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Direct melt compounding was used to prepare nanocomposites of organophilic montmorillonite (o-mmt) clay dispersed in maleated polypropylenes (PPgMA) as well as nanocomposites of organoclay and polypropylene (PP) modified with various grades of PPgMA compatibilizers. The thermal effect on the rheology and melt compounding was first investigated with a plasticorder. The shear viscosities and the melt flow indices (MFI) of the PPgMA compatibilizers were sensitive to the blending temperature, which had to be varied with the compatibilizer grade to achieve desirable level of torque for extensive exfoliation of organoclay in the plasticorder. However, for low molecular weight oligomer, the clay dispersion was poor because of low shear viscosity and thermal instability. Next, the PPgMA-modified PP/organoclay nanocomposites were prepared on a corotating twin-screw extruder. The nanoscale dimensions of the dispersed clay platelets led to significantly increased linear viscoelastic properties, which were qualitatively correlated with the state of exfoliation in the nanocomposites. The relative viscosity (relative to the silicate-free matrix) curves revealed a systematic trend with the extent of clay exfoliation. Furthermore, the degree of clay dispersion was found to increase with the loading of compatibilizers; however, high loading of compatibilizer compromised the final moduli of the nanocomposites. POLYM. ENG. SCI., 46:289-302, 2006. © 2006 Society of Plastics Engineers, INTRODUCTION Commercial exploitation of isotactic polypropylene has been expanded rapidly since the development of 'Zieglertype' catalysts in 1950s. It is because of polypropylene's attractive characters of low cost, low weight, [...]

Significant progress has been made in magnesium-based composites during recent decades, especially for the appearance of magnesium matrix composite reinforced by nanoparticles. The nanoparticles added not only exhibit a good strengthening effect, but also maintain the initial toughness of the matrix, effectively balancing the contradiction between the strength and plasticity in the traditional magnesium matrix composites. The magnesium matrix nanocomposites with excellent mechanical properties have pushed the development of magnesium matrix composites to a new stage. However, it is very difficult to disperse the nanoparticles in metal melt especially in magnesium melt which is different from other metal melts and dangerous during the cast processing. This means that the preparation of magnesium matrix nanocomposite is extremely challenging. Further, the magnesium matrix nanocomposites possess a distinctive characteristic in deformation behavior, strengthening and toughening mechanism due to their special size effect of nanoparticles. Accordingly, this review will focus on the new preparation technologies, deformation behavior, mechanical properties and strengthening and toughening mechanisms. The potential applications, development trends and future research ideas of magnesium matrix nanocomposite are also prospected. [ABSTRACT FROM AUTHOR]

In the present study, magnesium-based composites with three different volume percentages of nano-sized SiC particulates (SiCp) reinforcement were fabricated using a simple and inexpensive technique followed by hot extrusion. Microstructural characterization of the materials revealed uniform distribution of nano-size SiCp and obvious grain refinement. The tensile test result indicates a remarkable improvement on the strength for the as-extruded SiCp/AZ31B nanocomposite, while the elongation to fracture was decreased by comparing with the AZ31B alloy. Although, compared with the as-extruded AZ31B alloy, the ductility of the SiCp-reinforced AZ31B nanocomposite is decreased, but the ductility of the present SiCp-reinforced AZ31B nanocomposite is far higher than that of the conventional micron or submicron SiCp-reinforced magnesium matrix composites. It is concluded that, compared with the larger sized (micron or submicron) particles, the addition of nano SiCp in the AZ31B alloy resulted in the best combination of the strength and ductility. An attempt is made in the present study to correlate the effect of presence of nano-SiCp as reinforcement and its increasing amount with the microstructural and mechanical properties of magnesium. [ABSTRACT FROM AUTHOR]

In the present study, AZ31B magnesium matrix composites reinforced with two volume fractions (3 and 5 vol.%) of micron-SiC particles(1 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. The as-cast ingots were extruded at 350 °C with the extrusion ratio of 15:1 at a constant ram speed of 15 mm/s. The microstructure of the composites was investigated by optical microscopy, scanning electron microscope and transmission electron microscope. Microstructure characterization of the composites showed relative uniform reinforcement distribution and significant grain refinement. The presence of 1 μm-SiC particles assisted in improving the elastic modulus and tensile strength. The ultimate tensile strength and yield strength of the 5 vol.% SiCp/AZ31B composites were simultaneously improved. [ABSTRACT FROM AUTHOR]

