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Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels

The 𝜎 phase which exists in various series of stainless steels is a significant subject in steels science and engineering. The precipitation of the 𝜎 phase is also a widely discussed aspect of the science and technology of stainless steels. The microstructural variation, precipitation mechanism, prediction method, and effects of properties of 𝜎 phase are also of importance in academic discussions. In the first section, a brief introduction to the development and the precipitation characteristics (including morphologies and precipitation sites) of 𝜎 phase in stainless steels is presented. In the second section, the properties effect, prediction method, processing effect, elemental addition, retardation method and Thermo-Calc simulation of the 𝜎 phase in stainless steels are highlighted.

Catalysis effect of metal doping on wear properties of diamond-like carbon films deposited by a cathodic-arc activated deposition process

Abstract Diamond-like carbon (DLC) films containing various metal dopings were synthesized by using a cathodic-arc activated deposition process. Metal plasma with intensive ion energies catalyzes the decomposition of hydrocarbon gases (C 2 H 2 ), and induces the formation of hydrogenated DLC films with a mixture of sp 2 and sp 3 carbon bonds. The composite film structure consists of a metal-doped DLC film on top of a graded metal nitride interlayer, which provides enhanced mechanical and tribological properties. The catalysis effect of three common transition metal plasmas, including Cr, Ti, and Zr was investigated. This experiment depicts the advantage of the catalysis effect of Cr plasma in synthesizing DLC films with a higher sp 3 carbon bond ratio comparing with that of Ti and Zr plasma. The wear properties were correlated with the metal doping. The optimized Cr-C:H films with Cr and CrN interlayers give satisfactory wear performance in a ball-on-disk test with a lower wear rate of 2×10 −17 m 3 /Nm and a lower friction coefficient of 0.1 sliding against WC counterparts. Wear life of Cr plasma activated DLC (Cr-C:H) outperforms that of Ti and Zr by nine- and twofold, respectively. The Cr-C:H film exhibits a dense microstructure and a higher sp 3 bond ratio, showing great potential for wear applications.

Deposition and characterization of Ti(C,N,O) coatings by unbalanced magnetron sputtering

Several TiCNO coatings were deposited using an unbalanced magnetron sputtering system. The coating properties as a function of oxygen/nitrogen flow ratio were studied using glow discharge optical spectrometer, X-ray diffraction, scratch testing and nano-indentation measurement. The tribological properties of the coatings were then investigated using a ball-on-disk setup with alumina balls. The results show that coating properties and performance are greatly affected by the flow rate of oxygen. With oxygen flow rate at 4 sccm during deposition, the TiCNO coating shows the lowest wear rate among all. Further increase in oxygen flow rate caused a decrease of hardness, adhesion and wear resistance, while together with the increase of friction coefficient. Also found is that the color of these coatings changes as a function of oxygen flow rate, indicating that these coatings can be used as decorating thin films with wide variety of choices.

Microstructural characterization of Al/Mg alloy interdiffusion mechanism during accumulative roll bonding

In this study, the Accumulative Roll Bonding (ARB) process was used with a snap-stack procedure to reduplicate an Al (1100)/Mg (AZ31) alloy. Samples underwent four rolling and stacking cycles four times, which produced a 24-layer structure. The ARB process creates a multilayer compound between Al/Mg layers with excellent bonding characteristics. The excellent bonding characteristics were due to atomic diffusion. Diffusion couples between Al and Mg were investigated to study the composition of the formation in the Al−Mg system. Layers of intermetallic compound (IMC) Al3Mg2 and Al12Mg17 were observed. The composition-depth curves of the diffusion zone were determined by electron microprobe analyses of the IMCs.

Growth of intermetallic phases in Al/Cu composites at various annealing temperatures during the ARB process

The purpose of this study is to discuss the effect of annealing temperatures on growth of intermetallic phases in Al/Cu composites during the accumulative roll bonding (ARB) process. Pure Al (AA1100) and pure Cu (C11000) were stacked into layered structures at 8 cycles as annealed at 300 °C and 400 °C using the ARB technique. Microstructural results indicate that the necking of layered structures occur after 300 °C annealing. Intermetallic phases grow and form a smashed morphology of Al and Cu when annealed at 400 °C. From the XRD and EDS analysis results, the intermetallic phases of Al2Cu (θ) and Al4Cu9 (γ2) formed over 6 cycles and the AlCu (η2) precipitated at 8 cycles after 300 °C annealing. Three phases (Al2Cu (θ), Al4Cu9 (γ2), and AlCu (η2)) were formed over 2 cycles after 400 °C annealing.

