Experimental and Crystal Plasticity Simulation Study on the Deformation Behavior of Liquid Phase Sintered Tungsten Heavy Alloys
Creators: name: Kumar, Mirtunjay
Publisher: name: Zenodo
Year: 2022
Subjects: Tungsten Heavy Alloy, Deformation Behavior, VPSC, EBSD, DAMASK, CPFFT

description: Tungsten heavy alloys (WHA) are typical two-phase composites that include a nearspherical b.c.c. structured tungsten phase dispersed in a ductile f.c.c. Ni-Fe-W solidsolution matrix phase. Owing to the outstanding combination of the properties such as high strength, good wear and corrosion resistance, WHA are widely used in defence and civil industries. Conventionally, WHA are usually fabricated through liquid-phase sintering within the temperature range of 1450-1550 degree Celcius, and the obtained tungsten grain size is usually about 40-50 μm. The present thesis aims to decipher the governing deformation mechanisms in the ductile two-phase WHA to provide a way forward for improving the performance of WHA to fulfil the global demand for high-end defence related applications. In this regard, crystallographic texture has been used as a tool in the present dissertation to identify and estimate the contribution from the various deformation micro-mechanisms. Till date, the studies on WHA have focused primarily on alloy development by tailoring the tungsten content or alloying addition, with a few studies on mechanical properties. The systematic investigation of the deformation behaviour of two phase WHA is missing in the literature. In particular, the role of Ni-Fe-W solid-solution matrix on deformation behaviour has only been analyzed to a very limited extent in previous studies. Tungsten is isotropic in the elastic regime, but there is high degree of anisotropy in the plastic properties of tungsten in the quasistatic and dynamic strain rate regime with a strong dependence on crystallographic texture. In the current work, the evolution of texture during plastic deformation is systematically analysed by performing both postmortem and in situ studies using neutron diffraction, x-ray diffraction, and electron back scatter diffraction (EBSD). Furthermore, texture simulations have been conducted to supplement the experimental microstructure and texture measurements using mean field and full field crystal plasticity frameworks. The Eshelby inclusion based viscoplastic self-consistent simulations (VPSC) was employed for mean field simulations while the fast Fourier transform (FFT) based DAMASK (Düsseldorf Advanced Material Simulation Kit) was used for carrying out full field simulations. These simulations also provided information about key parameters such as slip activity, an average number of slip systems, stress and strain partitioning between the two phases, micro-mechanical Taylor factor and equivalent Von Mises stress and strain maps which shed light on the operative mechanisms of deformation of the WHA. In the first part of the thesis, deformation behaviour was analysed using axisymmetric compression tests using five model WHA with different tungsten content ranging from 80 to 98% by weight. Compression tests to a strain of ∼0.69 showed a slight increase in yield strength with increase in tungsten content and similar strain hardening behaviour. The flow behaviour of the WHA during compression was analysed based on experimental evidence of shear band formation and failure of W-W interface. A double fiber texture &lt;111&gt; and &lt;100&gt; parallel to compression direction was observed in the tungsten phase whereas in f.c.c. matrix phase &lt;110&gt; fiber evolved. The effect of relative content of matrix was manifested in terms of increase in relative proportion of &lt;100&gt; fiber over the &lt;111&gt; fiber. Texture simulation using mean field VPSC suggested that lattice corotation scheme considering the role of neighbouring orientations needs to be invoked to correctly replicate the experimental textures. The physical interpretation of the role of neighbours in increasing the &lt;100&gt; fiber is provided in terms of increasing contiguity of tungsten phase with increase in tungsten content, which in turn causes higher fluctuation in lattice rotation path. Microstructural observation from EBSD indicated an increase in intragranular misorientation in the tungsten phase indicating stress and strain partitioning in the tungsten phase with increase in volume fraction. Strengthening of the matrix phase due to solid solution strengthening by tungsten followed with high strain hardening enables similar hardness of the matrix and tungsten phase that facilitates co-rotation leading to unique texture evolution in the tungsten and matrix phase. In the second part of the work, deformation behaviour of WHA was analysed by changing the strain path from axisymmetric uniaxial compression to plane strain compression. A model WHA with composition 90W-10M (M-Matrix, Ni-Fe-W