Thermodynamic calculation of stacking fault energy in Fe–Mn–Si shape memory alloys

Based on thermodynamic considerations together with measurement of the stacking fault probability (Psf) by X-ray diffraction profile analysis, the stacking fault energy (SFE, γ) of austenite in Fe–Mn–Si shaped memory alloys can be estimated. For instance, the stacking fault energy of an fcc(γ) phase in an Fe–30.3Mn–6.06Si was calculated as 7.8 mJ/m². Compositional dependence of stacking fault energy in these alloys with certain composition range has also been derived as SFE(γ)=180.54+7.923 wt.% Mn–46.38 wt.% Si (J/mol), showing that the stacking fault energy increases with the addition of Mn and decreases with the addition of Si.     

Effects of thermal martensite content and deformation on damping capacity of a Co–32 wt.% Ni alloy

The damping capacity of Co–32 wt.% Ni alloy was investigated as a function of the amount of thermal and strain-induced martensite under non-magnetic and 900 Oe magnetic fields, respectively. The damping capacity of the Co–32 wt.% Ni alloy containing martensite without magnetic field consists of the magneto-mechanical damping capacity of mainly α phase, damping capacities of α and phases without magneto-mechanical damping effect. Under a magnetic field of 900 Oe, the more the thermal martensite mass fraction the higher the damping capacity. However, the damping capacity of the deformed Co–32 wt.% Ni alloy with the strain-induced martensite decreases with increasing deformation degree despite the increase in total martensite fraction, because the lattice defects like dislocations introduced during deformation act as barriers to movement of damping sources such as magnetic domain walls, stacking faults boundaries in both α and phases, and α/ interfaces.     

Characterization of prior cold worked and age hardened Cu–3Ti–1Cd alloy

The influence of cold deformation by 50%, 75% and 90% on the age-hardening behavior of a Cu–3Ti–1Cd alloy has been investigated by hardness, tensile tests and light optical as well as transmission electron microscopy. The hardness of Cu–3Ti–1Cd alloy increased from 111 Hv in the solution-treated condition to 355 Hv in 90% cold worked and peak aged condition. The yield and ultimate tensile strengths of Cu–3Ti–1Cd alloy reached maxima of 922 MPa and 1035 MPa, respectively, on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was brought about by the precipitation of ordered, metastable, coherent β′ Cu₄Ti phase, in addition to high dislocation density and deformation twins. Both the hardness and the strength of the alloy decreased on overaging due to the development of the incoherent equilibrium phase β Cu₃Ti in a cellular structure form. However, the morphology of the discontinuous precipitation was changed to globular form at high deformation levels.     

Tensile behavior change depending on the microstructure of a Fe–Cu alloy produced from rapidly solidified powder

The relationship between consolidating temperature and the tensile behavior of iron alloy produced from Fe–Cu rapidly solidified powder is investigated. Fe–Cu powder fabricated by high-pressure water atomization was consolidated by heavy rolling at 873–1273 K. Microstructural changes were observed and tensile behavior was examined. Tensile behavior varies as the consolidating temperature changes, and these temperature-dependent differences depend on the morphology of the microstructure on the order of micrometers. The sample consolidated at 873 K shows a good strength/elongation balance because the powder microstructure and primary powder boundaries are maintained. The samples consolidated at the higher temperatures have a microstructure of recrystallized grains, and these recrystallized samples show the conventional relationship between tensile behavior and grain size in ordinal bulk materials.     

Effect of prior cold work on age hardening of Cu–3Ti–1Cr alloy

The influence of 50%, 75% and 90% cold work on the age hardening behavior of Cu–3Ti–1Cr alloy has been investigated by hardness and tensile tests, and light optical and transmission electron microscopy. Hardness increased from 118 Hv in the solution-treated condition to 373 Hv after 90% cold work and peak aging. Cold deformation reduced the peak aging time and temperature of the alloy. The yield strength and ultimate tensile strength reached a maximum of 1090 and 1110 MPa, respectively, following 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation twins. The maximum strength on peak aging was obtained due to precipitation of the ordered, metastable and coherent β′-Cu₄Ti phase, in addition to high dislocation density and deformation twins. Over-aging resulted in decreases in hardness and strength due to the formation of incoherent and equilibrium β-Cu₃Ti phase in the form of a cellular structure. However, the morphology of the discontinuous precipitation changed to a globular form on high deformation. The mechanical properties of Cu–3Ti–1Cr alloy are superior to those of Cu–2.7Ti, Cu–3Ti–1Cd and the commercial Cu–0.5Be–2.5Co alloys in the cold-worked and peak-aged condition.     

