22 research outputs found
Effect of initial texture on deformation-induced grain growth in pulsed electrodeposited microcrystalline copper
No full textOwing to the presence of large fraction of grain boundaries, deformation-induced grain growth is commonly observed in fine-grained electrodeposited metals. Here we demonstrate that microcrystalline copper (d similar to 1-10 mu m) with different textures produced by electrodeposition exhibit significant deformation-induced grain growth in tension by coalescence along with twin boundary migration and detwinning. Oriented growth with the formation of a cube texture was noted in the deformed samples. There was an increased fraction of twin boundaries with large angular deviation from Brandon's criterion during deformation
Pulsed electrodeposition and hardness of microstructurally graded iron
No full textFunctionally graded materials (FGM) are designed to acquire a desired spectrum of properties within a material with a gradual variation in either microstructure, composition or porosity across the thickness. Microstructurally graded iron with grain sizes from nanocrystalline/sub-microcrystalline to microcrystalline range can facilitate novel applications which demand an optimum combination of strength and toughness with good corrosion resistance. The present study was aimed at providing methods to synthesize and engineer microstructurally graded iron with ultra-fine and coarse grained structure using pulsed electrodeposition. The effect of various parameters like CaCl2 and saccharin contents in the chloride electrolytic bath, pH of the bath and applied current density on developing the microstructurally graded iron deposits was studied. The CaCl2 concentration in the bath has a profound effect on development of microstructure and crystallographic texture. With increasing current density, pH of the bath and saccharin content, there was significant refinement of the grain structure. The microstructurally graded iron deposit produced under optimized electrodeposition conditions exhibited a gradual variation in hardness ranging from similar to 3 GPa to 10 GPa
Deformation-induced thermally activated grain growth in nanocrystalline nickel
No full textGrain growth during indentation at low temperatures has been taken to imply that grain growth is largely stress induced and athermal in nanometals. Indentation experiments on electrodeposited nano-Ni indicate clearly that the load required for grain growth decreases with an increase in temperature, suggesting strongly that concurrent grain growth is thermally activated. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Microstructure, texture and tensile behavior of pulsed electrodeposited Ni-Al composites produced using organic additive-free sulfamate bath loaded with Al nanoparticles
No full textNickel and Ni-Al composites with varying Al nanoparticle content were produced by pulsed electrodeposition using an organic additive-free sulfamate bath. The as-deposited Ni-Al composites exhibited increased compressive residual stresses with increasing Al nanoparticles. Detailed microstructural investigation revealed that Al codeposition led to significant refinement of the columnar morphology and grain structure of Ni matrix along with a texture change from strong to fiber texture parallel to the growth direction. The as-deposited Ni with bimodal grain structure displayed good tensile strength and ductility. The microhardness and tensile strength of the Ni deposits were increased initially with Al content and then decreased at higher particle content. Particle induced grain refinement and texture changes could be related to change in the growth of upcoming Ni nuclei on to the surface of well-dispersed Al nanoparticles. Significant grain refinement and lesser internal stresses along with good particle distribution and relatively harder fiber texture at lower particles concentration collectively contributed to the observed higher strengths in Ni-Al composites as compared to Ni deposits. There was substantial change in fracture morphology from spherical dimples to flat regions with increasing Al content due to agglomeration of nanoparticles at higher particle loading
Microstructure and mechanical behavior of annealed MP35N alloy wire
No full textIn a previous paper, the microstructure, monotonic, and cyclic response of as-drawn similar to 100 pm diameter MP35N low-Ti alloy wire were presented and discussed. In this sequel paper, the effects of annealing the same cold-drawn wire on microstructure and mechanical properties are examined. Specifically, segments of the wire were annealed for 1 h at 973 K, 1023 K, 1073 K, 1123 K and 1173 K in a vacuum furnace. The resulting microstructure was characterized by SEM, EBSD and TEM and compared to the asdrawn microstructure. In-situ heating in the TEM of MP35N ribbon in a similarly cold worked condition enabled corroboration of microstructure evolution during annealing. Annealed wires were tested monotonically and cyclically in uniaxial tension at room temperature, the latter using a stress ratio (R) of 0.3. In addition, the annealed wires were tested cyclically at R= -1 using the rotating beam bending fatigue test. Post-deformation structures and fracture surfaces were characterized using TEM and SEM respectively. Annealing the cold drawn wire results in recrystallization and grain growth; the extent is dependent on the annealing temperature. Deformation twin boundaries in the as-drawn structure illustrate faceted bulging and eventually complete elimination, the microstructure evolving into fine equiaxed grains containing coarser annealing twins with no significant change in texture. Yield strength decreases rapidly with recrystallization to almost half the value of the as-drawn condition, but is accompanied by an increase in modulus (by similar to 25%) and tensile elongation reaching 30%. Cyclic response by the way of S-N curves is not enhanced by annealing on an absolute stress scale (due to the loss in yield strength) although the annealed wires are cyclically superior when the stress data are normalized by yield stress. (C) 2015 Elsevier B.V. All rights reserved