solid solution) was unidirectionally rolled to 90% rolling reduction corresponding to a true strain (ε) of -2.3 at room temperature. Samples were retained after 30%,50%,75% and 90% rolling reduction to perform comprehensive microstructural and textural characterisation. Neutron diffraction was used to measure the texture of both tungsten and matrix phase, while microstructural characterisation was performed using EBSD. At low to intermediate stages of deformation ( 30%-50% reduction), a strong α- fiber (&lt;110&gt;∥RD) was observed in the tungsten phase, while a Copper type texture comprising of Copper ({11-2} &lt;111&gt;), Brass ({1-10}&lt;112&gt;) and S({12-3}&lt;634&gt;) components were observed in the f.c.c. matrix phase. With the increase in rolling reduction beyond 75%, the evolution of γ-fiber and rotated cube was observed in the b.c.c. tungsten phase, while a systematic decrease in the intensity of Copper component and the simultaneous increase in the intensity of brass component was observed in the f.c.c. matrix phase. The microstructural analysis illustrated the presence of strong orientation gradients (heterogeneous slip) at intermediate rolling reductions and widespread occurrence of micro and macro shear bands at higher rolling reductions. Local texture analysis inside the shear bands highlighted the presence of texture components such as Goss and Brass along with significant dislocation activity manifested in terms of higher intragranular misorientation in both the phases. The texture evolution at different rolling reductions was analysed using both mean field (VPSC) and full field FFT based model (DAMASK), which in turn enabled the determination of stress and strain partitioning as well as the local plastic work in the two phases. Based on the insights from the first two chapters where a substantial role of the matrix was observed on the deformation response of WHA, the subsequent chapters of the present thesis were focused on the deformation behaviour of relatively lesser-known f.c.c. matrix which is a concentrated ternary solid solution with a nominal composition of Ni-24W-22Fe solid solution. Like the previous chapters, the deformation behaviour was analysed along axisymmetric and plane strain compression strain paths. Axisymmetric deformation behaviour captured via real-time in situ EBSD studies provided crucial information about the role of neighbouring orientations on the deformation response of similarly oriented grains. It was observed that based on the characteristics of neighbours, the local velocity gradient of the individual grains deviated strongly from the externally imposed global velocity gradient, which in turn caused large scatter in lattice rotation path and the final stable end position in the orientation space. The experimental observation of the neighbour's role on the choice of slip systems and the resulting lattice rotation path was elegantly captured using full field model by using the real EBSD microstructure of the undeformed sample as input. Based on DAMASK simulations, the regimes in orientation space that are most susceptible to the effect of neighbours were determined. In the last part of the thesis, deformation behaviour of the matrix was studied in the plane strain compression mode. A thorough characterisation of microstructure and texture was performed by retaining samples after 30%,50%,75% and 90% rolling reduction. It was observed that texture evolution in the matrix is quite rapid with a fully developed Copper type texture after 30%-50% rolling reduction. Subsequently, a texture transition from Copper type to Brass type was observed at higher rolling reductions (75%). EBSD analysis illustrated the occurrence of micro shear bands as well as macroscopic shear bands cutting across several grains through-thickness at higher rolling reductions. Local texture analysis was performed by partitioning the orientations present within the shear bands, which in turn highlighted the role played by shear bands in causing the texture transition from Copper type to Brass type. Full field simulations were also performed to determine the distribution of shears on the individual slip systems to identify the possible mechanisms such as slip localisation to determine the susceptibility of shear band formation in different orientations. Thus, a thorough understanding of deformation behaviour of WHA with particular emphasis on that of Ni-Fe-W solid solution matrix using mechanical testing, processing along with comprehensive microstructural and textural characterisation coupled with a full field and mean field crystal plasticity simulations has been developed. The critical role of slip localisation in the formation of shear bands and micro shear bands in the composite and matrix alloy has been established that is expected to shed light on strain localisation and failure in ductile two-phase alloys.