Gamma -> Epsilon Martensite Transformation and Twinning Deformation in fcc Cobalt During Surface Mechanical Attrition Treatment

The deformation microstructure of face-centered cubic cobalt subjected to surface mechanical attrition treatment was studied as a function of strain levels. Strain-induced gamma --> epsilon transformation and twinning deformation were evidenced by transmission electron microscopy and were found to progress continuously in ultrafine and nanocrystalline grains as the strain increased.

Strain-Induced Grain Refinement of Cobalt During Surface Mechanical Attrition Treatment

The microstructural evolution during surface mechanical attrition treatment of cobalt (a mixture of hexagonal close packed (hep) and face-centered cubic (fcc) phases) was investigated. In order to reveal the mechanism of grain refinement and strain accommodation. The microstructure was systematically characterized by both cross-sectional and planar-view transmission electron microscopy. In the hcp phase, the process of grain refinement. Accompanied by an increase in strain imposed in the surface layer. Involved: (1) the onset of 110 111 deformation twinning, (2) the operation of (1 120) 110 1 0} prismatic and (1 120) (000 1) basal slip, leading to the formation of low-angle dislocation boundaries, and (3) the successive subdivision of grains to a finer and finer scale. Ressulting in the formation of highly misoriented nanocrystalline grains. Moreover. The formation of nanocrystalliies at the grain boundary and triple junction was also observed to occur concurrently with straining. By contrast. The fec phase accommodated strain in a sequence as follows: (1) slip of dislocations by forming intersecting planar arrays of dislocations, (2) {1 1 1} deformation twinning, and (3) the gamma(fcc) --> epsilon(hcp) martensitic phase transformation. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization occurring in the hep phase and the gamma --> E: martensitic transformation in the fcc phase as well.

Microstructural evolution and formation of nanocrystalline intermetallic compound during surface mechanical attrition treatment of cobalt

Nanocrystalline intermetallic Co3Fe7 was produced on the surface of cobalt via surface mechanical attrition (SMA). Deformationinduced diffusion entailed the formation of a series of solid solutions. Phase transitions occurred depending on the atomic fraction of Fe in the surface solid solutions: from hexagonal close-packed (<4% Fe) to face-centered cubic (fcc) (4-11% Fe), and from fcc to body-centered cubic (>11% Fe). Nanoscale compositional probing suggested significantly higher Fe contents at grain boundaries and triple junctions than grain interiors. Short-circuit diffusion along grain boundaries and triple junctions dominate in the nanocrystalline intermetallic compound. Stacking faults contribute significantly to diffusion. Diffusion enhancement due to high-rate deformation in SMA was analyzed by regarding dislocations as solute-pumping channels, and the creation of excess vacancies. Non-equilibrium, atomic level alloying can then be ascribed to deformation-induced intermixing of constituent species. The formation mechanism of nanocrystalline intermetallic grains on the SMA surface can be thought of as a consequence of numerous nucleation events and limited growth. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In Situ Synthesis of Nanocrystalline Intermetallic Compound Layer During Surface Mechanical Attrition Treatment of Zirconium

The surface mechanical attrition treatment (SMAT) technique was developed to synthesize a nanocrystalline (NC) layer on the surface of metallic materials for upgrading their overall properties and performance. In this paper, by means of SMAT to a pure zirconium plate at the room temperature, repetitive multidirectional peening of steel shots (composition (wt%): 1C, 1.5Cr, base Fe) severely deformed the surface layer. A NC surface layer consisting of the intermetallic compound FeCr was fabricated on the surface of the zirconium. The microstructure characterization of the surface layer was performed by using X-ray diffraction analysis, optical microscopy, scanning and transmission electron microscopy observations. The NC surface layer was about 25 mu m thick and consisted of the intermetallic compound FeCr with an average grain size of 25 +/- 10 nm. The deformation-induced fast diffusion of Fe and Cr from the steel shots into Zr occurred during SMAT, leading to the formation of intermetallic compound. In addition, the NC surface layer exhibited an ultrahigh nanohardness of 10.2 GPa.

Localized Solid-State Amorphization at Grain Boundaries in a Nanocrystalline Al Solid Solution Subjected to Surface Mechanical Attrition

Using high-resolution electron microscopy, localized solid-state amorphization (SSA) was observed in a nanocrystalline (NC) Al solid solution (weight per cent 4.2 Cu, 0.3 Mn, the rest being Al) subjected to a surface mechanical attrition treatment. It was found that the deformation-induced SSA may occur at the grain boundary (GB) where either the high density dislocations or dislocation complexes are present. It is suggested that lattice instability due to elastic distortion within the dislocation core region plays a significant role in the initiation of the localized SSA at defective sites. Meanwhile, the GB of severely deformed NC grains exhibits a continuously varying atomic structure in such a way that while most of the GB is ordered but reveals corrugated configurations, localized amorphization may occur along the same GB.

