Hard Brittle Phase Research Articles

Influence of second phases on fatigue crack growth behavior of nickel aluminum bronze

The influence of phases with different morphology and mechanical properties on fatigue crack growth behavior in nickel aluminum bronze (NAB) has been investigated. Annealing at 675°C and normalizing at 920°C heat treatments were used to produce different morphologies and fractions of second phases. This analysis shows that the coarse dendritic κII particles and κIII lamellae as hard brittle phases in as-cast and annealed NAB have an accelerative effect on the fatigue crack propagation where by cracks propagate through α and κII/κIII interface. Fatigue cracks in normalized NAB prefer to propagate through the ductile α grains, form fatigue striations and have the lowest crack growth rate. The uniformly distributed, fine κIV precipitates in the α grains improves fatigue crack growth resistance. This work identifies the role of NAB second phases on propagation of fatigue cracks, and provides suitable heat treatment for improving fatigue crack resistance in terms of controlling second phase type, distribution and percentage.

Phase stability, electronic, elastic and thermodynamic properties of Al-RE intermetallics in Mg-Al-RE alloy: A first principles study

Electronic structure and elastic properties of Al2Y, Al3Y, Al2Gd and Al3Gd phases were investigated by means of first-principles calculations from CASTEP program based on density functional theory (DFT). The ground state energy and elastic constants of each phase were calculated, the formation enthalpy (ΔH), bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio (ν) and anisotropic coefficient (A) were derived. The formation enthalpy shows that Al2RE is more stable than Al3RE, and Al-Y intermetallics have stronger phase stability than Al-Gd intermetallics. The calculated mechanical properties indicate that all these four intermetallics are strong and hard brittle phases, it may lead to the similar performance when deforming due to their similar elastic constants. The total and partial electron density of states (DOS), Mulliken population and metallicity were calculated to analyze the electron structure and bonding characteristics of the phases. Finally, phonon calculation was conducted, and the thermodynamic properties were obtained and further discussed.

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.

Effects of Annealing on Microstructure and Phase Transformation Temperature of NiTiNb Shape Memory Alloy after Equal Channel Angular Pressing

The effect of annealing temperature（300-600°C）on the microstructure and transformation temperature of NiTiNb shape memory alloy rods deformed with equal channel angular pressing（ECAP） method was analyzed. The results show that the hard brittle phase （Ti,Nb）2Ni was decreased after ECAP. With the rising of annealing temperature, the number of β-Nb phase increase while eutectic declines. The DSC analysis shows that the martensite and reverse transformation temperature increase with annealing temperature. After annealing with 500°C, the martensite/austenite transformation starting temperatures rise to -64.6°C and -30.1°C, respectively.

Effects of annealing process on interface and mechanical properties of silver clad aluminum wires

The billet was prepared by putting the aluminum bar into the silver tube, and the silver clad aluminum wire was fabricated through cold hydrostatic extrusion and subsequent drawing. The effects of annealing parameters on the interface and properties of the wires were investigated. The results showed that when the annealing temperature was below 150°C and annealing time was less than 30min, no new phase was observed obviously, while new phase could be observed when the temperature reached above 200°C. The results of energy spectrum analysis showed that new phase were mainly Ag3Al electronic compound (μ phase) which was hard brittle phase with the β-Mn structure. The effects of annealing temperature and time on the interface were different: for any annealing time in this study, the width of the interface rose markedly with the increase of annealing temperature, while annealing time had no significant effect on the width of interface when the temperature was low. Only when the temperature was comparatively high, the increment rate of width of the interface gradually increased with the rise of annealing time. The width of interfaces was 5∼7μm, 7∼9μm and 10∼12μm after annealing treatment under the temperature of 150°C, 200°C and 300°C respectively for 30minutes. If the annealing temperature was lower than 200°C, the main way was that silver diffused towards aluminum, but when the annealing temperature was up to 300°C, the reverse diffusion occurred. When annealing temperature was 150°C or 200°C, the tensile strength of composite wire decreased after increased with the increment of annealing time, while when it was 300°C, the tensile strength always increased. The elongation percentage of composite wire had an adverse change compared to that of tensile strength. The reason for the change of mechanical properties of composite wire is mainly that mechanical properties was synthetically influenced by three mechanisms called annealing softening, diffusion phase transformation of interface and alloying of sheath and core because of their interdiffusion. Annealing softening produced dominant effect in lower annealing temperature and shorter annealing time condition, while the other two mechanisms had main effect in higher annealing temperature and longer annealing time condition.

