Dilawar Ali | Government College University, Lahore (original) (raw)
Papers by Dilawar Ali
Journal of Materials Science, 2011
Tensile tests were performed on high-purity W and Mo polycrystals at room temperature for a range... more Tensile tests were performed on high-purity W and Mo polycrystals at room temperature for a range of axial strain-rates 2.1 × 10−4–2.1 × 10−2 s−1. The critical resolved shear stress (CRSS) data was analyzed by using the analytical formulation for the strain-rate dependence of the CRSS derived in the kink-pair nucleation (KPN) model of flow stress in crystals with high intrinsic lattice friction. On evaluation of various microscopic slip-parameters of the model, the active slip-system in both W and Mo polycrystals was identified as {110}〈111〉. This is in good agreement with that deduced from the published data on the temperature dependence of the CRSS of these crystals as well as from the observed slip-lines on the deformed crystals reported in the literature. Moreover, the available data on the temperature dependence of the CRSS of Mo, Nb, Fe, V, and K crystals were also analyzed within the framework of the KPN model of flow stress. Peierls mechanism was found to be responsible for the CRSS of these metals; the active slip-systems in refractory metals Mo, Nb, Fe, and V were {110}〈111〉 and {211}〈111〉 whereas that in alkali metal K was {321}〈111〉.
Journal of Materials Engineering and Performance, 2011
Strips of 99.95 at.% Mo polycrystals annealed at 700 °C as well as the ones annealed and then age... more Strips of 99.95 at.% Mo polycrystals annealed at 700 °C as well as the ones annealed and then aged for 6 months at room temperature were deformed in tension at various strain-rates in the range 2.1 × 10−4 to 4.2 × 10−3 s−1 till fracture. It is found that natural aging of the annealed specimens for 6 months leads to 20-30% reduction in the yield stress (YS), 18-22% reduction in the ultimate tensile strength (UTS), and 72-76% reduction in the ductility, i.e. the tensile strain εmax corresponding to UTS, depending on the value of $ \dot{\upvarepsilon } $ in the tensile strain-rate range referred to. Data analysis in terms of the kink-pair nucleation model of flow stress shows that the reduction in YS of the aged Mo specimens is a consequence of lowering of the Peierls energy per interatomic spacing along the length of screw-dislocation segments trapped in the Peierls valleys on the migration of point defects to the dislocation cores during the course of natural aging. The reduction in UTS and εmax is attributed to the variation in the relative contribution of the processes of dislocation multiplication and annihilation together with the reduction in the Peierls stress of the metal.
Journal of Materials Science, 2010
The temperature dependence of the critical resolved shear stress (CRSS), τ, of ultra-pure tantalu... more The temperature dependence of the critical resolved shear stress (CRSS), τ, of ultra-pure tantalum single crystals (RRR ≥ 14000) observed below 250 K for a range of shear-strain rates $ \dot{\gamma } = 2\times 10^{ - 5} - 6\times 10^{ - 3} \,{\text{s}}^{ - 1} $ was analyzed within the framework of a kink-pair nucleation model of flow stress. The CRSS/strain-rate data follow the model formulation $ \tau^{ 1/ 2} = C + D\,{ \ln }\dot{\gamma } $ , where C and D are positive constants, for each deformation temperature T in the range 78–250 K. Evaluation of the various slip-parameters of flow stress points to (211) $ [\bar{1}11] $ slip system responsible for the yielding of ultra-pure tantalum single crystals in the so-called stress/temperature regime III (T < 250 K). The value of the pre-exponential factor $ \dot{\gamma }_{\text{o}} $ in the Arrhenius-type equation for the shear-strain rate $ \dot{\gamma } $ is found to be of the order of 105 s−1, which is substantially lower than that $ \left( {\dot{\gamma }_{\text{o}} \sim 10^{7} \,{\text{s}}^{ - 1} } \right) $ determined in the stress/temperature regime II (250–400 K) and contradicts the assumption invariably made in most of the theoretical models of flow stress that $ \dot{\gamma }_{\text{o}} $ is a constant over a wide temperature range.