Grain-size distribution effects in plastic flow and failure
No full textThere has been considerable success over the past five decades in developing a phenomenological and micromechanism-based understanding of plastic flow, creep and superplasticity. Although it is widely known that grain sizes have a distribution in polycrystals and nanocrystals, this factor is usually not included in most analysis of deformation and failure. Experimental observations relating to the influence of grain size distributions are discussed briefly, and an analysis is developed to consider the influence of this factor on the transition from grain boundary strengthening to grain boundary weakening in nanocrystalline materials. The transition from grain boundary strengthening to weakening becomes broader with an increase in the standard deviation of the grain size distribution. It is demonstrated that the observed standard deviations for grain size distributions and nominal errors in grain size measurements can lead to substantially different experimental observations under nominally identical conditions
Microstructural evolution and mechanical characteristics in nanocrystalline nickel with a bimodal grain-size distribution
No full textAn attractive microstructural possibility for enhancing the ductility of high-strength nanocrystals is to develop a bimodal grain-size distribution, in which the fine grains provide strength, and the coarser grains enable strain hardening. Annealing of nanocrystalline Ni over a range of temperatures and times led to microstructures with varying volume fractions of coarse grains and a change in texture. Tensile tests revealed a drastic reduction in ductility with increasing volume fraction of coarse grains. The reduction in ductility may be related to the segregation of sulphur to grain boundaries
On the exothermic peak during annealing of electrodeposited nanocrystalline nickel
No full textNanocrystalline metals frequently exhibit poor thermal stability, and the exothermic peak in differential scanning calorimetry is usually attributed to grain growth. We show from experiments on electrodeposited nano-Ni with varying levels of S, and tests with microcrystalline Ni and S powders, that the exothermic peak is associated with the formation of a nickel sulfide phase and concurrent grain growth. Analysis suggests that segregation plays a more important role in limiting grain growth than second-phase particles in nano-Ni. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Microstructural stability and superplasticity in an electrodeposited nanocrystalline Ni-P alloy
No full textStabilization of nanocrystalline grain sizes by second phase particles can facilitate superplasticity at high strain rates and/or low temperatures. A metastable single phase nano-Ni-P alloy prepared by electrodeposition, with a grain size of similar to 6 nm, transforms to a nanoduplex structure at T> 673 K, with similar to 4 vol.% Ni3P particles at triple junctions and within Ni grains. The nanoduplex microstructure is reasonably stable up to 777 K, and the growth of Ni grains occurs in a coupled manner with the growth of Ni3P particles such that the ratio of the two mean sizes (Z) is essentially constant. High temperature tests for a grain size of 290 nm reveal superplastic behavior with an optimum elongation to failure of 810% at a strain rate of 7 x 10(-4) s(-1) and a relatively low temperature of 777 K. Superplastic deformation enhances both grain growth and the ratio Z, implying that grain boundary sliding (GBS) significantly influences the microstructural dynamics. Analysis of the deformation processes suggests that superplasticity is associated with GBS controlled by the overcoming of intragranular particles by dislocations, so that deformation is independent of the grain size. The nano-Ni-P alloy exhibits lower ductility than nano-Ni due to concurrent cavitation caused by higher stresses. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Superplasticity in electrodeposited nanocrystalline nickel
No full textElectrodeposited nanocrystalline Ni films were processed with different levels of S, to evaluate the role of S on superplasticity. All the materials exhibited high strain rate superplasticity at a relatively low temperature of 777 K. Microstructural characterization revealed that the S was converted to a Ni3S2 phase which melts at 908 K; no S could be detected at grain boundaries. There was no consistent variation in ductility with S content. Superplasticity was associated with a strain rate sensitivity of similar to 0.8 and an inverse grain size exponent of similar to 1 both of which are unusual observations in superplastic flow of metals. Based on the detailed experiments and analysis, it is concluded that superplasticity in nano-Ni is related to an interface controlled diffusion creep process, and it is not related to the presence of S at grain boundaries or a liquid phase at grain boundaries. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
