Formation Of Brittle Phases Research Articles

Liquid Nitriding of Stainless Steel 316L to improve fatigue properties for Orthopedic Screws

Liquid nitrate is an important method used to improve mechanical properties. One of these properties is resistance to fatigue. The aim of this study was to improve the fatigue resistance of the stainless steel 316L. The rotational bending method was used with constant and variable stresses at different times of (1, 3, 5) hours and at (530, 630) C0. These tests were performed before and after nitration.
 The results showed that the depth of the nitride layer was (0.21, 0.33, 0.45) mm, increasing with time nitriding when the temperature was 530 C0. While the depth of this layer at a temperature of 630 C0 (0.26, 0.39,0.5) mm with increasing time. As a result of these processes, a layer of solid chromium nitrides and other phases of iron nitride were formed on the outer surface. These layers helped to inhibit the growth of the cracks and their progress in addition to the generation of pressure stresses on the surface leading to obstructing the progress of the cracks.
 This study showed that the fatigue resistance was directly proportional to the increase in nitrate time due to the increased depth of the hardened layer, but this resistance decreased when the temperature was 630 C0 due to the formation of brittle phase with low resistance.

Dissimilar joining of TiAl alloy and Ni-based superalloy by laser welding technology using V/Cu composite interlayer

Joining TiAl alloy to Ni-based superalloy is of great interest for applications in aerospace and automobile field. In present study, we report the feasibility of TiAl alloy to Ni-based superalloy welding through V/Cu composite interlayer carried out by laser. Detailed microstructural characterization, mechanical properties and fractured behavior of the joints were performed. The V/Cu composite interlayer remarkably decreased the formation of brittle phases and eliminated subsequent cracking compared with no interlayer and eventually improvement of mechanical properties of the joints. The tensile strength of room temperature and high temperature (873 K) were 230 MPa (reaching about 60% of the TiAl base metal) and 145 MPa, respectively. The study results indicate that insertion of V/Cu composite interlayer is very useful for laser welding of TiAl/Ni-based superalloy.

Effect of Boron Content on Microstructure, Mechanical Properties, and Oxidation Resistance of Mo-Si-B Composites

Mo-Si-B composites with boron content from 0 to 8 wt.% were prepared by spark plasma sintering using Mo, Si, and B elemental powders. The effects of boron content on the phase composition, mechanical properties, and high-temperature oxidation resistance of the Mo-Si-B composites were investigated. The results show the evolution of the major phase constitution of the composites with increasing boron content is in the following sequence: (MoSi2, Mo5Si3) → (MoSi2, Mo5Si3, Mo5SiB2) → (MoSi2, MoB) → (MoSi2, MoB, MoB2). As the boron content increases, both the hardness and flexural strength of the composites increases, but the fracture toughness of the composites gradually decreases. The strengthening mechanisms for the composites are second-phase strengthening and grain refinement strengthening. However, the formation of brittle phases, i.e., Mo5SiB2, MoB, and MoB2, reduces the toughness of the composites. In addition, since a dense barrier layer of SiO2-B2O3 formed on the surface of the composites inhibits the inward diffusion of oxygen, B-doped Mo-Si-B exhibits better oxidation resistance compared with non-doped composites.

Effect of the Conditions of Crystallization of Rapidly Quenched TiNiCu Amorphous Alloys with High Copper Contents on the Characteristics of Martensite Transformations and Shape Memory Effect

Amorphous TiNiCu alloys with copper contents of 28 to 38 at % are fabricated via melt spinning. The isothermal crystallization of alloys is conducted at 500°C with variable durations of the heat treatment (100 to 300 s). It is shown that shortening the duration of crystallization prevents the formation of brittle phases of the Ti–Cu system and contributes to the martensitic B2 ↔ B19 transformation, with the temperature intervals of transformation shifting to higher values and a marked increase in the enthalpy of the martensitic transitions and the magnitude of the shape memory effect.