In this study, both AZ91 alloy and nano-SiCp/AZ91 composite were subjected to multi-pass forging under varying passes and temperatures. The microstructure and mechanical properties of the alloy were compared with its composite. After six passes of multi-pass forging at a constant temperature of 400 ℃, complete recrystallization occurred in both the AZ91 alloy and composite. The decrease of temperature and the increase of passes for the multi-pass forging led to further refinement of dynamic recrystallized grains and dynamic precipitation of second phases. The grain size of the nano-SiCp/AZ91 composite was smaller than that of the AZ91 alloy under the same multi-pass forging condition, which indicated that the addition of SiC nanoparticles were beneficial to grain refinement by pinning the grain boundaries. The texture intensity for the 12 passes of multi-pass forging with varying temperatures was increased compared with that after nine passes. The ultimate tensile strength is slightly decreased while the yield strength was increased unobviously for the AZ91 alloy with the decrease of temperature and the increase of the passes for the multi-pass forging. Under the same condition of multi-pass forging, the yield strength of the composite was higher than that of the AZ91 alloy due to the Orowan strengthening effect and grain refinement strengthening resulting from externally applied SiC nanoparticles and internally precipitated second phases. By comparing the microstructure and mechanical properties between the AZ91 alloy and nano-SiCp/AZ91 composite, the strength-toughness properties of the composites at room temperature were affected by the matrix grain size, texture evolution, SiC nanoparticles distribution and the precipitated second phases. [ABSTRACT FROM AUTHOR]

Grain boundary (GB) engineering via alloying opens a pathway to design the interfaces in alloys for tuning their mechanical properties, thus it is quite critical to determine the optimum addition of alloying element. Here, we prepared immiscible Cu-Cr alloyed thin films by non-equilibrium magnetron sputtering deposition to study Cr alloying effects on the microstructure and mechanical properties of Cu thin films. It is found that Cr doping can notably refine the grains and promote the formation of fully nanotwinned columnar grains at low Cr volume concentrations (≤∼3.7 at.%) associated with the average GB Cr concentrations ≤ ∼15 at.%, beyond which the formation of nanotwins is significantly suppressed. The role of Cr atoms/clusters played in the twinning process is rationalized in terms of dislocation rebound-promotion mechanism. Importantly, a maximum hardness is discovered at the optimum Cr addition of ∼2.0 at.%. The Cr concentration-dependent hardness of Cu-Cr alloyed films was quantified by a combined strengthening model. The achievement of the maximum hardness was related to the greatest GB solute segregation-induced strength contribution. It is further uncovered that the Cu-Cr system exhibits strain rate sensitivity (SRS, m ) monotonically reduced with increasing the Cr concentration, ranging from a positive m of 0.0314 (for pure Cu) to a negative m of −0.0245, which is attributed to a competition between the dislocation-boundary interactions (increasing m ) and the dislocation-solute atoms interactions (decreasing m ). These findings provide valuable insights into tuning the composition of alloyed thin films to achieve optimized mechanical performance. [ABSTRACT FROM AUTHOR]

Tuning the microstructure to optimize the mechanical performance of nanocrystalline Cu thin films via the alloying strategy is quite important for their application in microdevices. In this work, we prepared nanocrystalline miscible Cu-Ti and immiscible Cu-Mo alloyed thin films to investigate alloying effects on the microstructure and mechanical properties of Cu thin films in terms of mixing enthalpies. It is found that the dopants of both Ti and Mo can notably refine the grains, and in particular promote the formation of nanotwins below a critical content of solute, beyond which the formation of nanotwins is notably suppressed. The nonmonotonic solute concentration-dependent twinning behavior observed in Cu-Ti and Cu-Mo alloyed thin films is explained by the coupling effects between grain size and grain boundary segregated dopants that affects the stimulated slip process of partials. The increased hardness of both Cu-Ti and Cu-Mo systems with increasing the solute contents are quantitatively explained by combining several strengthening mechanisms, including solid solution strengthening, grain/twin boundary (GB/TB) strengthening, solute segregation-induced strengthening. It unexpectedly appears that with increasing the solute contents, the Cu-Ti system exhibits monotonically reduced positive strain rate sensitivity (SRS, m ), whereas the Cu-Mo system manifests almost constant negative SRS. The fundamental difference in SRS m between Cu-Ti and Cu-Mo is rationalized in terms of the interactions between solute atomic clusters and dislocations based on the cross-core diffusion mechanism. [ABSTRACT FROM AUTHOR]

