Grain Boundaries Of Matrix Research Articles

Wear Resistance of Boron-Modified Supermartensitic Stainless Steel Coatings Produced by High-Velocity Oxygen Fuel Process

The wear resistance of boron-modified supermartensitic stainless steel coatings produced by high-velocity oxygen fuel (HVOF) was investigated through pin-on-disk measurements. It was shown that addition of boron leads to the formation of an interconnected and rigid boride network delimiting the grain boundaries of the martensitic matrix. A very refined structure was formed as result of the high cooling rates imposed to the molten alloy during the HVOF process. The specific wear rates of the HVOF coatings were about tenfold lower than the boron-free supermartensitic stainless steel, lying in the order of 10−5 mm3/N m. The refined boride skeleton along the HVOF coating was found to be effective to reduce the materials’ removal from the exposed softer martensitic matrix. While the supermartensitic stainless steel master alloy and the mild steel substrate displayed severe adhesive wear, the HVOF coatings exhibited mild delamination wear at low sliding velocities (10 and 20 cm/s) and abrasive wear at the highest tested velocity (40 cm/s). The studied boron-modified supermartensitic HVOF coatings are an interesting approach to protect the surface of inexpensive steel substrates against wear.

Enhanced electrical resistivity and mechanical properties in BCTZ-based composite ceramic

Environmental and human health concerns about lead toxicity have prompted the development of lead-free piezoceramics. Among them, ([Formula: see text][Formula: see text]([Formula: see text][Formula: see text]O3 (BCTZ) with excellent piezoelectric properties has the most potential and attracts extensive attention. However, lack of concern toward electrical resistivity and mechanical properties has greatly hindered its practical application. Here, we report the achievement of enhanced insulation characteristics (grain electrical resistivity increased by one order of magnitude) and superior mechanical properties (Vickers hardness value increased by 40%) in Al2O3-added BCTZ composite ceramics. Such improvement can be attributed to specific composite microstructure, where the nonferroelectric second phase dispersed in the grain interior and grain boundary of BCTZ matrix results in blocking effect on the electric current paths as well as propagation of microcracks. These findings will pave a new way for the practical application of BCTZ ceramics.

A coupled, physics-based matrix-grain boundary model for creep of carbide strengthened nickel-based superalloys - II. Experimental results and model application

Materials which precipitate secondary phases in intergranular regions suppress grain boundary sliding directly as the carbides act as pinning sites resulting in dislocation accumulation and the generation of back stress. Materials which precipitate in intragranular regions suppresses grain boundary sliding indirectly by limiting the rate at which dislocations arrive to the boundary, Reference [1]. It is considered that grain boundary sliding necessitates a supply of extrinsic dislocations that are released from the matrix during creep. As carbides act as obstacles to dislocation motion, their presence in the matrix suppresses the rate at which dislocations can arrive to the boundary to facilitate grain boundary sliding.Aging and stress relaxation experiments have been conducted on the nickel-based superalloy, Inconel 617, to investigate the role of intragranular and intergranular carbides on grain boundary sliding. All specimens were solutioned for 2 h at 1200 °C and water quenched prior to testing to dissolve remaining secondary phases. Following solutioning, aging at 1000 °C for increasing durations produced unique microstructural parameters of carbide size, spacing, and volume fraction within the grain and along the grain boundary. Carbide size, spacing, and volume fraction and grain size were all observed to increase with increasing exposure; for up to 30 h of aging.Stress relaxation tests at 780 °C with an initial stress of 200 MPa were then conducted on the aged specimens to investigate and analytically model the rate controlling properties of matrix and grain boundary carbides as they pertain to grain boundary sliding according to the model formulation presented in Reference [1]. Relaxation rate of Inconel 617 drastically decreased after aging at 1000 °C for 3 h with little variation in relaxation rate up to 30 h of aging prior to testing. Using the model presented in Reference [1] combined with the experimental results presented below, a reload test has been simulated, considering the combined role of both the matrix and grain boundary on the overall sliding rate.