The relationship of the volume fraction of martensite vs. plastic strain in an Fe–Mn–Si–Cr–N shape memory alloy

The volume fractions of stress-induced martensite formed by certain plastic strains were determined by X-ray diffraction and quantitative metallography in an Fe–Mn–Si–Cr–N alloy at room temperature. The results are fitted by least square method and are well consistent with an exponential function f_M=1−exp{−β[1−exp(−η)]ⁿ} deduced by Olson and Cohen, who used it to fit with experimental data for AISI304 stainless steel. The similarity of and β, as well as the difference in n for these two alloys are discussed in relation to their nucleation mechanisms.     

Room temperature tensile behaviour of K640S Co-based superalloy

The room-temperature tensile properties of K640S cobalt-based superalloy were investigated by tensile tests and microstructure observation. The experimental results showed that the deformation mechanism of K640S alloy is dislocation slip; that the dislocation could be decomposed into continuous stacking faults with different orientations; and that with the increase of the number of dislocations, the dislocation tangle interacts with the decomposed stacking fault to increase the tensile strength of the alloy. As the stretching proceeded further, a plurality of slip systems were activated between different grains to coordinate the deformation, and the grains were gradually plastically deformed. The stress concentration occurred at the carbide interfaces, and microcracks were formed, causing mixed crystal fracture to the alloy.     

Heat pump cycles based on the AF–F transition in Fe–Rh alloys induced by tensile stress

The heat-pumping scheme based on the 1st order antiferromagnetism–ferromagnetism transition induced in FeRh alloy by one-dimensional tensile stress is proposed. Using the model S–T diagram for this alloy, the heat-pump cycles are drawn up based both on the transition latent heat absorption and emission when the transition is induced isothermally and on the change in alloy's temperature when the transition is induced adiabatically by applying tensile stress. The calculated values of heat coefficient φ for the cycles are 30 at ΔT=5 К and 20 at ΔT=10 К, where ΔT is the difference between the temperature surrounding and that of the heat receiver. These values are achieved using the tensile stress of 1·10⁹ Pa. The high values of φ make it possible to consider Fe–Rh alloys near the equiatomic composition as an effective refrigerant for elastocaloric heat-pumping near the room temperature.     

Austenite-to-ferrite phase transformation in low-alloyed steels

The kinetics of the austenite(γ)-to-ferrite() phase transformation in iron-based alloys with low amounts of interstitial and substitutional components has been simulated. The finite mobility and the diffusion of the components determine the γ/ transformation kinetics. Numerical difficulties may occur during simulations of diffusional phase transformations in such systems due to the fact that the diffusion of substitutional and of interstitial components occur on completely different time scales. However, substitutional alloying components (e.g. Mn, Cr, Ni) can be assumed to be immobile, if their amount can be completely dissolved in ferrite and the driving force for the transformation is sufficiently high. For lower, but not too small driving forces the site fractions of the substitutional components remain constant in both phases except a thin concentration spike which occurs at the austenite side of the interface. In a new model the Gibbs energy dissipation due to the diffusional motion of this spike has been considered and the transformation kinetics in the ternary Fe–C–Mn system has been calculated. The simulated transformation kinetics are compared to results obtained by isothermal dilatometer tests on a low-alloyed Fe–C–Mn steel.     

Effect of Mn on damping capacities, mechanical properties, and corrosion behaviour of high damping Mg–3 wt.%Ni based alloy

The effect of Mn on the damping capacities, mechanical properties, and corrosion behaviour of high damping Mg–3 wt.%Ni based alloys has been studied. The damping vs. strain amplitude spectrum of the studied alloys could be divided into three parts. The strain amplitude weakly dependent part appears again when the microplastic strain occurs at high strain amplitude. The mechanical properties of as-cast Mg–3 wt.%Ni alloy could be improved by the addition of Mn, which is due to the refinement of α-Mg dendrites and solid solution strengthening by Mn. In addition, the corrosion resistance of the alloys could also be improved remarkably by the addition of Mn.     

Remarkable improvement of damping capacity in FeMn-based alloys by a long annealing

ABSTRACT

Effects of a long annealing up to 10?h were investigated on damping capacity and phase evolutions in FeMn-based alloys. The damping capacity in the Fe17.5Mn alloy rose with the annealing time. A highest damping capacity was obtained in the Fe16.5Mn10.5Cr alloy after 2?h annealing. The highest damping capacity was comparable in Fe17.5Mn and Fe16.5Mn10.5Cr alloys. A positive relationship between the damping capacity and the stacking faults existed in both the Fe17.5Mn and Fe16.5Mn10.5Cr alloys annealed for above 2?h. To reduce the amount of α′-martensites was favourable to the further improvement of damping capacity. The α′-martensites clustered locally in the Fe17.5Mn alloy, but they distributed inside the ?-martensite bands in the Fe16.5Mn10.5Cr alloy.     