Interlayer engineering for titanium clad steel by hot roll bonding

Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 °C with a total reduction ratio of 86.7%. Interfacial microstructure and bonding properties of titanium clad steel plates were investigated by electron microscopy and mechanical tests. The results showed β-Ti, TiC and Fe2Ti reaction phases were generated at Ti/steel interface for the clad plates with no interlayer. Inserting Nb or Mo interlayer can effectively suppress the formation of brittle phases, while the weak bonding joint transferred to Nb/steel or Mo/steel interface. And some micro-voids were found at the interface of Nb/steel and Mo/steel. The improved shear strength of clad plates with Nb interlayer might be attributed to the elimination of brittle phases at bonding interface. The small size and little quantities of the micro-voids at Nb/steel interface had a relatively weak effect on shear strength. However, the large number and big size of micro-voids were responsible for the degradation of shear strength for the clad plates with Mo interlayer.

Effect of alumina on grain refinement of Al-Si hypereutectic alloys

The size, volume fraction and distribution of primary as well as eutectic silicon affect the mechanical properties of the Al-Si hypereutectic alloys. It is very difficult for the simultaneous refinement and modification of primary and secondary Si particles in hypereutectic Al-Si alloys through traditional processes. This paper explores the role of γ-Al2O3 nanoparticles on Si particles in the course of solidification in hypereutectic Al-Si alloys at particular pouring temperature. The present study involves incorporation of varying contents dispersed γ-Al2O3 nanoparticles into a molten base metal during stir casting and followed by solidification. It has been reported that the synthesized composites having good interfacial bonding (wetting) between the dispersed phase and the liquid matrix was achieved in order to provide improved mechanical properties of the composite. The cast product of hypereutectic Al-16Si alloy with the reinforcement of nanoparticles, illustrated a significant improvement in both wear behaviour and hardness. The dry sliding wear test has been performed on a group of specimens with varying parameters (different loads and sliding velocities) in a pin on disc wear testing machine. Moreover, the wear rate and specific wear rate also affected in different load and different sliding velocities. However in XRD analysis of the samples, the enhancement of wear resistance as well as hardness was due to the formation of brittle phases like SiO2, Al2O3 and Al-rich intermetallic compounds. The hardness value of the materials increases nearly 6% in addition to increase in the density of only 0.8%. As per literature, the large plate eutectic Si particles were modified in to the fine core particles and it acquires enough potential for various applications.

Preparation and Characterization of TiB2-(Supra-Nano-Dual-Phase) High-Entropy Alloy Cermet by Spark Plasma Sintering

This paper introduces the preparation method and characterization results of TiB2 ceramics with CoCrFeNiAl high-entropy alloy (HEA) as a sintering aid by Spark Plasma Sintering (SPS). Good wettability between HEA and TiB2 was proved by the sessile drop method, indicating promising prospects for this composite. The sintering results showed that the addition of HEA could dramatically promote the sinterability of TiB2. TiB2-5 wt. % HEA dense ceramics prepared at the optimal temperature of 1650 °C showed fine morphology without formation of brittle phases. The liquid phase in the ceramics was highly consistent with the so-called “supra-nano-dual-phase materials (SNDPM)”, with near-ideal strength. This study represents the first time that a ceramic-SNDPM composite has been fabricated since the invention of such structures.

Comparative study of mechanical and wear behavior of Cu/WS2 composites fabricated by spark plasma sintering and hot pressing

The mechanical and wear behavior of copper-tungsten disulfide (Cu/WS2) composites fabricated by spark plasma sintering (SPS) and hot pressing (HP) was investigated, comparatively. Results indicated that the addition of lubricant WS2 substantially reduced wear rate of the Cu matrix composites fabricated by SPS, and the optimum content of WS2 is 20 wt% with regard to the wear behavior. However, it affected a little to the wear rate while dramatically decreased the friction coefficient of the composite fabricated by HP. This difference in friction behavior of the self-lubricating composites fabricated by the two techniques was closely related to their different mechanical properties. Severe interfacial reaction occurred during spark plasma sintering, leading to brittle phase formation at interface.