A coupled, physics-based matrix-grain boundary model for creep of carbide strengthened nickel-based superalloys - I. Concepts and formulation

At elevated temperatures and moderate to high stresses, dislocation creep is considered the dominating mechanism of deformation governed by the motion of dislocations through the matrix and along the grain boundary. A local increase in dislocation activity within the matrix has been observed to accelerate grain boundary sliding along boundaries intersecting the active slip system. The increase in rate is considered here as a function of the increase in supply of intragranular dislocations to the grain boundary, thereby facilitating grain boundary sliding as the dislocations glide under the applied shear stress. As such, the rate at which dislocations arrive to the boundary from the matrix controls the rate at which the grain boundary can slide. One means of suppressing dislocation mobility through both the matrix and along the grain boundary is the precipitation of secondary phases.Materials which precipitate secondary phases in intergranular regions suppress grain boundary sliding directly, acting as pinning points, while materials which precipitate in intragranular regions suppress grain boundary sliding indirectly by limiting the rate at which dislocations arrive to the boundary. To consider the rate controlling properties of both intra- and intergranular carbides on grain boundary sliding. A physics-based model for creep is presented, which considers the microstructural rate controlling features of both the matrix and grain boundary as it pertains to dislocation mobility and the effect on the overall grain boundary sliding rate.Mobility of dislocations through the matrix and along the grain boundary is modeled as a function of microstructural condition – namely M23C6 carbide size, spacing, and volume fraction. The motion of dislocations within the grain boundary is modeled as a viscous flow process. It is considered that grain boundary sliding requires a supply of extrinsic dislocations from the matrix which, upon absorption into the grain boundary, glide and subsequently pile-up at irregularities (triple points, carbides, etc.) under the influence of an applied shear stress (or traction). With increasing number of dislocations in the pile-up, back stress accumulates at the interface, reducing the effective stress and subsequent sliding rate. Dislocation mobility through the matrix as it relates intragranular microstructure has been simulated, from which the rate controlling properties of intragranular particles is investigated.

Normal ve Kriyojenik Şartlarda Öğütmenin Aktive Edilmiş Sinterleme Süreçleri ile Geliştirilen Tungsten Esaslı Kompozitlerin Mikroyapı ve Özellikleri Üzerindeki Etkisi

Tungsten matrix composites reinforced with TiB2 and Y2O3 particles were fabricated by milling under ambient/cryogenic conditions and Ni activated sintering. Powder blends constituting the W - 1 wt. % Ni - 2 wt. % TiB2 - 1 wt. % Y2O3 composition were mechanically milled for 12 h under ambient condition or cryomilled for 10 min or sequentially milled under ambient and cryogenic conditions. Milling was carried out in a high-energy ball mill under ambient condition whereas cryogenic milling was conducted in externally circulated liquid nitrogen. Milled powders were compacted using a hydraulic press and the pellets were sintered at 1400°C for 1 h under Ar / H2 gas flowing conditions. The effects of different milling types on the microstructural and mechanical properties of the sintered composites were investigated. After sintering, in addition to dominant W phase, small amounts of WB and NiW phases were detected in all sintered samples. The application of cryomilling after milling at ambient condition provided the disappearance of the clustered TiB2 and Y2O3 particles in the sintered sample: They were located at the grain boundaries of W1Ni matrix and homogeneously distributed through the microstructure. Sequentially milled and sintered composite had the highest relative density (95.77 %) and the highest microhardness (7.23 GPa) values among the samples. Nano-indentation tests showed that there was an improvement in the hardness and elastic modulus of W matrix phase, which yielded the values of 8.9 and 373.7 GPa, respectively.

Effect of niobium addition on the flux of submerged arc welding of low carbon steels

The present work evaluates the effect of adding 3 wt.% of NbC and Nb2O5 to a commercial SAW flux on the microstructure and mechanical properties of the weld metal of ASTM 131 AH31 plates. The analyses of the weld bead microstructure indicated the presence of three weld zones, as expected, enhancing the presence of fine and coarse HAZ. Microscopy and EDS indicated that Nb dissociates from the original compound and precipitates along the grain boundaries of the iron matrix as niobium oxide. Mechanical characterization evidenced that NbC and Nb2O5 added to a commercial SAW flux increased the microhardness values by ∼40% and ∼140%, respectively. The addition of NbC decreased the ductility of the weld metal, whereas the addition of Nb2O5 increased it. The results obtained indicate a potential product for high wear resistance applications.