Nanocrystalline Formation During Mechanical Attrition of Cobalt

The nanocrystalline (nc) formation was studied in cobalt (a mixture of c (hexagonal close packed) and gamma (face-centered cubic) phases) subjected to surface mechanical attrition treatment. Electron microscopy revealed the operation of {10(1) over bar 0}< 11(2) over bar 0 > prismatic and {0001}< 11(2) over bar 0 > basal slip in the E phase, leading to the successive subdivision of grains to nanoscale. In particular, the dislocation splitting into the stacking faults was observed to occur in ultrafine and nc grains. By contrast, the planar dislocation arrays, twins and martensites were evidenced in the gamma phase. The strain-induced gamma ->epsilon martensitic transformation was found to progress continuously in ultrafine and nc grains as the strain increased. The nc formation in the gamma phase was interpreted in terms of the martensitic transformation and twinning.

Nanocrystalline Formation during Mechanical Attrition of Cobalt

The nanocrystalline (nc) formation was studied in cobalt (a mixture of c (hexagonal close packed) and gamma (face-centered cubic) phases) subjected to surface mechanical attrition treatment. Electron microscopy revealed the operation of {10(1) over bar 0}< 11(2) over bar 0 > prismatic and {0001}< 11(2) over bar 0 > basal slip in the E phase, leading to the successive subdivision of grains to nanoscale. In particular, the dislocation splitting into the stacking faults was observed to occur in ultrafine and nc grains. By contrast, the planar dislocation arrays, twins and martensites were evidenced in the gamma phase. The strain-induced gamma ->epsilon martensitic transformation was found to progress continuously in ultrafine and nc grains as the strain increased. The nc formation in the gamma phase was interpreted in terms of the martensitic transformation and twinning.

Grain Refinement at the Nanoscale Via Mechanical Twinning and Dislocation Interaction in a Nickel-Based Alloy

A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.

In Situ Synthesis Of Nanocrystalline Intermetallic Layer During Surface Plastic Deformation Of Zirconium

By means of a surface plastic deformation method a nanocrystalline (NC) intermetallic compound was in situ synthesized on the surface layer of bulk zirconium (Zr). Hardened steel shots (composition: 1.0C, 1.5Cr, base Fe in wt.%) were used to conduct repetitive and multidirectional peening on the surface layer of Zr. The microstructure evolution of the surface layer was investigated by X-ray diffraction and scanning and transmission electron microscopy observations. The NC intermetallic layer of about 25 gm thick was observed and confirmed by concentration profiles of Zr, Fe and Cr, and was found to consist of the Fe100-xCrx compound with an average grain size of 22 nm. The NC surface layer exhibited an extremely high average hardness of 10.2 GPa. The Zr base immediately next to the compound/Zr interface has a grain size of similar to 250 nm, and a hardness of similar to 3.4 GPa. The Fe100-xCrx layer was found to securely adhere to the Zr base. (c) 2007 Elsevier B.V All rights reserved.

The main factor influencing the tensile properties of surface nano-crystallized graded materials

The improved mechanical properties of surface nano-crystallized graded materials produced by surface severe plastic deformation ((SPD)-P-2) are generally owing to the effects of the refined structure, work-hardened region and compressive residual stress. However, during the (SPD)-P-2 process, residual stress is produced simultaneously with work-hardened region, the individual contribution of these two factors to the improved mechanical properties remains unclear. Numerical simulations are carried out in order to answer this question. It is found that work hardening predominates in improving the yield strength and the ultimate tensile strength of the surface nano-crystallized graded materials, while the influence of the residual stress mainly emerges at the initial stage of deformation and decreases the apparent elastic modulus of the surface nano-crystallized graded materials, which agrees well with the experimental results. (C) 2010 Elsevier B.V. All rights reserved.

The mechanical properties of an aluminum alloy by plasma-based ion implantation and solution-aging treatment

The age-strengthening 2024 aluminum alloy was modified by a combination of plasma-based ion implantation (PBII) and solution-aging treatments. The depth profiles of the implanted layer were investigated by X-ray photoelectron spectroscopy (XPS). The structure was studied by glancing angle X-ray diffraction (GXRD). The variation of microhardness with the indenting depth was measured by a nanoindenter. The wear test was carried on with a pin-on-disk wear tester. The results revealed that when the aluminum alloys were implanted with nitrogen at the solution temperature, then quenched in the vacuum chamber followed by an artificial aging treatment for an appropriate time, the amount of AIN precipitates by the combined treatment were more than that of the specimen implanted at ambient temperature. Optimum surface mechanical properties were obtained. The surface hardness was increased and the weight loss in a wear test decreased too.