Effect of Ti interlayer on the bonding quality of W and steel HIP joint

Tungsten (W) and steel bonding is one of the key technologies for blanket First Wall (FW) manufacture in thermal fusion reactor. The W/Steel joints are prone to fail without interlayer for the different thermo physical properties. To study the effect of titanium (Ti) interlayer on the bonding quality of W and steel joints, W/Steel Hot Isostatic Pressing (HIP) experiments with Ti interlayer were conducted under 930 °C, 100 MPa for 2 h. Intermetallics caused by atom interdiffusion would affect the bonding quality of W/Ti/Steel HIP joints, the bonding quality was evaluated by microstructure analysis and mechanical tests. All the HIP joints were well bonded and results showed no intermetallics occurred between W/Ti interfaces, meanwhile multiply phases were found between Ti/Steel interfaces. Shear tests indicated when Ti thickness was 100–500 μm, the maximum shear strength of W/Ti/Steel HIP joints would be up to around 151 MPa. Charpy impact tests showed the W/Ti/Steel HIP joints all broke in a brittle manner and the maximum Charpy impact energy was ∼0.192 J. Nano-indentation tests demonstrated W/Ti interfaces could be enhanced by solid solution hardening and formation of brittle phases has conducted high hardness across the Ti/Steel interfaces.

Optimization of Process Parameters of Hybrid Laser–Arc Welding onto 316L Using Ensemble of Metamodels

Hybrid laser–arc welding (LAW) provides an effective way to overcome problems commonly encountered during either laser or arc welding such as brittle phase formation, cracking, and porosity. The process parameters of LAW have significant effects on the bead profile and hence the quality of joint. This paper proposes an optimization methodology by combining non-dominated sorting genetic algorithm (NSGA-II) and ensemble of metamodels (EMs) to address multi-objective process parameter optimization in LAW onto 316L. Firstly, Taguchi experimental design is adopted to generate the experimental samples. Secondly, the relationships between process parameters (i.e., laser power (P), welding current (A), distance between laser and arc (D), and welding speed (V)) and the bead geometries are fitted using EMs. The comparative results show that the EMs can take advantage of the prediction ability of each stand-alone metamodel and thus decrease the risk of adopting inappropriate metamodels. Then, the NSGA-II is used to facilitate design space exploration. Besides, the main effects and contribution rates of process parameters on bead profile are analyzed. Eventually, the verification experiments of the obtained optima are carried out and compared with the un-optimized weld seam for bead geometries, weld appearances, and welding defects. Results illustrate that the proposed hybrid approach exhibits great capability of improving welding quality in LAW.

Activities of HIP joining of plasma-facing armors in the blanket first-wall in Korea

Joining technology for dissimilar materials was developed for the fabrication of an ITER blanket first-wall, which consisted of Be, CuCrZr, and stainless steel (SS). The Be/CuCrZr/SS joint was fabricated using a hot isostatic pressing (HIP) method. Beryllium armor was joined to the CuCrZr/SS block at 580°C under 100MPa. The optimal interlayer coatings of Cr/Cu and Ti/Cr/Cu were developed using an ion-beam assisted physical vapor deposition. Beryllium is also a candidate armor material for the TBM first-wall. Successful joining of Be to ferritic-martensitic steel (FMS) was accomplished using an HIP method by introducing the thick diffusion barrier. A thick diffusion barrier of a Cu foil(10μm) limited the excessive diffusion and prevented the formation of brittle phases at the Be/FMS interface. Be and FMS were bonded at 650–850°C; however, a temperature of lower than 750°C was recommended to avoid material degradation of FMS. In addition, the joining of W to FMS has been developed. Tungsten is another armor material applicable to more severe plasma conditions. The large difference in the thermal expansion between W and FMS was resolved by introducing the Ti interlayer and Mo separator. Moreover, the double-staged HIP (the first stage at 900°C and 100MPa and the second stage at 750°C and 70MPa) was applied to suppress the edge delamination of W/FMS joints during thermal history.

Classification and microstructural stability of high generation single crystal Nickel-based superalloys

This paper presents a brief review on various types of single crystal Ni-based superalloys designed over the recent years. The design principles to achieve required mechanical properties and microstructural stability over the long time exposure to high temperatures are emphasized. The main attention is given to alloys with a high content of refractory elements. In particular the formation of brittle phases, like topologically close packed (TCP) phases, degrading mechanical properties of the Ni-based superalloys is emphasized.