Notch toughness of Fe-based bulk metallic glass and composites

The notch fracture toughness of Fe 75Mo 5P 10C 8.3B 1.7 monolithic bulk metallic glass (BMG) and Fe 77Mo 5P 9C 7.5B 1.5 and Fe 79Mo 5P 8C 6.7B 1.3 BMG matrix composites with α-Fe as reinforcing phase, fabricated by suction mould casting, were evaluated. It was found that the monolithic BMG has a toughness of 27 MPa m 1/2, while the Fe 77Mo 5P 9C 7.5B 1.5 BMG composite reinforced by single α-Fe dendrite phase exhibits a higher toughness of nearly 40 MPa m 1/2. However, for the Fe 79Mo 5P 8C 6.7B 1.3 alloy with more dendrites, the toughness decreased up to 25 MPa m 1/2. Microstructure investigation reveals that the simultaneous formation of Fe–Mo–P hard brittle phase apart from α-Fe dendrites in the Fe 79Mo 5P 8C 6.7B 1.3 alloy is the reason for the degradation of the fracture toughness.

Effects of Deformation and Aging on Microstructure and Performances of CuSn<sub>10</sub>Zn<sub>2</sub>FeCo Alloy by Centrifugal Casting

CuSn10Zn2FeCo alloy has been fabricated by using vacuum melting and centrifugal casting technology. The effect of deformation and aging on microstructure and mechanical properties of the alloy is investigated by SEM, XRD and TEM. The results show that Cu-Sn-Zn-Fe-Co alloy reinforced with iron nanoparticles has been fabricated at condition of enforced flow, most of them with diameters form 2 to 20 nm, with strong strengthening effect. The precipitation particles have coherent or semi-coherent relationship to the copper matrix. The optimum cold rolling and heat treatment processing is proved to be solution treatment at 1023-1070K for 1.5h followed by 40% cold rolling deformation then aged at 673K for 4h, under which the tensile strength 635 MPa, yield strength 505 MPa and elongation 20% can be obtained. The amounts of hard brittle phase significantly reduce and the segregation of Sn element eliminates. The high strength of the alloy can be explained by substructure strengthening due to cold rolling and precipitation strengthening of the iron nanoparticles and other metallic compounds such as CoCu2Sn phase, the strengthening mechanism obeys Orowan mechanism.

On the weldability of grey cast iron using nickel based filler metal

Shielded metal arc welding process using nickel based filler metal was used to join grey cast iron. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was studied. PWHT included heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. By using nickel based filler metal, formation of hard brittle phase (e.g. carbides and martensite) in the fusion zone is prevented. Before PWHT, heat affected zone exhibited martensitic structure and partially melted zone exhibited white cast iron structure plus martensite. Applied PWHT resulted in the dissolution of martensite in heat affected zone and graphitization and in turn the reduction of partially melted zone hardness. Results showed that welding of grey cast iron with nickel based filler metal and applying PWHT can serve as a solution for cast iron welding problems.

Repair welding process of friction stir welding groove defect

The groove defect formed in the friction stir welding dramatically deteriorates weld appearances and mechanical properties of the joints owing to its larger size and penetration. Therefore, the friction stir repair welding was utilized to remove such a groove defect, and the focus was placed on the mechanical properties and microstructural characteristics of the repair joints so as to obtain an optimum repair welding process. The experimental results indicate that the groove defect can be removed by friction stir repair welding, and the offset repair welding process is superior to the symmetrical repair welding process. In the symmetrical repair welding process, a large number of fine cavity defects and an obvious aggregation of hard-brittle phase Al 2Cu occur, accordingly the mechanical properties of the repair joint are weakened, and the fracture feature of repair joint is partially brittle and partially plastic. A good-quality repair joint can be obtained by the offset repair welding process, and the repair joint is fractured near the interface between the weld nugget zone and thermal-mechanically affected zone.

