EPMA mapping of small particles of α-AlFeSi and β-AlFeSi in AA6063 alloy billets

It is well known that the spatial distribution and the spatial density of the particles o-AlFeSi and -AlFeSi in the billets of Al-Mg-Si alloys, such as AA6063 alloys affect the quality of anodizing performance of their extrusions. For this reason it is very important to control the spatial distribution and the spatial density of both AlFeSi particles at extrusion plants. The X-ray diffraction method (XRD) has been used for discrimination between -AlFeSi and -ALFeSi particles. However it is not an appropriate method for determining the spatial distributions of particles in the alloys. As an alternative method an electron probe microanalyzer (EPMA) has been used for determining the spatial distributions of each element in the microstructures. However, unfortunately it is difficult to discriminate between the particles composed of the same elements like -AlFeSi and -AlFeSi particles. Thus, we tried to develop a convenient method to discriminate between -AlFeSi and -AlFeSi particles in the microstructure of AA6063 alloys and developed the EPMA mapping of -AlFeSi and -AlFeSi particles. First, in order to discriminate between the two particles, we tried to use the relative X-ray intensity ratio, the I _Fe/I _Si ratio instead of the Fe/Si mass ratio. Then, we calculated the value of the I _Fe/I _Si ratio from -AlFeSi and -AlFeSi by using Monte Carlo calculations and obtained the critical value of the I _Fe/I _Si ratio, to distinguish between -AlFeSi and -AlFeSi. After that, using the discrimination value, we developed the EPMA mapping program (EPMA method) to observe the distributions of -AlFeSi and -AlFeSi, and to calculate the areas (%) of -AlFeSi and -AlFeSi. Finally, we checked the correlation between the EPMA and the XRD methods. Consequently, the two methods were in good agreement. Today, this EPMA method instead of the XRD method is successfully used in the quality control of 6063 aluminum alloy billets after heat treatment at our aluminum extrusion works.     

Interfacial structure and mechanical properties of surface iron–nickel alloying layer in pure iron fabricated by surface mechanical attrition alloy treatment

By surface mechanical attrition alloy treatment (SMAAT) and subsequent low temperature anneal treatment, a refined Fe/Ni alloy surface layer, about 50 μm in length, was fabricated on a pure iron plate. Micro-hardness and the friction and wear behavior of alloy surface layers were studied in comparison with those of their SMATed-only nanocrystallization counterpart. The interface microstructure indicated that the nickel powders had been permeated and welded into the pure iron surface in some localized regions by plastic deformation. The SMAAT process includes impacting with high strain velocity, grain refinement and synchronous diffusion. Atomic diffusion has been accelerated by the generation of high density defects through severe plastic deformation. Quick formation of the Fe/Ni intermetallic and solid solution phase alloy layer during SMAAT can be detected. Subsequent annealing treatment further accelerates the diffusion of Ni and Fe elements and leads to the formation of alloy phases. The results of friction and wear tests show that the properties of the alloy layer were remarkably improved. The main reason for this result may originate from its microstructures, i.e. an alloy layer with smaller grains, which reduces the effect of fatigue wear.     

Evolution of Al–19·4Si alloy surface structure after electron beam treatment and high cycle fatigue

Abstract

Processing of Al–19·4Si alloy by high intensive electron beam has been carried out, and multiple increase in fatigue life of the material has been revealed. Investigations of structure and surface modified layer destruction of Al–19·4Si alloy subjected to high cycle fatigue tests to fracture have been carried out by methods of scanning electron microscopy. The factors responsible for the increase in fatigue life of Al–19·4Si alloy have been revealed and analysed.     

Mössbauer study of iron in β-silicon nitride — effects of heat treatment and γ-irradiation

Mössbauer spectra of iron in -silicon nitride, prepared from a mixture of silica and carbon and from metallic silicon, were measured under various conditions. Iron was introduced into the crystal as an impurity in carbon, and where necessary was further doped by means of thermal diffusion mainly in a nitrogen atmosphere. As a result, the spectra were observed to change in a complicated way, including a sudden change when the samples were highly exposed to -rays or heated at high temperatures. In addition, they were found to show significant ageing at room temperature. All these results were dependent on the doping procedure. Also, in the processes of doping different kinds of species containing iron were found to form inside and outside the crystal depending on the state of iron before the doping, even though the doping itself was performed for all the samples under almost the same conditions. Explanations are given for the former results in terms of dynamics of the iron and silicon, along with residual carbon atoms and vacancies which were left behind by those atoms in the heating process or newly generated in the irradiation process.     