Fabrication, Microstructure and Shear Properties of Al Foam Sandwich

Aluminum foam sandwich (AFS) structure materials have drawn much attention due to their unique structural and functional properties. However, the use of AFS materials as an attractive candidate for some applications was limited. In this work, AFS was fabricated by the brazing method, using Al-Si-based alloy, aluminum foam and steel plates as filler, matrix and panels, respectively. The microstructures of the soldered interfacial region, elemental distributions and phase identification were determined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The diffusion behavior of Si, Al, Fe and Cr was investigated. The effect of brazing time on the shear strengths of soldered joints was also analyzed. The results showed that the microstructures of joints were changed on increasing the brazing time. When the brazing time was 5 min, the solder started melting, which led to low shear strength. When increasing the brazing time to 10 min, the shear strength reached the peak value of 6.26 MPa due to the formation of a layer plate structure in the joints. When further increasing the brazing time, a number of Al/Fe intermetallic compound were formed in the joints, leading to a decrease of the shear strength due to the formation of brittle phase.

Microstructural and mechanical characterisation of laser-welded high-carbon and stainless steel

Laser welding is becoming an important joining technique for welding of stainless steel to carbon steel and is extensively used across various sectors, including aerospace, transportation, power plants, electronics and other industries. However, welding of stainless steel to high-carbon steel is still at its early stage, predominantly due to the formation of hard brittle phases, which undermine the mechanical strength of the joint. This study reports a scientific investigation on controlling the brittle phase formation during laser dissimilar welding of high-carbon steel to stainless steel. Attempts have been made to tailor the microstructure and phase composition of the fusion zone through influencing the alloying composition and the cooling rate. Results show that the heat-affected zone (HAZ) within the high-carbon steel has significantly higher hardness than the weld area, which severely undermines the weld quality. To reduce the hardness of the HAZ, a new heat treatment strategy was proposed and evaluated using a finite element analysis-based numerical simulation model. A series of experiments has been performed to verify the developed thermo-metallurgical finite element analysis (FEA) model, and a qualitative agreement of predicted martensitic phase distribution is shown to exist.

Compound materials by PM-HIP

There are many applications where compound materials can be of interest, for example when different properties are needed in different parts of a component. Compound materials can be produced by hot isostatic pressing (HIP) of powder metallurgical materials. One aspect that should be considered in the design is the quality of the interface between the two different material compositions. Diffusion during HIP can cause formation of brittle phases in the interface or deteriorate properties by diffusion of alloying elements. The present work shows results from a study where different steel types were joined (quench and temper steel/air hardening steel/bearing steel with a tool steel/corrosion resistant martensitic steel). The evaluation was performed by computational predictions and by small scale HIP experiments that were evaluated by microstructure analysis and chemical analysis.

Laser welding of Ti–6Al–4V to Nitinol

Formation of brittle intermetallic phases in addition to different thermal expansion coefficients associated with dissimilar welding leads to the formation of transverse cracks in weld metal and eventually restricts widespread applications of dissimilar joints. Therefore, joining technology should be expanded in field of dissimilar welding in order to solve its difficulties. In the present study, an experimental work with pulsed Nd:YAG laser was performed for dissimilar welding of Ti–6Al–4V and Nitinol. Autogenous welding of these two alloys resulted in joints with poor strength and ductility due to the formation of transverse cracks in the weld metal. Therefore, the chemical composition of the weld metal has to be modified in order to reduce the formation of brittle phases and eliminate subsequent cracking. In this work, this was done by insertion of a copper interlayer with a thickness of 75μm between the base metals. The results indicated that insertion of copper interlayer has a great influence on the reduction of the amount of Ti2Ni brittle intermetallic phase, elimination of transverse cracks through the weld metal and eventually improvement of mechanical properties of the joints. Insertion of copper interlayer was very useful since it altered the cracked autogenous joint to a joint which could withstand a tensile stress of 300MPa.