Laser 3D printing of CoCrFeMnNi high-entropy alloy

Due to their superior properties, high-entropy alloys (HEAs) are considered as novel structural materials that can substitute conventional alloys. From the viewpoint of future applications, it is important to explore methods for producing complex shaped and homogeneous HEAs. In this study, laser 3D printing technology is employed to fabricate CoCrFeMnNi HEA. The microstructure and mechanical properties of laser 3D printed HEA are also evaluated. An equiaxed-to-columnar transition structure can be observed in the melt pool of the printed sample. The fine BCC phase is found to distribute at the grain boundaries of the FCC matrix, which is the major phase of the printed sample. The printed HEA exhibits an outstanding combination of high strength and excellent ductility.

The microstructures, mechanical properties, and temperature distributions in nodular cast iron friction-welded joint

The paper focuses on an experimental topic relating to the field of friction welding process of a nodular cast iron. The microstructures, phase transformation, temperature distributions, microhardness, and tensile test are all studied within the framework of the paper in question. Maximum temperature measurements in the axial center and periphery of the analyzed joints were equal to 950 and 840 °C, respectively. Both temperature and increasing temperature gradient at the axial center were higher than those at the periphery. The maximum tensile strength of the examined friction-welded nodular cast iron joints was 53% of that of the parent metal. The welding region was composed of deformed graphite nodules, coarse pearlite, proeutectoid ferrite, and acicular martensite. Highly deformed graphite nodules were distributed along the weld interface due to the material flow in the thermo-mechanically affected zone (TMAZ). In the central zone, graphite displayed a striped configuration and ferrite transformed into a martensite structure. In the peripheral region, graphite surrounded by martensite kept the form of individual granules. Maximum hardness at the interface in the TMAZ and the heat-affected zone reached 603 HV and 345 HV, respectively. The executed microstructure analysis showed that the cracks started occurring mostly at the interface of the deformed graphite nodules and then spread through the grain boundaries of metal matrix. The fracture surface appearance showed a cleavage fracture in the peripheral zone and a little dimple fracture around graphite nodules in the central zone.

Experimental and numerical investigations of interface characteristics of copper/steel composite prepared by explosive welding

Copper-steel composite is an attractive candidate for cooling staves applied in the blast furnace because of their lower cost and high deformation resistance ability. However, systematic researches on the application effect of composites are insufficient. In this work, a novel copper-steel composite stave was developed by explosive welding process. The experimental and numerical investigations were carried out on the interface of copper steel composite. The results showed that copper-steel composite cooling stave possesses the similar cooling capacity and higher anti-deforming capability than copper cooling staves. The interface is characterized by a wave structure and melted zones in the vortex region are surrounded by intensely deformed bulk materials. The grain structure in the interface indicated some grains were elongated in the direction of vortex. Refined grains and nano-sized grains of 56 nm were formed in the melted zones. The smaller nanocrystalline iron grains are dispersed uniformly at the grain boundary of the copper matrix and in the vicinity of the nanometer iron grains were tiny copper grains. The higher hardness in the deformed regions, superior local properties of the interface owing to nanocrystalline grains and typical dimple fracture confirm the good metallurgical bonding of the copper-steel composite prepared by explosive welding.

Synthesis and Thermoelectric Properties of WO<sub>3</sub>/Cu<sub>2</sub>SnSe<sub>3</sub> Composites

In this study, nanometer WO3 powder was uniformly dispersed into the Cu2SnSe3 powder by ball milling process, and the WO3/Cu2SnSe3 thermoelectric composite was prepared by spark plasma sintering (SPS). The results showed that the nano-WO3 particles were mainly distributed in the grain boundary of Cu2SnSe3 matrix, and the grain growth of Cu2SnSe3 was inhibited. The addition of nano-WO3 could enhance the electrical conductivity of Cu2SnSe3, and while the Seebeck coefficient increased slightly for the 0.4% WO3/Cu2SnSe3 composite. The thermal conductivity was not decreased until the content of WO3 exceeded 1.6%. The highest thermoelectric figure of merit ZT of 0.177 was achieved at 700 K for 0.4% WO3/Cu2SnSe3 composite. The enhancement of ZT value of WO3/Cu2SnSe3 thermoelectric material was mainly attributed to the improvement of the electrical properties.

