16 is imparted throughout the sheet after one pass.
In the present investigation, the applicability of constrained groove pressing technique to process pure nickel sheets is studied. Neishi et al. (2002) have concluded that the homogeneity of microstructural evolution during severe plastic deformation depends on the stacking fault energy of the processed material and pure nickel with moderate stacking fault energy can be considered as an ideal model material for severe plastic deformation processing to obtain homogenous microstructure at relatively lower strains; hence pure nickel sheet is chosen to understand the deformation characteristics during CGP. Total effective plastic strain of 3.48 is imparted through three passes of CGP and severely deformed nickel sheets are characterized by tensile tests, hardness tests and X-ray diffraction. The influence of CGP processing on the structural evolution and mechanical properties is examined and correlated.
Section snippets
Experimental materials and methods
Vacuum annealed 5 mm thick pure nickel sheets of dimensions 130 mm × 50 mm are severe plastically deformed by constrained groove pressing at room temperature. The initial microstructure of vacuum annealed nickel sheets consists of homogenous equiaxed grains (Fig. 3) with definite grain boundaries and the average grain size measured by line intercept method is found to be 34 μm. The constrained groove pressing experiments are carried out in 250 T oil hydraulic press and thin layer of Molykote D-321R is
Analysis of X-ray diffraction profile
X-ray diffraction peak profile analysis is considered as a complimentary tool to transmission electron microscopy (TEM) for describing the microstructure of polycrystalline materials. Even though TEM can provide a direct image of the grain sizes and distribution, the grain overlapping may lead to uncertainty (Zhang et al., 2003). Also the reliability depends on whether the thin TEM foil represents the microstructure of the entire sample. In contrast, methods based on XRD peak profile analysis
Results and discussion
A total effective plastic strain of 3.48 is imparted successfully in pure nickel sheets at room temperature by CGP technique through three passes. The surface of the CGP processed sheets revealed impressions (marks) which are imparted by grooved die profiles onto sheet surfaces during pressing. The photograph of the CGP processed sheet after first pass, illustrated in Fig. 4 clearly depicts the impressions (marks) formed during CGP processing. Further processing of nickel sheets beyond third
Conclusions
The applicability of CGP technique for severe plastically deforming nickel sheets is investigated in the present study. Successfully processed nickel sheets by CGP up to three passes are characterized for mechanical and microstructural properties. The findings can be summarized as follows:
- 1.
Significant increase in yield strength and tensile strength is noticed after first pass, whereas subsequent increase in strength is marginal. Marginal drop in strengths observed after third pass is attributed
Acknowledgements
The authors thank Dr. G. Malakondaiah, Director, Defence Metallurgical Research Laboratory (DMRL) for permitting us to publish this work. The authors also acknowledge the assistance rendered by Mechanical Behaviour Group (MBG) for tensile testing of specimens and Surface and Failure Analysis Group (SFAG) for X-ray diffraction studies and metallography. The authors acknowledge the funding provided by Defence Research and Development Organization (DRDO).
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