Effects of energetic particle bombardment on residual stress, microstrain and grain size of plasma-assisted PVD Cr thin films

Residual stress, microstrain and grain size of ion plated Cr films deposited on molybdenum substrates were analyzed and studied. A model based on the effects of incident particle energy and/or momentum on the properties of these films is proposed and discussed. It was concluded that the film properties may be affected by both the momentum and energy transfer of the incident particles, or by either one.

Characterization of nitrogen-doped a-C:H films deposited by cathodic-arc activated deposition process

Nitrogen-doped CrC:H/N films containing chromium were synthesized by using cathodic-arc activated deposition (CAAD) process. Metal plasma with intensive ion energies catalyzes the decomposition of hydrocarbon gas (C2H2), and results in the deposition of amorphous carbon films. Nitrogen was introduced to form nitrogen-containing CrC:H/N films, which contained a mixture of sp2 and sp3 carbon bonds. The deposited carbon films consist of nano-composite CrC:H/N films on top of a graded chromium nitride interlayer. The wear properties were correlated with the nitrogen doping. CrC:H/N films exhibits steady state friction coefficients lower than 0.11 when deposited at low N2/C2H2 flow rate ratio (<15%). CrC:H/N possesses improved wear properties, resulting from its adhered dense microstructure and the nature of the transferred layer induced by the friction condition between the CrC:H/N coatings and the sliding counterpart.

Precipitation behavior of σ phase in fusion zone of dissimilar stainless steel welds during multi-pass GTAW process

The purpose of this study is to investigate the precipitation characteristics of σ phase in the fusion zone of stainless steel welds at various welding passes during a tungsten are welding (GTAW) process. The morphology, quantity, and chemical composition of the δ-ferrite and σ phase were analyzed using optical microscopy (OM), a ferritscope (FS), a X-ray diffractometer (XRD), scanning electron microscopy (SEM), an electron probe micro-analyzer (EPMA), and a wavelength dispersive spectrometer (WDS), respectively. Massive δ-ferrite was observed in the fusion zone of the first pass welds during welding of dissimilar stainless steels. The σ phase precipitated at the inner δ-ferrite particles and decreased δ-ferrite content during the third pass welding. The σ and δ phases can be stabilized by Si element, which promoted the phase transformation of σ→ϱ+λ2 in the fusion zone of the third pass welds. It was found that the σ phase was a Fe−Cr−Si intermetallic compound found in the fusion zone of the third pass welds during multi-pass welding.

Effect of vibration on microstructures and mechanical properties of 304 stainless steel GTA welds

This study investigates the microstructures and mechanical properties of 304 stainless steel at various vibration frequencies during simultaneous vibration welding. The experimental results demonstrated that simultaneous vibration welding could accelerate the nucleation and grain refinement of the microstructures. The effect of the grain refinement was more evident at the resonant frequency (375 Hz) and a minimum content of residual δ-ferrite (4.0%). The γ phase grew in the preferential orientation of the (111) direction with and without vibration. The full width at half maximum of the diffraction peak widened after the vibration, which was attributed to the grain refinement. The residual stress could be efficiently removed through simultaneous vibration welding when the amplitude of the vibration was increased. Furthermore, the lowest residual stress (139 MPa) was found when the vibration frequency was 375 Hz. The hardness and Young’s modulus exhibited slight increases with low and medium frequencies. The hardness values were increased by 7.6% and Young’s modulus was increased by 15% when the vibration frequency was resonant (375 Hz).

Dispersion strengthening behavior of σ phase in 304 modified stainless steels during 1073 K hot rolling

The dispersion strengthening behavior of the σ phase in 304 modified stainless steel as hot-rolled at 1073 K has been investigated in this study. The morphology, quantity and chemical composition of the δ phase were analyzed using optical microscopy (OM), X-ray diffractometry (XRD), ferritscope (FS), and image analysis (IA). The amounts of σ phase in the stainless steels increased gradually at 1073 K as the reduction ratio increased from 0% to 75%. The XRD analyses showed that a higher reduction ratio enhanced the conversion of δ-ferrite (110) to σ phase (542). The σ phase was precipitated homogeneously at the recrystallized ferrite grains when the reduction ratio was increased from 0% to 75%.