Applied Surface Science, 2011
Journal of Physics D-applied Physics, 2009
An analytical expression for the strain-rate (\dot{\gamma}) dependence of the critical resolved s... more An analytical expression for the strain-rate (\dot{\gamma}) dependence of the critical resolved shear stress (CRSS) (τ) of crystals with high intrinsic lattice friction has been derived within the framework of a kink-pair nucleation model of flow stress. It predicts a linear relationship between τ1/2 and \ln\dot{\gamma} at a given deformation temperature, as embodied in \tau^{1/2}=C+D\ln\dot{\gamma} , where C and D are positive constants. Comparison of theory and experiment, using the wealth of data available in the literature, shows an excellent agreement between the predicted and the observed response of the CRSS to the strain rate. Evaluation of the low-temperature slip parameters of the model leads to identification of the slip system responsible for yielding of crystals.
Journal of Alloys and Compounds, 2010
Mechanically polished 4 N pure iron specimens were irradiated under a vacuum of 10 −3 Torr with Q... more Mechanically polished 4 N pure iron specimens were irradiated under a vacuum of 10 −3 Torr with Q-switched pulsed Nd:YAG laser for a number of laser shots ranging from 500 to 1500 with an increment of 250 shots. Surface morphology of laser irradiated specimens was examined by both optical and scanning electron microscopes. Heat affected area and its perimeter were found to increase with the increase in number of laser shots. Scanning electron micrographs revealed the formation of cracks, pits, and ripples as well as hydrodynamic and exfoliational sputtering of the material. Rose-like structure was developed on the target surface exposed to 500 laser shots due to the molten material movement caused by laser-induced plasma-recoil pressure. Substantial amorphization in the target occured on irradiation with 1000 laser shots. XRD study of the irradiated specimens revealed that crystallite size decreases while dislocation line density and microstrain increase on increasing the number of laser shots.
Journal of Materials Science, 2011
Tensile tests were performed on high-purity W and Mo polycrystals at room temperature for a range... more Tensile tests were performed on high-purity W and Mo polycrystals at room temperature for a range of axial strain-rates 2.1 × 10−4–2.1 × 10−2 s−1. The critical resolved shear stress (CRSS) data was analyzed by using the analytical formulation for the strain-rate dependence of the CRSS derived in the kink-pair nucleation (KPN) model of flow stress in crystals with high intrinsic lattice friction. On evaluation of various microscopic slip-parameters of the model, the active slip-system in both W and Mo polycrystals was identified as {110}〈111〉. This is in good agreement with that deduced from the published data on the temperature dependence of the CRSS of these crystals as well as from the observed slip-lines on the deformed crystals reported in the literature. Moreover, the available data on the temperature dependence of the CRSS of Mo, Nb, Fe, V, and K crystals were also analyzed within the framework of the KPN model of flow stress. Peierls mechanism was found to be responsible for the CRSS of these metals; the active slip-systems in refractory metals Mo, Nb, Fe, and V were {110}〈111〉 and {211}〈111〉 whereas that in alkali metal K was {321}〈111〉.
Journal of Materials Engineering and Performance, 2011
Strips of 99.95 at.% Mo polycrystals annealed at 700 °C as well as the ones annealed and then age... more Strips of 99.95 at.% Mo polycrystals annealed at 700 °C as well as the ones annealed and then aged for 6 months at room temperature were deformed in tension at various strain-rates in the range 2.1 × 10−4 to 4.2 × 10−3 s−1 till fracture. It is found that natural aging of the annealed specimens for 6 months leads to 20-30% reduction in the yield stress (YS), 18-22% reduction in the ultimate tensile strength (UTS), and 72-76% reduction in the ductility, i.e. the tensile strain εmax corresponding to UTS, depending on the value of $ \dot{\upvarepsilon } $ in the tensile strain-rate range referred to. Data analysis in terms of the kink-pair nucleation model of flow stress shows that the reduction in YS of the aged Mo specimens is a consequence of lowering of the Peierls energy per interatomic spacing along the length of screw-dislocation segments trapped in the Peierls valleys on the migration of point defects to the dislocation cores during the course of natural aging. The reduction in UTS and εmax is attributed to the variation in the relative contribution of the processes of dislocation multiplication and annihilation together with the reduction in the Peierls stress of the metal.