Effect of Zn addition on microstructure and mechanical properties of Mg–9Gd–3Y–0.5Zr alloy

Abstract

Inert atmosphere processing of hydroxyapatite in the presence of lithium iron phosphate

The present study describes sintering behaviour of hydroxyapatite (HAp) upon addition of lithium iron phosphate (LFP) (1–10 wt.%) system in inert (Ar) atmosphere. The interaction between materials and melting of LFP influenced early and intermediate stages of HAp sintering, shifting the densification curves towards low-temperature side. Analysis of densification process indicated significant differences upon LFP addition. The reaction mechanism that assumes the initial interaction between phosphates from LFP and calcium from HAp was proposed, generating calcium vacancies and contributing to HAp densification. Cross-sections of sintered samples showed changes in microstructural properties, with uniform atomic distribution and presence of Li2Fe3O4 spherical inclusions (200 nm) located at grain boundaries of calcium phosphate matrix. The Rietveld refinement analysis indicated changes in structural and microstructural parameters like crystallite size, anisotropy and microstructural strain of HAp upon LFP addition. Mechanical characterisation indicated improvements in fracture behaviour upon LFP addition.

Study of (DNPs) x /CuTl-1223 Nanoparticle-Superconductor Composites

Sol-gel and solid-state reaction methods were used to synthesize diamond nanoparticles (DNPs) and (DNPs) x /CuTl-1223 (x = 0, 0.25, 0.50, and 1.00 wt.%) nanoparticle-superconductor composites, respectively. Effects of these DNPs on structural, morphological, compositional, and transport properties of CuTl-1223 superconducting phase were investigated by different experimental techniques such as X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), and resistivity versus temperature (R-T) measurements. The unchanged crystal structure and stoichiometry of host CuTl-1223 superconducting matrix with addition of DNPs gave evidence about the dispersion of nanoparticles at the grain boundaries of the host matrix, which may heal up the inter-granular voids and pores resulting in enhanced inter-grain connectivity. Critical transition temperature T c (0) and hole concentration of CuTl-1223 superconductor were observed to be increased with addition of DNPs up to a certain optimum value (i.e. x = 0.5 wt.%).

Microstructure and property evolutions of a novel Super304H steel during high temperature creeping

ABSTRACTHigh-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M23C6 carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization – desensitization process of the steel during creeping.

Investigation on the microstructure, room and high temperature mechanical behaviors and strengthening mechanisms of the (TiB+TiC)/TC4 composites

A hybrid of (TiB+TiC) reinforced Ti-6Al-4V (TC4) matrix composite was produced by solution ball milling and spark plasma sintering process. The reinforcements were uniformly dispersed into the β grain boundary of the TC4 matrix. The size of β-grain is markedly reduced from more than 300 μm to about 23 μm after adding 9.5 vol % reinforcements to the matrix. The room-temperature tensile test results show that yield strength of the composites was enhanced and eventually saturated with the addition of reinforcements. Among all the strengthening mechanisms, grain-refinement effect plays the most important role and contributes 58.5–76.3% increment of the room-temperature yield strength. TiC-particles contribute 15.9–32.2% and TiB-whiskers share the rest contribution. However, the existence of TiBw clusters weakens the load-bearing effect of TiB. The high-temperature yield strength of the composites was improved with increasing volume fraction of the reinforcements and decreased with increasing test temperature. The grain-refinement strengthening efficiency decreases with increasing temperature, while by contrast, the strengthening efficiency of TiC particles goes up. Owing to the combination effect of matrix softening and dynamic recovery process, no saturation of high-temperature yield strength is observed even when the maximum volume fraction of reinforcements (9.5 vol %) were added to the composites.

Grain boundary engineering with nano-scale InSb producing high performance InxCeyCo4Sb12+z skutterudite thermoelectrics

Thermoelectric semiconductors based on CoSb3 hold the best promise for recovering industrial or automotive waste heat because of their high efficiency and relatively abundant, lead-free constituent elements. However, higher efficiency is needed before thermoelectrics reach economic viability for widespread use. In this study, n-type InxCeyCo4Sb12+z skutterudites with high thermoelectric performance are produced by combining several phonon scattering mechanisms in a panoscopic synthesis. Using melt spinning followed by spark plasma sintering (MS-SPS), bulk InxCeyCo4Sb12+z alloys are formed with grain boundaries decorated with nano-phase of InSb. The skutterudite matrix has grains on a scale of 100–200 nm and the InSb nano-phase with a typical size of 5–15 nm is evenly dispersed at the grain boundaries of the skutterudite matrix. Coupled with the presence of defects on the Sb sublattice, this multi-scale nanometer structure is exceptionally effective in scattering phonons and, therefore, InxCeyCo4Sb12/InSb nano-composites have very low lattice thermal conductivity and high zT values reaching in excess of 1.5 at 800 K.

