Nanocrystalline Grain Size Research Articles

Effect of annealing treatment on the microstructure and mechanical properties of bulk nanostructured Ti/AZ61 composites prepared by hot pressing

Magnesium alloys, as the lightest structural alloys, provide the possibility of solving environmental problems by improving energy efficiency. Developing and maintaining nanocrystalline matrix is a huge challenge, and the key to achieving ultra-high strength. The nanocrystalline Ti/AZ61 composite prepared by hot pressing was annealed, and its microstructural evolution was characterized via SEM, XRD, and TEM, including the changes in magnesium matrix, Ti dispersions, and nano-scale precipitates. The density and mechanical properties of annealed composites were tested and analyzed. After annealing treatment at 623 K for 80 h, the average grain size of nanocrystalline Ti/AZ61 composites was 102 nm. 97% of Ti dispersions were submicron sized, and Ti3Al precipitates remained nanometer-sized. Ti dispersions and Ti3Al precipitates were uniformly distributed in the magnesium matrix. Compared with hot pressed composites, the crack that originated from the prior particle boundary (PPB) was restrained for annealed composites. The improved density and PPB bonding strength of annealed Ti/AZ61 composites had a positive impact on strength and plasticity. After annealing treatment for 40 h, the density increased from ∼2.041 g/cm3 to 2.066 g/cm3. The hardness, yield strength, and FFS were 146 HV, 493 MPa, and 5.6%, respectively, with corresponding increasing rates of 7%, 12%, and 65%, respectively.

Investigation on thickness size effect on ductility of magnetron sputtered Niobium coatings on SS316L substrate for forming of precoated metallic bipolar plates

Metallic bipolar plates (BPPs) are key components of the proton exchange membrane fuel cell (PEMFC). To lower the fabrication cost of metallic BPPs, precoated BPPs have attracted much attention due to the high efficiency of precoating-stamping process. However, precoatings on metallic substrate tend to crack during the forming process, leading to deterioration or even complete loss of corrosion resistance. Therefore, to avoid micro cracks of formed precoated BPPs, development of coatings with high ductility is necessary. In this study, Niobium coatings with different thicknesses on SS316L substrate are prepared with magnetron sputtering process, and uniaxial tensile tests are then conducted for the precoated specimens to evaluate their ductility. The microstructure and fracture behaviour of the Niobium precoatings are characterized by XRD, SEM, TEM, laser confocal microscope analysis. It is found that with the increase of coating thickness, the number of micro cracks at the same strain decreases significantly, and the strain for the first crack to appear also increases. Furthermore, a brittle-to-ductile transition of fracture mechanism is observed. The grain size of Niobium nanocrystalline coating increases with the thickness, which leads to the improvement of plasticity and failure strain. Therefore, the application of precoated metallic BPPs is further advanced.

Microstructure, mechanical properties and tribological behaviors of TiAlN-Ag composite coatings by pulsed magnetron sputtering method

TiAlN-Ag composite coating was prepared by pulsed magnetron sputtering method on cemented carbide surface. The effect of silver content on the microstructure, mechanical properties and tribological behaviors of TiAlN-Ag composite coatings was studied. Results show that doping an appropriate amount of Ag into TiAlN coating can significantly refine the grain size of TiN nanocrystalline, reduce the internal stress, and improve hardness, plasticity and toughness, as well as the coating/substrate bond strength and tribological properties of the coating. Under dry friction conditions, the wear forms of TiAlN-Ag coating transfers from severe adhesion wear and brittle spalling of TiAlN coating to slight adhesion wear and abrasive wear, whereas the wear mechanism under oil lubrication changes from the comprehensive form of brittle fracture, fatigue peeling and abrasion to the mainly abrasive wear with slight fatigue spalling. Overall, the TiAlN-Ag composite coating with 2.40 at.% Ag exhibits the best comprehensive mechanical properties and wear resistance.

Effect of Heterogeneous Composite Structure on the Microstructure and Properties for Nanostructured 2205 Duplex Stainless Steel

2205 duplex stainless steel (DSS) is a kind of complex structure stainless steel, which has advantages of both austenitic and ferritic stainless steel. Compared with 300 series stainless steel, it has better performance and can be used in more severe environment. Bulk 2205 DSS with heterogeneous composite structure is prepared via aluminothermic reaction. The volume percentage of ferrite phase is 32.3%, that of austenite phase is 59.7%, and σ phase is 8.1%. Average grain size of nanocrystalline and ultrafine austenite is 20.9 nm, accounting for 74.2%. The relatively mechanical properties deteriorate, which belongs to brittle fracture. The joint action mechanism of dual-phase structure and micro/nanostructure is analyzed by fracture observation. At present, the tensile strength and fracture elongation of as-cast heterogeneous composite structure 2205 DSS are poor. Therefore, different rolling and annealing systems are needed to coarsen grains and adjust mechanical properties through structure design.

