Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract

A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.     

Time and temperature dependent mechanical characterization of polymer‐based materials in electronic packaging applications

Abstract

The thermo‐mechanical testing of high performance polyimide films Type HPPST supplied by Dupont^® was conducted at different strain rates and in different temperature environments. The stress‐strain behavior of materials was investigated, and the dependence of Young's modulus on temperature and strain rate is reported. In view of the uncertainty of the Young's modulus determination, the specimens were tested with unloading‐reloading to verify the test results. Constant strain rate uniaxial tensile tests and long‐time creep tests at various temperatures were performed to characterize the time‐temperature‐dependent mechanical property precisely. Cyclic loading tests were also implemented on specimens to investigate cyclic stress‐strain behaviors. This research is expected to enhance finite‐element‐modeling accuracy and characterize material properties precisely.     

Effects of Undercooling and Transformation Time on Microstructure and Strength of Fe–C–Mn–Si Superbainitic Steel

Strength of Materials - A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation,...     

Fabrication conditions and transformation behavior of epitaxial Ni–Mn–Ga thin films

Epitaxial Ni–Mn–Ga films have been grown onto heated substrates by sputtering. Their chemical composition depends on the sputtering argon pressure. Representative epitaxial films of Ni_52.3Mn_26.8Ga_20.9, 0.5 μm-thick, transform martensitically at about 120 °C, accompanied by sharp changes in the lattice parameter and resistivity, and orders ferromagnetically below 98°. The observed high transformation temperature, orthorhombic martensitic structure, twinning mode and film morphology, indicate a potential multifunctional behavior of the film, such as high-temperature shape-memory effect and magnetic field actuation.     

Martensitic transformation and microstructure of polycrystalline Ni–Mn–Ga and C-doped Ni–Mn–Ga Heusler alloys

Abstract

The martensitic transformations of Ni–21·7Mn–23·8Ga (at.-%) (NiMnGa) and Ni–19·4Mn–22·7Ga–1·6C (at.-%) (NiMnGaC) alloys were investigated by the measurement of resistivity. Two kinds of martensitic transformations occur in NiMnGa alloy. The first martensitic transformation is thermoelastic, which exhibits a steep increasing in resistivity. The second transformation exhibits a larger thermal hysteresis compared with the first transformation. NiMnGaC alloy only shows a single martensitic transformation and the C addition increases the first martensitic transformation temperatures. The first martensitic phase of NiMnGa alloy is of five layered structure while the martensitic phase of NiMnGaC alloy is of non-modulated structure. Combined with the observation of optical microscopy and TEM, NiMnGa alloy exhibits much wider martensite twins than NiMnGaC alloy does.     

Formation and transformation of metastable phases during electrodeposition and annealing of cobalt–iron alloy films

During the electrodeposition of Co–Fe alloy films from a CoSO₄·7H₂O-FeSO₄·7H₂O bath, the formation of metastable phases, such as a complex cubic Co–Fe phase isostructural to α-Mn and the HCP ε-Co/Fe and Ω-Co/Fe phases, appears to be related to the incorporation of metal hydroxide/oxide precipitates into the plated alloy films. In the absence of the incorporated precipitates, the plated films are the equilibrium α-Fe solid solution BCC phase. Thus, the addition of stabilizing reagents (such as ammonium citrate), and/or a lowering of solution pH, prevents the formation of the precipitates and promotes the formation of the BCC phase. On the other hand, increasing temperature causes the formation of metastable phases, possibly through the weakening of the stabilizing effect of the ammonium citrate, or the promotion of the formation of metal hydroxides/oxides precipitates. The BCC phase has higher saturation magnetic flux densities and lower coercivities than the metastable phases. Annealing of the films transforms the metastable phases, if present, into the BCC phase, leading to a decrease in the coercivity. An increase in the magnetic flux density after annealing is, however, not observed, possibly due to the cracking or delamination of the films as a result of annealing. Cracking and delamination make the determination of the film volume, which is required for magnetic flux density calculation, questionable.     

