Papers by Keyword: Nano Grain

Abstract: Transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1mm diameter as-drawn Ni-Ti wires subjected to a non-conventional heat treatment by controlled electric pulse current. This method enables a finer control of the recovery and recrystallisation processes taking place during the heat treatment and accordingly a better control on the final microstructure. The best functional properties were obtained for heat-treated Ni-Ti wires having a nanograined microstructure (20-50 nm) partially recovered through polygonization and partially recrystallized. Such microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer time and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 100 nm. The density of dislocation defects increased significantly with increasing grain size of the microstructure. The activity of three <100>/{011} slip systems was identified in the largest grains of 500-1200 nm. An additional mode of plastic deformation, {114} compound austenite twinning, was observed in the largest grains of fully recrystallized microstructures. It is proposed that dislocation slip (and possibly deformation twinning) occurring in superelastic cycling is coupled with the stress-induced martensitic transformation.

Abstract: Fabrication of advanced electronic components requires high-quality powders. In this work, nano-powders of Li or Na niobates are synthesized from (Li or Na)-Nb ethoxide by a sol-crystal method. A single crystal of (Li or Na)-Nb ethoxide is decomposed to an amorphous matrix below 473 K. Next, small crystals are grown by heating at the appropriate temperature for each specimen. The sol-crystal method provides homogeneous quality and fine grains by heating at lower temperature. Structural analysis of the powders is performed by a transmission electron microscope (TEM) and X-ray diffraction. As a result, LiNbO3 turns to dense-powders, but NaNbO3 forms nano-porous powders. In order to understand this difference, we try to observe in-situ the crystallization and grain growth processes by high-temperature TEM. We successfully observe in-situ this processing and discuss the structural change and formation mechanism of LiNbO3, comparing these features with those of NaNbO3.

Abstract: Recently, the necessity to grade grain size to ultrafine and nano scale for understanding the mechanical behavior of these materials has been recognized. However, the nature of such classification has remained unclear. As an example, ultrafine (100 nm -1 μm) and nano (<100 nm) grained FCC metals, compared to their coarse grained counterparts, exhibit a grain size strengthening that may deviate from the Hall-Petch relationship. To explain the mechanism of such deviation, previous dislocation theories seem insufficient. To solve this problem, a critical grain size criterion governing the shift of deformation mechanism is proposed in this work. This model employs an energetic approach; it relates the grain boundary energy density to certain critical energy values; and it permits, for the first time, a quantitative grading of grain sizes. Predictions based on this model were evaluated. The prediction on copper polycrystals of various grain sizes showed a very good agreement with experimental results. It is thus wished that the grain size theory on plastic deformation mechanism could be unified with the dislocation theory. In this study, such unification is attempted by using a parameter defined as the defect energy density. The possibility of such generalization is further reasoned upon the fact that the defect energy approach should be a unique but common form applicable for both dislocations and grain boundaries.

Abstract: Bulk dense nanocrystalline BaTiO3 ceramics ranging from 15 nm to 100 nm have been successfully prepared by the spark plasma sintering method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometre scale for BaTiO3 ceramics. Scanning nonlinear dielectric microscopy measurements revealed temperature-dependent variations in contrast, which were attributed to domain rearrangements in BaTiO3 ceramics below 100 nm. Furthermore the piezoresponse hysteresis loops showed that the nanocrystalline BaTiO3 ceramics were switchable and ferroelectricity was retained at the high temperature of 290 oC, demonstrating the existence of nano ferroelectric domains and the ferroelectric phase still retained above Curie temperature, which confirmed the diffused phase transition character in nanograin BaTiO3 ceramics. The dielectric data also show a ferroelectric to paraelectric phase transition in nanograin BaTiO3 ceramics.

Abstract: The present work is devoted to the investigation of the influence of the grain size on the main mechanical characteristics of nanopolycrystals of different metals. The Hall-Petch parameter behaviour for Al, Cu, Ni, Ti and Fe was examined in the wide grain size interval. The stages of plastic deformation and the parameters of work hardening for nanocrystalline copper were analysed in detail. The deformation mechanisms and critical grain sizes accounting for the transition from the dislocation slip to the grain boundary sliding were described.