Surface mechanical attrition treatment of additively manufactured 316L stainless steel yields gradient nanostructure with superior strength and ductility

Surface severe plastic deformation (S2PD) of additively manufactured/three-dimensional (3D) printed metallic parts is gaining increased attention as a post-manufacturing operation to enhance the material performance in a wide variety of applications. Surface mechanical attrition treatment (SMAT) is an S2PD technique that can yield a nanostructured surface layer induced by compressive stresses and work hardening. In the present study, SMAT was performed on 316L (austenitic) stainless steel (SS) processed by selective laser melting (SLM), and the consequent effects on mechanical response were investigated. The underlying mechanisms of microstructural evolution leading to the formation of nanocrystalline grains resulting from SMAT in SLM 316L SS are elucidated. The interactions between twins and deformation bands act as potential sites for impeding the movement of dislocations, which in turn leads to the formation of stacking faults, twinning, and occasionally transform to a different crystal structure. Twin-twin and/or twin-deformation band intersections sub-divide the matrix grains into smaller cells or low-angle disoriented blocks, which result in the formation of low-angle grain boundaries and finally in nanocrystallization at the surface. The size of nanocrystalline grains increases progressively with depth from the surface to micrometer size grains in bulk. The gradient nanostructure in the additively manufactured alloy after SMAT imparts an unusual combination of strength and ductility that markedly exceeds that of conventional, bulk nanostructured, or even high-performance 316L SS (containing nanoscale deformation twins embedded in submicron-sized austenitic grains obtained by dynamic plastic deformation processes). Analytical models revealed that strengthening results from a combination of grain boundaries and dislocations. The results of the present investigation pave the way for engineering high-performance SS for a variety of engineering applications.

Crystallization behavior and soft magnetic properties of Fe–B–P–C–Cu ribbons with amorphous/α-Fe hierarchic structure

In this work, Fe83+x(B4P8C4)16-xCu1 (x = 0, 1, 1.5 and 2 at. %) alloy system with amorphous/α-Fe hierarchic structures in as-cast state were artificially designed. The microstructure, crystallization kinetics, nanocrystalline structure, and magnetic properties of the alloys were systematically investigated. And the structural mechanism for the magnetic properties of these alloys was discussed. It is shown that the structure is evolved from full amorphous to amorphous/α-Fe dual-phase microstructure in the free surface side, and/or roller side layer with Fe content increasing. The formation of hierarchic structure decreases the activation energies of onset and peak crystallization of α-Fe phase, and increases the size of nanocrystalline grains, and subsequently is beneficial to improve the Bs of alloys. The Hc of alloys increases due to the formation of hierarchic structure, whereas this effect is gradually eliminated by the annealing process at the suitable crystallization annealing temperature (Ta). Typically, Fe85B3.5P7C3.5Cu1 nanocrystalline alloy exhibit the Bs of 1.86T and Hc of 5.7 A/m after annealing at 723 K. High Fe bearing alloys have lath domain structure with stronger orientation preference in as-cast state, and transformed into labyrinth packing with thinner spacing after structural relaxation annealing, demonstrating that hierarchic structure leads to the high magnetic anisotropy in the alloys.

Simulating Thermodynamic Stabilization in Nanocrystalline Binary Alloys Using a Novel Cellular Automaton Model

Predictive models for grain growth of nanocrystalline binary alloys are designed to select appropriate solutes and assess thermodynamic stabilization in nanoscale alloy systems. The available models incorporate concepts of free energy, solute segregation, size-misfit elastic strain energy, grain boundary energy and phase field framework. Besides the above factors, the present work proposes a novel cellular automaton model by considering normal grain boundary (GB) diffusion, triple junction GB kinetics and grain misorientation. The experimental data for two kinds of binary alloy system were used to validate the reasonability of the proposed model. For nanocrystalline Fe-4% Zr alloy with large atomic size mismatch and negative mixing enthalpy, compared with the available models, the proposed model shows a better fit to the experimental results for grain size as a function of annealing temperatures. For another binary alloy system with small atomic size mismatch and positive mixing enthalpy, the proposed model also captures well the measurements for grain size of nanocrystalline W-20% Ti alloy. The comparisons reveal that the proposed model has a wide application in addressing the thermal stabilization of nanocrystalline grain size.