High-Frequency Shear Modulus and Relaxation Time of Soft-Sphere and Lennard–Jones Fluids

The high-frequency shear modulus, G, and shear relaxation time, _shear, are obtained using the Zwanzig–Mountain equation for soft-sphere and Lennard-Jones potentials. The Hansen and Weis soft-sphere radial distribution function and the Matteoli–Mansoori Lennard-Jones radial distribution function are used in the equation. The shear relaxation times of different isotherms for both of these fluids pass through a minimum at a reduced density of about 0.7, which indicates a change from fluid-like behavior to viscoelastic behavior. The origins of this common density point are discussed. It is also shown that for the Lennard-Jones fluid, if the ratio of the reduced relaxation time to a power of the reduced temperature is plotted as a function of the reduced density, all isotherms become superimposed on a single curve.     

Heating rate dependent delamination of metal–polymer interfaces: experiments and modeling

Bimaterial interfaces in microelectronics packages are the most common regions of failure under thermo-mechanical excursions. In this work, we report experimentally observed role of heating rate on the delamination initiation and propagation across a metal-polymer interface in a microelectronic package. We observe that the rate of delamination propagation increases with increasing heating rate. When the heating rate increases, in addition to the higher amount of delamination growth per unit time, experimental results suggests that higher growth will also incur per unit temperature (loading). Correspondingly, the temperature at which complete delamination occur decreases. Using finite element modeling with cohesive interfaces, we provide a plausible explanation to this observed phenomenon. The analyses indicate that the mechanical behavior of the bimaterial interface is sensitive to both temperature and thermal rate.     

Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels

Abstract

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α′ (bcc)- and ε (hcp)-martensite in the γ (fcc) matrix during plastic deformation was analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. Depending on the content of the alloying elements different phase transformations γ → ε, γ → α′ (TRIP effect), or the formation of deformation twins (TWIP effect) occurred. Additions of Al increased the stacking fault energy (γ_fcc) and suppressed the γ → ε transformation while Si decreased γ_fcc and sustained the γ → ε transformation. These steels with reduced densities of about 7.3 Mg m^?3 exhibit high tensile ductility up to 95% with true tensile strength of about 1100 MPa. The excellent plasticity induced by twinning or phase transformation up to extremely high strain rates of about <disp-formula><graphic href="splitsection2-m1.tif"/></disp-formula> results in an extraordinary shock resistance and allows for deep drawing and backward extrusion operations of parts with complex shapes.     

Spatial transformation of Laguerre–Gaussian laser modes

Abstract

A method is described to spatially transform the annular profile of an arbitrary high-order Laguerre-Gaussian (LG) laser mode into an ultranarrow annulus using a combination of an axicon and a lens. The method is shown to conserve the azimuthal phase variation of the illuminating LG mode. The thin annular (hollow) light beam generated possesses orbital angular momentum and is suitable for experimental studies with cold atoms.     

Phase transformation during aging and resulting mechanical properties of two Ti–Nb–Ta–Zr alloys

Abstract

Phase transformations and mechanical properties of both Ti–29Nb–13Ta–4·6Zr and Ti–39Nb–13Ta–4·6Zr (wt–%) alloys were investigated. The microstructure of the 29Nb alloy is sensitive to solution and aging treatment. Ice water quenching from the solution treatment temperature resulted in (β+α") microstructure but air or furnace cooling led to a mixture of (β+ω). The formation of the orthorhombic α" martensite thus suppresses ω formation in the ice water quenched 29Nb alloy. Cooling rate from the solution treatment temperature also has a significant effect on the formation of α and ω phases during subsequent isothermal aging below the ω start temperature: slow cooling enhances ω but depresses α formation. This cooling rate dependence of aged microstructure was attributed to α" martensite acting as precursor of the α phase, thus providing a low energy path to the precipitation of a at the expense of ω. Phase transformation in the 39Nb alloy is more sluggish than that in the 29Nb alloy, owing to the presence of the higher content of β stabiliser Nb. For the 29Nb alloy, Young's modulus and mechanical properties are sensitive to the fraction of phases, and change significantly during aging, in contrast with the 39Nb alloy.     