Damping mechanisms of Fe–Mn alloy with (γ + ɛ) dual phase structure

Abstract

The objective of the present study is to propose damping mechanisms for Fe–Mn alloys containing austenite (face centred cubic) and ? martensite (hexagonal close packed), and to analyse the individual contribution of damping mechanisms to the total damping capacity of an Fe–17 wt-%Mn alloy with respect to volume fraction of ? martensite. On the basis of substructural characteristics of γ and ? phases, it is suggested that damping mechanisms of Fe–Mn alloys with (γ + ?) dual phase structures involve stress induced movement of various boundaries such as stacking fault boundaries in austenite and ? martensite, ?martensite variant boundaries, and γ/? interphase boundaries. The damping capacity of the Fe–17Mn alloy increases with increasing ? martensite content. The quantitative analysis shows that in the as quenched state, the ? martensite phase is responsible for a major part of the damping capacity.     

Magneto-elastic interchange in ferromagnetic alloys

The internal friction δ, exchange integral A, magnetocrystalline anisotropic constant K_I and saturation magnetization M_s of Fe–Cr–Al and Fe–Cr–Al–Si alloys annealed at 1373 and 1473 K are measured. The energy density and volume fraction of domain walls (DWs) of these alloys are calculated based on the theories of ferromagnetism and the magnetic parameters measured. The physical process of irreversible movement of 90° DWs is suggested. The results indicate the dissipated elastic energy per unit volume due to the irreversible movements of 90° DWs is equal in value to the energy density of DWs, that is γ_w/δ_w=λ_sE/2. It is an effect of magneto-elastic interchange in ferromagnetic alloys.     

Synthesis of fly ash particle reinforced A356 Al composites and their characterization

A356 Al–fly ash particle composites were fabricated using stir-cast technique and hot extrusion. Composites containing 6 and 12 vol.% fly ash particles were processed. Narrow size range (53–106 μm) and wide size range (0.5–400 μm) fly ash particles were used. Hardness, tensile strength, compressive strength and damping characteristics of the unreinforced alloy and composites have been measured. Bulk hardness, matrix microhardness, 0.2% proof stress of A356 Al–fly ash composites are higher compared to that of the unreinforced alloy. Additions of fly ash lead to increase in hardness, elastic modulus and 0.2% proof stress. Composites reinforced with narrow size range fly ash particle exhibit superior mechanical properties compared to composites with wide size range particles. A356 Al–fly ash MMCs were found to exhibit improved damping capacity when compared to unreinforced alloy at ambient temperature.     

Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet

The grain size dependence of the tensile properties and the deformation mechanisms responsible for those properties are examined for Mg alloy, AZ31B, sheet. Specifically, the Hall–Petch effect and strain anisotropy (r-value) are characterized experimentally, and interpreted using polycrystal plasticity modeling. {1 0 . 2} extension twins, {1 0 . 1} contraction twins, and so-called “double-twins” are observed via microscopy and diffraction-based techniques, and the amount of twinning is found to increase with increasing grain size. For the sheet texture and tensile loading condition examined, {1 0 . 2} extension twinning is not expected, yet the polycrystal plasticity model predicts the observed behavior, including this ‘anomalous’ tensile twinning. The analysis shows that the Hall–Petch strength dependence, of the polycrystal as a whole, is primarily determined by the grain size dependence of the strength of the prismatic slip systems.     

Formation of amorphous and nanocrystalline phases in high velocity oxy-fuel thermally sprayed a Fe–Cr–Si–B–Mn alloy

High velocity oxy-fuel (HVOF) thermal spray was used to deposit a Fe–Cr–Si–B alloy coating onto stainless steel (1Cr18Ni9Ti) substrate. Microstructures of the powder and the coating were investigated by X-ray diffraction (XRD), scanning election microscopy (SEM), transmission election microscopy (TEM) and differential scanning calorimeter (DSC). The coating had layered morphologies due to the deposition and solidification of successive molten or half-molten splats. The microstructures of the coating consisted of a Fe–Cr-rich matrix and several kinds of borides. The Fe–Cr-rich matrix contained both amorphous phase and nanocrystalline grains with a size of 10–50 nm. The crystallization temperature of the amorphous phase was about 605 °C. The formation of the amorphous phase was attributed to the high cooling rates of molten droplets and the proper powder compositions by effective addition of Cr, Mn, Si and B. The nanocrystalline grains could result from crystallization in amorphous region or interface of the amorphous phase and borides by homogeneous and heterogeneous nucleation.     