Balancing strength and ductility of cylindrical-shaped Cu64Zr36 nanoglass via embedded Cu nanocrystals

Large-plasticity can be realized in nanoglass at the expense of strength, and such a loss can be compensated by embedding stronger nanocrystals. Here, the effects of average grain size (Daverage) and volume fraction of nanocrystalline (ϕ) on deformation behaviors of cylindrical-shaped Cu64Zr36 nanoglass/crystalline Cu composites (NGCCs) are investigated by molecular dynamics simulations of tensile loading. Our results reveal that the necking behavior of NGCCs persistently starts from the glass grains, followed by the glass fracture. In particular, there exists a critical Daverage of 7.96 nm, which is related to the diameter of nanowires. With Daverage larger than the critical value, NGCCs are more likely to achieve both high-strength and large-ductility in comparison with their NG counterparts. It also confirms the significance of the surface effect on stretching NGCC nanowires. As the volume fraction of the Cu nanograins increases, the peak stress of NGCCs increases while the ductility decreases. Therefore, the tweaking of appropriate Daverage and ϕ makes it possible to overcome the long-term strength-ductility trade-off dilemma.

Magnetic and magnetocaloric properties of Gd melt-spun ribbons

Structural features, magnetic properties and magnetocaloric effect of pure Gd ribbons prepared by melt spinning method were carefully analyzed. The X-ray data show that there is no change in the cell parameters for the samples prepared at different copper-wheel speed. Average size of nanocrystalline grains was close to 30 nm. As compared to the bulk Gd sample, the Curie temperature was the same in the case of the ribbons. From the magnetic isotherms, the magnetic entropy change was derived using the Maxwell relation follow the standard procedure. Its value was comparable with the value of the bulk Gd. Good mechanical properties of fabricated Gd ribbon and their flexibility can be useful for design of flexible refrigerating elements.

In-situ tensile study of annealed bimodal nano/micro grained medical 316L stainless steel

The tensile fracture behavior of the annealed 316L austenitic stainless steel at room temperature was studied by in-situ tensile platform loaded on the field emission scanning electron microscope with 4.2e−4 s−1 of initial strain rate. The results showed that tensile strength and yield strength of annealed specimens of 316L for annealing treatment for 28 h at 800 °C were higher than those of as-cast specimens. After annealing, microstructures of the specimens were mainly micron crystals. When the applied loading was 162 MPa, initiation source appeared. Loading reached 380 MPa, main cracks continued to expand and became deeply, and specimen yielded at this time. When loading reached up to 533 MPa, specimens reached the maximum stress value. After breaking completely, included angle was approximately 70° between fracture surface and direction of tensile axis. The composition of the fracture was more pure and continuity of the matrix was good. This showed that micro/nanostructure stainless steel prepared by aluminothermic reaction casting, which could improve the mechanical properties through controlling size of nanocrystalline and microcrystalline austenite grains and their volume fraction.

Role of Nanostructure in Electrochemical Corrosion and High Temperature Oxidation: A Review

The extremely fine grain size of nanocrystalline (nc) metallic alloys results in significantly different mechanical, electrochemical and oxidation properties as compared to the coarse-grained alloys of the same composition. Although the synthesis and attractive mechanical properties of nanocrystalline materials have been investigated and reviewed in great depth, the high temperature oxidation and electrochemical corrosion of these materials has received limited attention. The difference in the active dissolution and passivation behavior of nc alloys as compared to their microcrystalline counterparts varies for each alloy system. However, a unified theory explaining these phenomena still eludes us. In this context, this article reviews the progress in the electrochemical corrosion behavior of different nanocrystalline alloys, and hence, develops a better understanding of the effect of grain size, composition, interfacial phenomena and selective dissolution on corrosion of nanocrystalline alloys. This review also presents the role of nanometric grain size and the associated increase in grain boundary diffusion on the high temperature oxidation of nc alloys. The attractive possibility of enhanced oxidation resistance at lower alloying additions as compared to the coarse-grained alloys has been discussed. Although the primary focus of the article is on ferrous alloy systems, however, the lead studies on the role of ultrafine grain size in oxidation/corrosion behavior of other alloys systems have also been reviewed.