Effect of negative bias voltage on mechanical and electrochemical nature in Ti–W–N coatings

Mechanical and electrochemical surface properties of Si (100) and AISI D3 steel substrates-coated Ti–W–N, deposited by r.f. magnetron sputtering process from a binary (50% Ti, 50% W) target in an Ar/N₂ (90%/10%) mixture, have been studied using nanoindentation, Tafel polarization curves and electrochemical impedance spectroscopy (EIS). The crystallinity of the coatings was analyzed via X-ray diffraction (XRD) and the presence of TiN(111), TiN(200), WN₂(107), and W₂N(220) phases were determined. Depth sensing nanoindentation measurements were used to investigate the elasto-plastic behavior of Ti–W–N coatings. Each group of samples was deposited under the same experimental conditions (power supply, Ar/N₂ gas mixture and substrate temperature), except the d.c. negative bias voltage that varied (0, ?50, and ?100 V) in order to study its effect on the mechanical and electrochemical properties of AISI D3 steel coated with Ti–W–N coatings. The measurements showed that the hardness and elastic modulus increase from 19 to 30 GPa and from 320 to 390 GPa, respectively, as a function of the increasing negative bias voltage. Coating track and coating-substrate debonding have been observed with atomic force microscopy (Asylum Research MFP-3D^®) on the indentation sites. Finally, the corrosion resistance of Ti–W–N coatings in 3.5 wt% NaCl solution was obtained from electrochemical measurements in relation to the increase of the negative bias voltage. The obtained results have shown that at the higher negative bias voltage (?100 V), the steel coated with Ti–W–N coatings presented the lower corrosion resistance. The corrosion resistance of Ti–W–N in 3.5 wt% NaCl solution was studied in relation to the increase of the bias voltage.     

Temperature dependent electrical conductivity of CNT–PEEK composites

The effect of temperature on electrical conductivity of nanocomposites consisting of Chemical Vapor Deposition (CVD)-grown multi-walled Carbon Nanotube (MWCNT) and Poly Ether Ether Ketone (PEEK) is presented in this paper. Different weight percentages of carbon nanotubes (CNT) were dispersed in PEEK through shear mixing by calendaring technique in Brabender. Percolation limit of the system was measured and discussed. The resulting nanocomposites were molded into round shaped pieces of 25.4 mm diameter and 1.4 mm thickness. The samples were then heated from room temperature to 140 °C while electrical conductivity is measured. It is found that electrical conductivity increases significantly with the increase in temperature and the rate of increase in conductivity with temperature depends on the content of CNT. The change in electrical conductivity does not follow the same route for heating and cooling cycle, thus resulting in electrical hysteresis.     

Temperature and stress–regime dependent primary–secondary–tertiary creep constitutive model

Abstract

High temperature deformation analysis of components such as steam turbine rotors requires a knowledge of the material deformation response for a wide range of stresses and temperatures. Deformation analysis of steam turbine rotors deals with stresses ranging above the material proof strength (shortly after plant start-up) down to those responsible for very long rupture durations (for the steady running phase of operation) at various temperatures. This study describes the construction of a temperature and stress–regime dependent (primary–secondary–tertiary) creep constitutive model to provide a more reliable representation for the material deformation response over wide ranges of stresses and temperatures. The adopted equation set is a refinement of the ‘Characteristic Strain’ model and depends in its formulation mainly upon creep rupture data. Successful application of the model for a 1CrMoV steel for a wide range of stresses over the temperature range of 450–675°C is demonstrated.