Journal of Materials Science, 2010
The temperature dependence of the critical resolved shear stress (CRSS), τ, of ultra-pure tantalu... more The temperature dependence of the critical resolved shear stress (CRSS), τ, of ultra-pure tantalum single crystals (RRR ≥ 14000) observed below 250 K for a range of shear-strain rates $ \dot{\gamma } = 2\times 10^{ - 5} - 6\times 10^{ - 3} \,{\text{s}}^{ - 1} $ was analyzed within the framework of a kink-pair nucleation model of flow stress. The CRSS/strain-rate data follow the model formulation $ \tau^{ 1/ 2} = C + D\,{ \ln }\dot{\gamma } $ , where C and D are positive constants, for each deformation temperature T in the range 78–250 K. Evaluation of the various slip-parameters of flow stress points to (211) $ [\bar{1}11] $ slip system responsible for the yielding of ultra-pure tantalum single crystals in the so-called stress/temperature regime III (T < 250 K). The value of the pre-exponential factor $ \dot{\gamma }_{\text{o}} $ in the Arrhenius-type equation for the shear-strain rate $ \dot{\gamma } $ is found to be of the order of 105 s−1, which is substantially lower than that $ \left( {\dot{\gamma }_{\text{o}} \sim 10^{7} \,{\text{s}}^{ - 1} } \right) $ determined in the stress/temperature regime II (250–400 K) and contradicts the assumption invariably made in most of the theoretical models of flow stress that $ \dot{\gamma }_{\text{o}} $ is a constant over a wide temperature range.
Applied Surface Science, 2011
Journal of Physics D-applied Physics, 2009
An analytical expression for the strain-rate (\dot{\gamma}) dependence of the critical resolved s... more An analytical expression for the strain-rate (\dot{\gamma}) dependence of the critical resolved shear stress (CRSS) (τ) of crystals with high intrinsic lattice friction has been derived within the framework of a kink-pair nucleation model of flow stress. It predicts a linear relationship between τ1/2 and \ln\dot{\gamma} at a given deformation temperature, as embodied in \tau^{1/2}=C+D\ln\dot{\gamma} , where C and D are positive constants. Comparison of theory and experiment, using the wealth of data available in the literature, shows an excellent agreement between the predicted and the observed response of the CRSS to the strain rate. Evaluation of the low-temperature slip parameters of the model leads to identification of the slip system responsible for yielding of crystals.
Journal of Alloys and Compounds, 2010
Mechanically polished 4 N pure iron specimens were irradiated under a vacuum of 10 −3 Torr with Q... more Mechanically polished 4 N pure iron specimens were irradiated under a vacuum of 10 −3 Torr with Q-switched pulsed Nd:YAG laser for a number of laser shots ranging from 500 to 1500 with an increment of 250 shots. Surface morphology of laser irradiated specimens was examined by both optical and scanning electron microscopes. Heat affected area and its perimeter were found to increase with the increase in number of laser shots. Scanning electron micrographs revealed the formation of cracks, pits, and ripples as well as hydrodynamic and exfoliational sputtering of the material. Rose-like structure was developed on the target surface exposed to 500 laser shots due to the molten material movement caused by laser-induced plasma-recoil pressure. Substantial amorphization in the target occured on irradiation with 1000 laser shots. XRD study of the irradiated specimens revealed that crystallite size decreases while dislocation line density and microstrain increase on increasing the number of laser shots.