Surface nanocrystallization by mechanical punching process for improving microstructure and properties of Cu-30Ni alloy

A punching process was employed to produce nanocrystalline on the surface of Cu-30Ni alloy. Compared with the original sample, corresponding changes of the grain size, mechanical properties and corrosion resistance of surface nanocrystalline induced by punching process were characterized by atomic force microscopy, nanoindentation, microhardness, electrochemical behavior and electron work function. It was observed that the grain size was about 40 nm in the topmost surface. The increasing mechanical properties were also demonstrated. Additionally, the dynamic polarization test showed that the surface nanocrystallization improved the resistance to electrochemical attacks. Such increasing corrosion resistance was identified with the higher EWF value. This study demonstrates that surface nanocrystallization can be generated by punching process, and thus mechanical properties and corrosion resistance of alloy can be improved.

Deformation-induced alloying of Cu sheet with Al using ball collisions

• Balls were coated with Al by mechanical alloying. • Cu sheet was treated by balls coated with Al. • Al was introduced into the Cu sheet. • Deformation induced intermixing of elements. • Hardness increased more than four times. Cu sheet was alloyed with Al using ball collisions at room temperature and ambient atmosphere. Al was introduced into the Cu sheet as contamination from the grinding media. The idea here was first to coat the balls with Al by mechanical alloying and treated the surface of the Cu sheet using ball collisions. The Al debris from the grinding media was first to adhere to the Cu sheet under the ball collisions and then the deformation-induced diffusion and intermixing of the elements resulted in the formation of the Cu(Al) solid solution. The size of nanocrystalline grains after treatment ranged between 10 nm and 50 nm and an average of about 20 nm. The hardness of the Cu sheet after the treatment was increased more than four times.

Effects of the initial microstructure of Ti–Cu alloys on final nanoporous copper via dealloying

A Ti60Cu40 ribbon alloy before and after annealing was dealloyed in 0.03M HF and 0.13M HF solutions for 3.6–43.2ks, and a bi-continuous nanoporous copper (NPC) from tenths of nanometers to hundredths of nanometers was obtained. The as-spun amorphous Ti60Cu40 ribbon alloy has a glass transition temperature of 562K and crystallization temperature of 657K. The as-spun ribbon alloy has an amorphous structure, and that after annealing at 633K has a heterogeneous crystalline structure with coexistence of a particle size of 17nm and amorphous phases. The alloy after annealing at 713K has a completely crystalline structure consisting of dendritic Ti2Cu3 and Ti2Cu intermetallic phases. The phase segregation occurs at higher annealing temperatures. The dealloyed crystalline Ti60Cu40 ribbons exhibits multimodal nanoporous structure, which is corresponding to the initial microstructure. The order in uniformity in NPCs is confirmed to be as follows: amorphous alloys>nanocrystalline alloys>completely crystalline alloys, while their characteristic scale length of nanopores and ligaments differs from region to region where the initial microstructure and chemical composition are different. The diffusivity of Ti2Cu3 phase was lower than that of Ti2Cu phase by a factor of 13 due to their difference in chemical composition and microstructure. The difference in the NPC structure became weak in concentrated HF solution due to the higher diffusion in relatively large scales which might exceed the size of nanocrystalline grains. The final NPCs reflected the initial microstructure of the starting materials.

Influence of annealing temperature on the structure and properties of the nanograined NiAl intermetallic coatings produced by using mechanical alloying

Nanostructured NiAl intermetallic coatings were produced on carbon steel by using mechanical alloying in ambient temperature and pressure. Ni and Al powders were mixed with the composition of Ni–50at.% Al. Coatings were examined using X-ray diffraction and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was also used to observe coating particles. Samples prepared after 480min ball milling were subjected to annealing treatment at temperatures range of 400°C to 600°C. Continued ball milling reduced the coarse grains of the coating to the nanoscale. The size of nanocrystalline grains in the Ni–Al coating ranged between 17nm and 96nm. The nanocrystalline grains had a tendency to grow with increasing annealing temperature. After annealing at 400°C, a solid state reaction between Al and Ni leads to the formation of new phases. With increasing annealing temperature to 600°C only NiAl peaks were observed on XRD pattern. After annealing the hardness of the coating initially decreased and then increased. Annealing treatment also improved wear resistance of the coating.