Magnetron sputtering technique was employed to synthesize crystalline/amorphous high-entropy alloy (AHEA) X/AHEA (X = Cu and Ni, AHEA = Fe 20 Co 20 Cr 20 Ni 20 B 10 Si 10) nanostructured multilayers (CANMs) with equal layer thickness h ranging from 5 to 150 nm, aiming at systematically investigating their plastic deformation and microstructural evolution. Unlike traditional crystalline/crystalline multilayers often with shear banding at small h , the present X/AHEA CANMs exhibit h -independent homogeneous deformation via the thinning of both constituent layers, while associated with h -dependent deformation-induced devitrification in AHEA layers with thickness below 25 nm. This homogeneous plasticity of AHEA layers with the large incubation size for mature shear bands can be ascribed to the coupling between dislocations in X layers and shear transformation zones in AHEA layers. Meanwhile, the absorption of abundant dislocations emitted from X layers by AHEA layers with small thickness facilitates the occurrence of devitrification behavior, implying the microstructural instability under strong constraint conditions. Moreover, both of Cu/AHEA and Ni/AHEA CANMs manifest size-dependent hardness with the fashion that "smaller is stronger", which is captured well by the rule-of-mixture, based on the size-dependent strength of X nanolayers and size-independent homogenous flow of AHEA nanolayers. [Display omitted] [ABSTRACT FROM AUTHOR]

The (micron[m]+submicron[s]+nano[n]) SiC particles (SiCp) reinforced AZ31B magnesium matrix composites (AZ31B/SiCp/m + s + n composites) were fabricated by stirring casting technology followed by hot extrusion. This method has successfully solved the homodisperse problem of SiCp/m + s + n in the magnesium matrix composites. The influence of SiCp/m + s + n on the dynamic recrystallization (DRX) behavior of the magnesium matrix was investigated by researching the TEM microstructures of composites. The results indicated that the addition of SiCp/m + s + n was helpful to increase the ratio of DRXed nucleation and reduce the average grain size of matrix. The particle deformation zone (PDZ) can be formed in the vicinity of micron SiCp, while the region with high dislocation density can be created around submicron SiCp owing to pin dislocation and deformation mismatch between matrix and particles. These regions are ideal sites for the formation of recrystallization nucleus. Meanwhile, the addition of fine SiCp(nano and submicron) may delay or hinder the growth of matrix grain through the pinning effect on the grain boundaries during hot extrusion. The tensile test indicates that the strength of the AZ31B/SiCp/m + s + n composites has a significant improvement. Compared with the single sized or bimodal sized particles reinforced magnesium matrix composites, the tensile strength of the AZ31B/SiCp/m + s + n composites were highlighted. [ABSTRACT FROM AUTHOR]

High strong magnesium matrix composites can be obtained by refining grain size, heat treatment and severe plastic deformation methods. However, most of the composite enhancing approaches result in the disappointingly poor tensile ductility. Thus, the designing and fabricating of composites with simultaneously good ductility and high strength have become burning issues for the application of light metals/alloys. Simply adding particles or changing particle parameters can not solve the problem of combination of high strength and good ductility. A new method has been developed, wherein the bimodal size grained microstructure formed by adding the bimodal sized SiC particles (SiCp) was selected as favorable microstructure for achieving good ductility and high strength in present work. The fine grains (grain size: <1 μm) were obtained through the particle stimulate of nucleation (PSN) and pin grain boundary effect. However, the coarse grains (grain size: ∼4 μm) were obtained through forming SiCp free zones in the present magnesium matrix composites. The tensile test indicates a significant improvement in the ductility (∼8.3%) and strength (UTS: ∼402 MPa, YS: ∼323 MPa) of the composites. Compared with the conventional single-sized particles (micron or nano) reinforced magnesium matrix composites, the tensile ductility and strength of present composite (AZ31B/SiCp/1n + 9m composite) were highlighted in the current literature. [ABSTRACT FROM AUTHOR]

A particulate reinforced magnesium matrix nanocomposite was subjected to multidirectional forging (MDF). The results showed that the microstructure of the nanocomposite mainly consisted of some large initial grains and fine recrystallized grains along initial grain boundaries after 1 MDF pass. With increasing the MDF passes, the degree of dynamic recrystallization increased and ultrafine grains with average size of about 20 nm were observed in the nanocomposites after 6 MDF passes. The distribution of SiC nanoparticles in the nanocomposite was homogeneous outside some SiC nanoparticle dense zones after different MDF passes. Compared with typical basal texture obtained for 1 MDF pass, the basal pole intensity decreased firstly for 3 MDF pass and then increased for 6 MDF passes. Both ultimate tensile strength and elongation to fracture of the nanocomposite were enhanced with increasing the MDF passes, while yield strength decreased after 3 MDF passes and then increased slightly after 6 MDF passes. [ABSTRACT FROM AUTHOR]