Microstructure and tensile properties of in situ polymer-derived particles reinforced steel matrix composites produced by powder metallurgy method

In this study, steel matrix composites were fabricated using 316L/ polycarbosilane (PCS) system via a novel semi powder metallurgy method including solution-assisted wet mixing and spark plasma sintering (SPS). PCS/316L powders could be obtained by coating the starting 316L powder with PCS, then followed by SPS. After sintering, M7C3 and iron silicide dispersion are in situ formed by the reaction between the 316L and PCS-derived SiC phase, and dispersed along the grain boundaries of the austenitic matrix forming a network structure and refining the microstructures of the metal matrix. Tensile tests revealed that the yield strength (YS) and ultimate tensile strength (UTS) are improved with increasing PCS content, within certain range of PCS content (0–3wt%). The 316L-3wt% PCS composite exhibits a YS of 526 ± 21MPa and a UTS of 898 ± 9MPa, representing a 49.4% and 38.6% improvement, respectively, when compared to pure 316L. The enhanced tensile properties of the composite is due to the network structure, refined microstructures and solid solution effects as well as good compatibility between M7C3 and austenitic matrix. However, with increasing amount of reinforcements above 3wt%, a slight decreased strength is observed.

Optimization of process parameters for spark plasma sintering of nano structured SAF 2205 composite

This research optimized spark plasma sintering (SPS) process parameters in terms of sintering temperature, holding time and heating rate for the development of a nano-structured duplex stainless steel (SAF 2205 grade) reinforced with titanium nitride (TiN). The mixed powders were sintered using an automated spark plasma sintering machine (model HHPD-25, FCT GmbH, Germany). Characterization was performed using X-ray diffraction and scanning electron microscopy. Density and hardness of the composites were investigated. The XRD result showed the formation of FeN0.068. SEM/EDS revealed the presence of nano ranged particles of TiN segregated at the grain boundaries of the duplex matrix. A decrease in hardness and densification was observed when sintering temperature and heating rate were 1200°C and 150°C/min respectively. The optimum properties were obtained in composites sintered at 1150°C for 15min and 100°C/min. The composite grades irrespective of the process parameters exhibited similar shrinkage behavior, which is characterized by three distinctive peaks, which is an indication of good densification phenomena.

Effects of copper doping on the structure, electrical and low-field magnetoresistance properties of (1–x)La0.67Sr0.33MnO3/xCu (x=0–0.15) composite coatings

The doping effect of metallic Cu on the performances of manganite coatings has been investigated. The (1–x)La0.67Sr0.33MnO3/xCu (x=0–0.15) composites were prepared by the sol-gel based screen printing method. XRD and SEM measurements revealed CuO phase to form by oxidation and to distribute mainly at grain boundaries (GBs) of the manganite matrix. Excess Cu entered the manganite lattice without changing its rhombohedral structure, resulting in a slight lattice expansion. The surface morphology of composite samples changed dramatically with different Cu contents, and average particle size refined with a low concentration of Cu. As the Cu content increased, the resistivity of the samples increased while the metal–insulator transition temperature decreased. The enhanced magnetoresistance (MR) effect close to the percolation threshold has been ascribed to the introduction of local disorder at GBs.

Fabrication of magnesium-boron carbide metal matrix composite by powder metallurgy route: Comparison between microwave and spark plasma sintering

Microwave and spark plasma sintering processes were used to produce magnesium-B4C metal matrix composites by powder metallurgy route. Magnesium powder with 5 wt% B4C in ethanol media was blended in a high-energy ball mill for 10 min. The sintering process was performed at 500° and 670 °C using spark plasma sintering and microwave furnace, respectively. XRD investigations revealed the crystallization of MgB2 and Mg2C3 compounds during both sintering processes. The mechanical examinations showed higher bending strength (191 ± 15 MPa) and microhardness (92 ± 7 Vickers) of SPS samples rather than microwave samples. FESEM images demonstrated uniform distribution of B4C reinforcements at grain boundaries of metallic matrix. The MAP analysis of SPS samples approved the formation of new compounds as a result of the reaction between magnesium and B4C.