Abstract: Some nano grained Al materials were produced by mechanical milling/alloying followed by vacuum hot pressing: nano grained pure Al, Al-1.5Mg and Al-0.7Mg-1.0Cu alloys in wt%. The nano bulk materials had average grain sizes of 90-150 nm and ball-on-disk wear equipment was used to investigate the effects of grain size comparing to coarse grained pure Al and T6-treated Al 6061 alloy. In comparison of coarse and nano grained pure Al materials, nano grained specimens showed much higher wear resistance and size of wear debris was very fine as much as 100 nm at applied load of 100g. Wear in nano grained materials proceeded by micro fracturing mechanism like abrasion in low applied and sliding velocity and the mechanism resulted in high wear resistance. Nano grained Al-1.5Mg and Al-0.7Mg-1.0Cu alloys showed much superior wear characteristics due to nano grains and high hardness. As a result, size of grains was a predominant factor for high resistance at low applied load and/or sliding velocity, while hardness at higher applied load or velocity.

Abstract: The static recrystallization (SRX) behavior of nano grained (NGed) Cu-30mass%Zn alloy processed by Multi-directional forging (MDF) was investigated. The NGed Cu-Zn alloy showed characteristic annealing behavior. The SRX nucleation and its grain growth occurred more readily in the samples deformed to higher cumulative strain at lower temperature. The frequency of new grain formation in the samples MDFed at 77 K was much higher than that at 300 K. The new grains were composed of fine annealing twins with thickness from 10 to 200 nm. The average grain size fully recrystallized was less than 300 nm.

Abstract: Nacre is a natural composite material making up the inner structure of mollusk shells. It has been of great interest in materials research due to its mechanical properties far exceeding that of its individual components: well ordered plates of aragonite (a CaCO3 polymorph) within an organic polymer matrix. Generally the aragonite plates had been treated as single crystals and mechanical behavior explained as the result of micro-scale mechanisms between plates and matrix. However, recent work has shown that the plates themselves are made up of smaller nano-scale structures, which are also thought to contribute to the bulk properties. In this work, transmission electron microscopy (TEM) was used to observe the nano-scale structure of nacre from abalone. “Nanograins” of aragonite surrounded by organic material was observed, showing composite structure within aragonite plates.

Abstract: On the basis of the Landau-Ginzburg models established by Falk and other authors, a new model for martensitic transformation in nanograined powder is developed, in which the influence of the interface on martensitic transformation in nanograined powder is considered. The interface strain coefficient K, the factor describing the difference of the strain between the interface and the core of a nano-grain, was suggested in this paper. The equation in the present model was solved numerically, and the size effect was observed. The calculation results show that the critical size increases with the drop in temperature, which means that the lower temperature will promote martensitic transformation in nanograined materials.

Abstract: Vjg"rtgrctcvkqp"cpf"ejctcevgtkucvkqp"qh"YQ50315J4Q"rjcug."yjkej"ku"cp"kpvgtogfkcvg"vq" jgzciqpcn"vwpiuvgp"qzkfg"ycu"rgthqtogf0"Vwpiuvgp"qzkfg"fkj{ftcvg"ycu"rtgekrkvcvgf"htqo"cekfkhkgf" uqfkwo" vwpiuvcvg" uqnwvkqp0" Vyq" cpf" vjtgg" ycujkpi" uvgru" jcxg" dggp" crrnkgf" vq" vjg" cu/qdvckpgf" coqtrjqwu" ign0" YQ50315J4Q" pcpqitckpu" jcxg" dggp" qdvckpgf" chvgt" vjg" j{ftqvjgtocn" rtqeguu" rgthqtogf"cv"362̇E."hqt"46"jqwtu"kp"cp"cekf"fkiguvkqp"dqod0"C"urkp"eqcvgt"ycu"wugf"hqt"vjkp"hkno" rtgrctcvkqp0" Uvtwevwtcn." oqtrjqnqikecn" cpf" qrvkecn" ejctcevgtkucvkqp" cu" ygnn" cu" gngevtqejgokecn" rgthqtocpeg" qh" vjkp" hknou" ygtg" fgvgtokpgf" d{" uecppkpi" gngevtqp" oketqueqr{." cvqoke" hqteg" oketqueqr{." Tcocp" urgevtqueqr{" cpf" e{enke" xqnvcoogvt{0" Vjg" kqp" kpugtvkqp" rtqrgtvkgu" qh" YQ50315J4Q" hknou" ujqy" uvtqpi" eqttgncvkqp" vq" vjg" enqug" rcemgf" uvtwevwtg" cpf" vq" vjg" rtgugpeg" qh" uvtwevwtcn"ycvgt0"