The present study dealt with the AZ91 alloy subjected to multidirectional forging (MDF) under decreasing temperature conditions with emphasizing on relationship between microstructures and mechanical properties generated by MDF. The obtained results showed that grains of the alloy were gradually refined while the amount of the second phase precipitated along the grain boundaries was increased with increasing the MDF passes and decreasing the MDF temperature. Compared with the as-cast alloy, yield strength, ultimate tensile strength and elongation of the AZ91 alloy after MDF under decreasing temperature conditions were simultaneously increased. As the numbers of MDF passes were gradually increased and the MDF temperature was gradually decreased, the yield strength was gradually decreased, suggesting that texture softening may overwhelm the strengthening due to the grain refinement as well as the homogeneous distribution of precipitates. [ABSTRACT FROM AUTHOR]

The hardness, tensile ductility and fatigue lifetime of nanostructured (NC) Cu-Zr alloyed thin films have been systematically measured at different Zr additions (0, 0.5, 2.0, 4.0, 8.0at.%). Experimental results showed that the Cu-0.5at.% Zr film exhibited the highest hardness, largest ductility and longest fatigue lifetime, which are increased by 120%, 80%, and above an order of magnitude, respectively, in comparison with its pure Cu counterpart. The simultaneous improvements of mechanical properties are rationalized with respect to microstructural evolutions related to Zr segregation at grain boundaries (GBs). Besides refinement in NC grains and enhancement in nanotwins, grain orientation was apparently modified by the GB Zr segregation. (110) grains were promoted and layer-like microstructure was formed with coexistence of three orientation of grains at the upper layer, facilitating Cu grain growth at room temperature. GB doping strengthening worked as an additional strengthening mechanism and mechanically driven grain growth became the predominant deformation mechanism and fatigue mechanism, which accounted for the optimal combination of mechanical properties achieved in the Cu-0.5at.% Zr film. When the Zr content was >2.0at.%, however, amorphous phases were formed at the GBs due to locally increased Zr concentration. The GB doping strengthening effect was concomitantly weakened and strength was gradually decreased. In addition, stress/strain localization occurred readily at the amorphous phase regions, triggering intergranular fracture and reducing ductility. During cyclic testing in the low stress range, GB amorphous phases strengthened rather impaired the GBs, which gave the Cu-Zr films a fatigue lifetime greater than that of the pure NC Cu film. The present results clearly indicate that, once solute content is suitably explored, the solute segregation at GBs in NC metals can not only retain NC grains in loading-free conditions, but also impart enhanced mechanical properties when exposed to applied stress. [ABSTRACT FROM AUTHOR]

Highlights: [•] Friction stir processing (FSP) was applied to Mg–Gd–Y–Zn–Zr casting. [•] The grain boundary Mg3RE and LPSO phases in the casting dissolved after FSP. [•] Fine grains with LPSO phase distributed only within the grains was achieved. [•] High mechanical properties were obtained in FSP Mg–Gd–Y–Zn–Zr. [ABSTRACT FROM AUTHOR]

Abstract: In this research, the as-cast and as-homogenized Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr alloy were subjected to the hot rolling with the same processing parameters. The microstructure and mechanical properties of the two kinds of as-rolled sheets were investigated. The sheet rolled from the as-cast alloy was mainly composed of recrystallized α-Mg grains surrounded by Mg3(Gd,Y) eutectic compounds. While the sheet rolled from the as-homogenized alloy was composed of large deformed grains with long-period stacking ordered (LPSO) phase inside and fewer recrystallized α-Mg grains, and exhibited higher strength due to the deformed grains with stronger basal texture and larger volume fraction of long-period stacking ordered (LPSO) phase. [Copyright &y& Elsevier]

Abstract: Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr alloy sheets containing long period stacking ordered (LPSO) phase were prepared by hot rolling at 400°C with total reduction of 96%. Microstructure evolution of the sheets during hot rolling was investigated, and its influence on mechanical properties was discussed. Twinning occurred during the early stage of hot rolling, and disappeared after total reduction higher than 89%. Average grain size was gradually refined, microstructure became much more homogeneous and volume fraction of LPSO phase decreased with increasing rolling reduction. Furthermore, the type of LPSO phases far from and near the block shaped phases were identified to be different. Basal texture was obtained during rolling process, but the intensity declined with the further rolling, which is mainly due to the dynamic recrystallization and the addition of RE elements. The as-rolled sheet with 96% reduction shows excellent mechanical properties: yield strength of 318MPa, ultimate tensile strength of 403MPa and elongation to failure of 13.7% at ambient temperature along the rolling direction. [Copyright &y& Elsevier]