Strain Rate Dependence of Tensile Properties of Extruded Mg–9Y–3Zn–1Mn Alloy

Tensile mechanical properties of an extruded Mg–9Y–3Zn–1Mn alloy are investigated at a wide range of strain rates. It is demonstrated that the tensile ductility of the alloy exhibits a pronounced strain rate dependence, while its tensile strength is less sensitive to strain rate. As strain rate decreases from to , the yield and ultimate tensile strengths of the alloy only decrease by 13% and 7% (from 237 and 320 MPa to 206 and 295 MPa), while its uniform and total elongations increase by 173% and 176% (from 10.12% and 11.53% to 27.62% and 29.32%), respectively. The results of the above tensile tests are analyzed by the characterizations of microstructure and morphologies of deformation surface and the measurements of strain hardening rate and strain rate sensitivity. It reveals that the less sensitive strain rate dependence of the tensile strengths arises from the reversed contributions of the precipitation strengthening role and the softening role caused by the grain boundary (GB) deformation; the pronounced strain rate dependence of the tensile ductility is attributed to the enhanced stress relaxation role of the GB deformation and the high deformation accomodation capability caused by the long period stacking ordered (LPSO) phase and stacking faults (SFs).

     

Microtwin formation in the α phase of duplex titanium alloys affected by strain rate

The effect of tensile strain rate on deformation microstructure was investigated in Ti-6-4 (Ti-6Al-4V) and SP700 (Ti-4.5Al-3V-2Mo-2Fe) of the duplex titanium alloys. Below a strain rate of 10⁻² s⁻¹, Ti-6-4 alloy had a higher ultimate tensile strength than SP700 alloy. However, the yield strength of SP700 was consistently greater than Ti-6-4 at different strain rates. The ductility of SP700 alloy associated with twin formation (especially at the slow strain rate of 10⁻⁴ s⁻¹), always exceeded that of Ti-6-4 alloy at different strain rates. It is caused by a large quantity of deformation twins took place in the α phase of SP700 due to the lower stacking fault energy by the β stabilizer of molybdenum alloying. In addition, the local deformation more was imposed on the α grains from the surrounding β-rich grains by redistributing strain as the strain rate decreased in SP700 duplex alloy.     

Effect of beta flecks on mechanical properties of Ti–10V–2Fe–3Al alloy

The effects of beta flecks on tensile properties and low-cycle fatigue life were investigated at room temperature for Ti–10V–2Fe–3Al alloy. It was found that beta flecks had a significant influence on tensile ductility and low-cycle fatigue life. The greater the volume fraction of beta flecks (P_A) or maximum area of beta flecks (S_max), the lower the tensile ductility and low-cycle fatigue life. Extensive scanning electron microscopy (SEM) and light microscopy (LM) observation showed that under tensile load, cracks preferentially nucleated at β grain boundaries of beta flecks, then grew, connected and propagated along grain boundaries to form characteristics of intergranular fracture and quasi-cleavage fracture. While under an alternating load, beta flecks acted as sites for low-cycle fatigue crack nucleation due to inhomogeneous alternating strains between soft GB and aged beta matrix. The presence of beta flecks accelerates both the crack nucleation and early crack propagation.     

Quantitative microstructural characterisation of Fe–30Ni alloy after martensitic transformations by means of stereological and magnetic methods

In the Fe–30Ni alloy investigated a martensitic transformation can occur both during quenching or plastic deformation. Martensite formed during plastic deformation, depending on the thermo-mechanical treatment applied, exhibits a different morphology from that achieved during quenching and forms the so-called composite-like structure. The morphology and volume fraction of martensite depends both on strain and temperature. In the present studies Fe–30Ni alloy was deformed by monotonic rolling in one path and perpendicular rolling in the temperature range M_D–M_S. The aim of the investigations was a determination of martensite volume fraction depending on the strain and temperature. To examine the influence of strain, the alloy was deformed by rolling in one path or perpendicular rolling at a temperature of − 30 °C, in the strain range of 10–30%. The dependence of temperature was investigated by rolling with 30% strain in a temperature range from − 30 °C to − 80 °C. The variants of thermo-mechanical treatment performed enabled us to achieve different martensite morphologies and volume fractions. Microstructural analysis was performed by means of light microscopy and transmission electron microscopy. The results of quantitative microstructural analysis of martensite and retained austenite volume fractions formed in different thermo-mechanical treatments were compared with those obtained by magnetic measurements. The fraction of deformation-induced martensite determined varied from 2% to 86%. The partial volume fractions V_V^MF of martensite formed in different deformation directions were also determined. It was found that the influence of the temperature on the martensite volume fraction is more pronounced than the influence of strain.