Formation of Brittle Phases During Pulsed Current Gas Tungsten Arc Welding of Titanium to Aluminum Alloys

Welding of titanium alloy TA15 to aluminum alloy Al 2024 was conducted by pulsed current gas tungsten arc welding using AlSi12 filler metal. Formation process of phases near the Ti/Al interface was discussed. Titanium and aluminum were partially fusion welded in the upper part while brazed together in the middle and bottom parts of the joint. In the upper part of the joint, intermetallics Ti3Al + Ti5Si3, TiAl + Ti5Si3, and TiAl3 were formed as three layers orderly from the titanium side to the weld metal. In the middle and bottom parts of the joint, intermetallics Ti5Si3 and TiAl3 were formed as two layers near the Ti/Al interface.

Fabrication of Bulk Metallic Glass Composites at Low Processing Temperatures

Bulk metallic glasses (BMGs) are amorphous alloys that exhibit unique mechanical properties such as high strength due to their liquid-like structure in the vitreous solid state. While they usually exhibit low ductility, they can be toughened by incorporating secondary phase particles within the amorphous matrix via composite fabrication to generate amorphous metal matrix composites (MMCs). Traditional MMCs are fabricated at high temperature in the liquid state with tedious blending processes. This high temperature processing route often leads to unwanted reactions at the reinforcement/matrix interface, creating brittle intermetallic by-products and damaging the reinforcement. In the present work, novel bulk metallic glass composites (BMGCs) were fabricated at low processing temperatures via thermoplastic forming (TPF) above the glass transition temperature of the amorphous matrix. Here, the unique thermophysical features of the matrix material allow for TPF of composites in non-sacrificial moulds incorporating various types of reinforcement, via processing in the solid state at low temperatures (less than 200 °C), within a short timeframe (less than 10 minutes); this avoids the formation of brittle phases at the reinforcement/matrix interface leading to efficient bonding between particles and matrix, thereby creating a tough, low density composite material.

Using graphitic foam as the bonding material in metal fuel pins for sodium fast reactors

The study evaluates the possible use of graphite foam as the bonding material between U–Pu–Zr metallic fuel and steel clad for sodium fast reactor applications using FEAST-METAL fuel performance code. Furthermore, the applicability of FEAST-METAL to the advanced fuel designs is demonstrated. Replacing the sodium bond with a chemically stable foam material would eliminate fuel clad metallurgical interactions, and allow for fuel swelling under low external stress. Hence, a significant improvement is expected for the steady state and transient performance. FEAST-METAL was used to assess the thermo-mechanical behavior of the new fuel form and a reference metallic fuel pin. Nearly unity conversion ratio, 75% smear density U–15Pu–6Zr metallic fuel pin with sodium bond, and T91 cladding was selected as a reference case. It was found that operating the reference case at high clad temperatures (600–660°C) results in (1) excessive clad wastage formation/clad thinning due to lanthanide migration and formation of brittle phases at clad inner surface, and (2) excessive clad hoop strain at the upper axial section due mainly to the occurrence of thermal creep. The combination of these two factors may lead to cladding breach. The work concludes that replacing the sodium bond with 80% porous graphite foam and reducing the fuel smear density to 70%, it is likely that the fuel clad metallurgical interaction would be eliminated while the fuel swelling is allowed without excessive fuel clad mechanical interaction. The suggested design appears as an alternative for a high performance metallic fuel design for sodium fast reactors.

Brazing of Boron Carbide by Cu-Alloys: Interface Interaction and Mechanical Properties of Joints

Boron carbide is one of the most promising structural ceramics; therefore its joining to other ceramics and metals is of technological importance. In this study, interface interactions and joints strength were studied in various cases of B4C/Cu based fillers (Cu-9at%B, Cu-4.5at%Ti and Ticusil®). The fillers partially wet B4C (q < 500), but the interface composition and morphology are different. In the B4C/Cu-9at.%B system no new phases are detected at the interface, while the formation of TiB2 takes place in the case B4C/Cu-4.5at%Ti, and a double layer containing TiB2 and TiC is formed at the B4C/Ticusil® interface. The maximal shear stress for B4C/B4C joints breakage was obtained for the Cu-9at.%B brazing alloy which displays partial wetting without the formation of brittle phases at the ceramic/metal interface. This study shows that improved wetting is essential but not sufficient in order to obtain strong bonding between B4C and the fillers examined.