Formation of nanostructured NiAl coating on carbon steel by using mechanical alloying

Nanostructured NiAl intermetallics coatings were generated on carbon steel by using mechanical alloying in ambient temperature and pressure. Ni and Al powders were mixed with the composition of Ni–50at% Al. The process involved powder particles trapped between the ball and cold welding at surface. Coatings were examined using X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the phase transmutation during process and subsequent heat treatment are strongly dependent on mechanical alloying time. After 480min heat treatment of mechanically alloyed sample, only NiAl peaks were viewed. Ball encounters and failures consecutively reduced the coarse grains of the coating to the nanoscale. The size of nanocrystalline grains in the NiAl coating ranged between 28 and 130nm. The creation of the coatings was studied at milling intervals between 60 and 600min. The average thickness and hardness of the surface layers at 480min, were 470μm and 930HV, respectively.

Bright upconversion emission of Nd 3+ in LiLa 1−xNd xP 4O 12 nanocrystalline powders

Upconversion emission of Nd 3+ doped LiLa 1− x Nd x P 4O 12 nanocrystals powders upon irradiation with 808 nm laser diode has been investigated. This emission of bright orange color was dependent on concentration and excitation incident light intensity. It was found that this emission was threshold phenomenon characterized by long build-up times which were dependent on energy of upconversion emission, concentration of active ions, size of nanocrystalline grains and power of incident light. The mechanism of observed upconversion was discussed in terms of thermally stimulated avalanche process. The emission intensity bistability of upconversion emission was demonstrated.

Study of Nd2–x Ce x CuO4 ± δ (x = 0.1–0.25) as cathode material for intermediate-temperature solid oxide fuel cells

The solid solubility limit of Ce in Nd2–xCexCuO4 ± δ, prepared by sol–gel process, is established up to x = 0.2. The transition from negative temperature coefficient to positive temperature coefficient, within the solid solubility region, is observed at 620 °C. The area-specific-resistance (ASR) is optimized for electrochemical cell sintered at 800 °C. ASR enhances with increase in sintering temperature of cell. ASR value of 0.93 ohm cm2 at 700 °C, determined by electrochemical impedance spectroscopy is comparable against that by voltage versus current (V–I) characteristics at 0.98 ohm cm2 at the same temperature. Electrochemical performance and ASR of Nd1.8Ce0.2CuO4 ± δ is improved when prepared by sol–gel route over solid-state reaction, which is attributed to uniform size and shape of nanocrystalline grains.

Theoretical calculation of molar heat capacity at constant pressureof nanoparticle and nanocrystalline

In this paper the theoretical calculation methods for CP(T) (molar heat capacity at constant pressure) values of nanoparticle and nanocrystalline are investigated. Theoretical formulas for calculating CP(T), Debye temperature and volume expansion coefficient of nanoparticle using melting point of nanoparticle are set up and the calculated values of CP(T) for copper nanoparticle are fairly consistent with the experimental ones. A relationship between Debye temperature and density of nanocrystalline is proposed, and Debye temperature can decrease with the reduction of nanocrystalline density. The calculated values of CP(T) of copper nanocrystalline are also quite consistent with the experimental ones. The bulk modulus of elasticity of nanocrystalline will drop off with the decrease of grain size of nanocrystalline, which should be taken into account when the CP(T) of nanocrystalline is calculated. The calculation methods proposed in this paper are feasible.

Fabrication of nanostructured Mo coatings on Al and Ti substrates by ball impact cladding

Nanostructured multicomponent Mo coatings were fabricated on Ti and Al substrates by ball impact cladding at room temperature in an ambient atmosphere. The process involved subjecting the substrate and Mo foil fixed at the top of a vibration chamber to high-energy collisions with balls. The coating formation was the result of a simultaneous process of mechanically induced plastic flow, nanocrystallization, and interdiffusion caused by the ball collisions. Plastic deformation refined the grains at the Mo foil/substrate interface to the nanometre scale. The size of nanocrystalline grains in the Mo coatings ranged between 2nm and 10nm. The ball collisions caused atomic level intermixing of different elements, introduced into the surface from the steel balls used for milling, and solid solubility improved remarkably. The hardness of the Mo coatings on the Al and Ti substrates was 552 and 1010 HV, respectively. The initial hardness of the Mo foil was 287 HV. The high hardness of the Mo claddings was attributed to the fine grain structure, formation of supersaturated solid solutions, and high residual compressive stresses.