Structure and mechanical properties of ceramic coatings fabricated by plasma electrolytic oxidation on aluminized steel

Ceramic coatings were formed by plasma electrolytic oxidation (PEO) on aluminized steel. Characteristics of the average anodic voltages versus treatment time were observed during the PEO process. The micrographs, compositions and mechanical properties of ceramic coatings were investigated. The results show that the anodic voltage profile for processing of aluminized steel is similar to that for processing bulk Al alloy during early PEO stages and that the thickness of ceramic coating increases approximately linearly with the Al layer consumption. Once the Al layer is completely transformed, the FeAl intermetallic layer begins to participate in the PEO process. At this point, the anodic voltage of aluminized steel descends, and the thickness of ceramic coating grows more slowly. At the same time, some micro-cracks are observed at the Al2O3/FeAl interface. The final ceramic coating mainly consists of gamma-Al2O3, mullite, and alpha-Al2O3 phases. PEO ceramic coatings have excellent elastic recovery and high load supporting performance. Nanohardness of ceramic coating reaches about 19.6 GPa. (c) 2007 Elsevier B. V. All rights reserved.

A Mechanical Model For The Quantification Of The Effect Of Laser Quenching On Ctod In Steels

The technology of laser quenching is widely used to improve the surface properties of steels in surface engineering. Generally, laser quenching of steels can lead to two important results. One is the generation of residual stress in the surface layer. In general, the residual stress varies from the surface to the interior along the quenched track depth direction, and the residual stress variation is termed as residual stress gradient effect in this work. The other is the change of mechanical properties of the surface layer, such as the increases of the micro-hardness, resulting from the changes of the microstructure of the surface layer. In this work, a mechanical model of a laser-quenched specimen with a crack in the middle of the quenched layer is developed to quantify the effect of residual stress gradient and the average micro-hardness over the crack length on crack tip opening displacement (CTOD). It is assumed that the crack in the middle of the quenched layer is created after laser quenching, and the crack can be a pre-crack or a defect due to some reasons, such as a void, cavity or a micro-crack. Based on the elastic-plastic fracture mechanics theory and using the relationship between the micro-hardness and yield strength, a concise analytical solution, which can be used to quantify the effect of residual stress gradient and the average micro-hardness over the crack length resulting from laser quenching on CTOD, is obtained. The concise analytical solution obtained in this work, cannot only be used as a means to predict the crack driving force in terms of the CTOD, but also serve as a baseline for further experimental investigation of the effect after laser-quenching treatment on fracture toughness in terms of the critical CTOD of a specimen, accounting for the laser-quenching effect. A numerical example presented in this work shows that the CTOD of the quenched can be significantly decreased in comparison with that of the unquenched. (C) 2008 Elsevier B.V. All rights reserved.

Effects of surface treatment on sapphire substrates

The influence of mechanical polishing, chemo-mechanical polishing (CMP), as well as CMP and subsequent chemical etching on the properties of sapphire substrate surfaces has been studied. The sapphire substrates have been investigated by means of polarizing microscopy, atomic force microscopy (AFM). X-ray diffraction rocking curves (XRCs) and micro-Raman spectroscopy. The results show that CMP with subsequent chemically etching yields the best-quality sapphire substrate surfaces. (C) 2004 Elsevier B.V. All rights reserved.

Influences of CO2 laser annealing on mechanical and laser-induced damage properties of ZrO2 thin films

Zirconium dioxide (ZrO2) thin films were deposited on BK7 glass substrates by the electron beam evaporation method. A continuous wave CO2 laser was used to anneal the ZrO2 thin films to investigate whether beneficial changes could be produced. After annealing at different laser scanning speeds by CO2 laser, weak absorption of the coatings was measured by the surface thermal lensing (STL) technique, and then laser-induced damage threshold (LIDT) was also determined. It was found that the weak absorption decreased first, while the laser scanning speed is below some value, then increased. The LIDT of the ZrO2 coatings decreased greatly when the laser scanning speeds were below some value. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was defect-initiated both for annealed and as-deposited samples. The influences of post-deposition CO2 laser annealing on the structural and mechanical properties of the films have also been investigated by X-ray diffraction and ZYGO interferometer. It was found that the microstructure of the ZrO2 films did not change. The residual stress in ZrO2 films showed a tendency from tensile to compressive after CO, laser annealing, and the variation quantity of the residual stress increased with decreasing laser scanning speed. The residual stress may be mitigated to some extent at proper treatment parameters. (c) 2007 Elsevier GmbH. All rights reserved.