Effect of drying catalysts on the properties of thermal copolymers from conjugated linseed oil–styrene–divinylbenzene

The effect of addition of a varying concentration of a drying catalyst (cobalt salt as primary drier) and a combination of catalysts such as cobalt, zirconium (secondary drier) and calcium (auxiliary drier) in a fixed concentration (1%) to a 50:20:30 compositions of 87% conjugated linseed oil, styrene (ST), and divinylbenzene (DVB) has been studied by characterizing the resulting polymers from thermal polymerization with various techniques such as soxhlet extraction, 1H NMR ( 1H nuclear magnetic resonance) spectroscopy, dynamic mechanical analysis (DMA), mechanical and thermogravimetric analysis. The thermal polymerization is performed in the temperature range of 85–160 °C. By soxhlet extraction, it is observed that the polymers contain approximately 64–77% crosslinked materials and the crosslinked insoluble fraction increases with an increase in cobalt catalyst concentration. For fixed concentration (1%) of catalysts, the insoluble fraction from the soxhlet extraction is maximum for the cobalt–zirconium mixture and minimum for the cobalt–calcium mixture. The micro-composition of these polymers obtained from the 1H NMR spectroscopy indicates that the crosslinked materials are composed of both soft oily and hard aromatic phases. The polymers with varying cobalt concentrations up to 0.6 wt% exhibit two separate glass transition temperatures, indicating the presence of two separate phases, one soft rubbery phase with sharp glass transition temperature of −50 °C and a hard brittle plastic phase of broadened glass transition temperature of 70–120 °C. On the other hand, instead of sharp peaks, the polymers with 0.8 and 1.0% cobalt salts exhibit two humps and a distinct peak in between the humps in the tan δ plots, indicating the presence of an additional phase comprised of a copolymer of linseed oil–styrene and DVB. For fixed concentration (1%) of catalysts, the cobalt–calcium combination follows the similar trend in the tan δ as that for the polymers with 0.8–1.0% cobalt whereas other combinations exhibit two phases. These polymers possess crosslink densities of 0.63–0.91 × 10 4 mol/m 3 and compressive strengths of 2.0–26.6 MPa. The catalyzed polymers are thermally stable below 300 °C and exhibit a major thermal degradation with a maximum degradation of 82–88% at a temperature of 500 °C.

Novel Conjugated Linseed Oil−Styrene−Divinylbenzene Copolymers Prepared by Thermal Polymerization. 1. Effect of Monomer Concentration on the Structure and Properties

A variety of novel opaque, white polymers ranging from rubbery materials to tough and rigid plastics have been prepared by the thermal polymerization at 85-160 degrees C of varying amounts of 87% conjugated linseed oil, styrene, and divinylbenzene. Gelation of the reactants typically occurs at temperatures higher than 120 degrees C, and fully cured thermosets are obtained after postcuring at 160 degrees C. The fully cured thermosets have been determined by Soxhlet extraction to contain approximately 35-85% cross-linked materials. The microcomposition of these polymers, as determined by 1H NMR spectroscopy, indicates that the cross-linked materials are composed of both soft oily and hard aromatic phases. After solvent extraction, the insoluble materials exhibit nanopores well distributed throughout the polymer matrixes. Dynamic mechanical analysis of these polymers indicates that they are phase separated with a soft rubbery phase having a sharp glass transition temperature of -50 degrees C and a hard brittle plastic phase with a broadened glass transition temperature of 70-120 degrees C. These polymers possess cross-link densities of 0.15-2.41 x 10(4) mol/m3, compressive Young's moduli of 12-438 MPa, and compressive strengths of 2-27 MPa. These materials are thermally stable below 350 degrees C and exhibit a major thermal degradation of 72-90% at 493-500 degrees C.