Effect of heat treatment on microstructure and mechanical properties of a new β high strength titanium alloy

Microstructure and mechanical properties of a new β high strength Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe titanium alloy were investigated in this paper. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 440 °C to 560 °C for 8 h were introduced to investigate the relationship between microstructures and properties. Microstructure observation of α/β solution treatment plus aging condition shows that the grain size is only few microns due to the pinning effect of primary α phase. The β solution treatment leads to coarser β grain size and the least stable matrix. The size and volume fraction of secondary α are very sensitive to temperature and strongly affected the strength of the alloy. When solution treated at 775 °C plus aged at 440 °C, the smallest size (0.028 μm in width) of secondary α and greatest volume fraction (61%) of α resulted in the highest yield strength (1624 MPa). And the yield strength decreased by an average of 103 MPa with every increase of 40 °C due to the increase of volume fraction and decrease of the size of secondary α. In β solution treatment plus aging condition, tensile results shows that the strength if the alloy dramatically decreased by an average of 143 MPa for every increase of 40 °C because of larger size of secondary α phase than α/β solution treated plus aged condition.     

Influence of heat treatment on stress–strain behaviour and lattice parameters of Sn–In alloy

Abstract

Sn based alloys have important industrial applications specially as pewters and soldering materials. One of these alloys, Sn–5·2 wt-%In alloy, is designed to be examined in the present work. The differential thermal analysis of this alloy gives a melting temperature value of 493 K. An empirical equation that can be used to determine the melting temperature of some Sn–In alloys is derived. Two different heat treated groups of samples, slowly cooled and quenched, are prepared. The phases present in these two groups of samples are determined from their X-ray diffraction patterns. The isothermal tensile stress–strain tests of all samples are reported at temperatures between 343 and 403 K. The changes of the work hardening parameters and also of the lattice parameters of the β-Sn phase with the deformation temperature are discussed. The values of the activation energy characterise a dislocation fracture mechanism.     

Effects of yttrium and heat treatment on the microstructure and tensile properties of Al–7.5Si–0.5Mg alloy

The effects of yttrium (Y) additions (0, 0.1, and 0.3 wt.%) and T6 heat treatment on the microstructure and tensile properties of Al–7.5Si–0.5Mg alloy have been investigated in the present work. The microstructures and fracture surfaces of as-cast and heat treated samples were examined by scanning electron microscopy (SEM). It was found that Y modified the eutectic silicon from a coarse plate-like and acicular structure to a fine branched and some fibrous one with a better uniform distribution. In addition, T6 heat treatment played a crucial role in the fragmentation and spheroidization of eutectic silicon, especially in the well modified alloys. The tensile properties were improved by the addition of Y followed by the T6 heat treatment, and a good combination of ultimate tensile strength (353 MPa), yield strength (287 MPa) and elongation (12.1%) was obtained when the Y addition was 0.3 wt.%. Furthermore, fractographic examinations revealed that dimple-like mechanism was responsible for ductile fracture.     

Phase transformation and microstructural evolution after heat treatment of a terbium-doped lithium–aluminum phosphate glass

The crystallization kinetics and phase transformation of a transparent Tb³⁺-doped lithium–aluminum phosphate glass, prepared by melt quenching, were investigated. The energy associated to the glass transition and the crystallization parameters (activation energy for crystallization and Avrami exponent) were evaluated by different methods using the experimental data obtained by differential thermal analysis performed at different heating rates. Using an isoconversional method to determine the change of the activation energy for crystallization with the fraction of crystallization, it was verified that with the increase in the fraction of crystallization from 0.1 to 0.9, the value of the activation energy decreased slightly from ~370 to ~310 kJ mol^?1 and that the Avrami exponent varied from 0.8 to 1, suggesting a surface crystal growth mechanism. Observation of the microstructural evolution of heat-treated glass samples confirmed a surface crystallization process revealing spherulitic crystals constituted mainly by aluminum metaphosphate.     

The relation between heat treatment and corrosion behavior of Mg–Gd–Y–Zr alloy

The corrosion behavior of Mg–7Gd–3Y–0.4Zr (GW73K) was investigated in as-cast (F), solution-treated (T4) and peak-aged (T6) conditions using immersion tests and electrochemical measurements in NaCl solution (5 wt.%). Microstructure analyses were carried out on GW73K after different heat treatments by optical microscope (OM), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction. It is found that GW73K alloy exhibits higher corrosion resistance in T4 than in F and T6 conditions due to the fully dissolution of cathodic coarse (Gd + Y) rich eutectic compound. The corrosion products of GW73K have different morphologies for F, T4 and T6 conditions. The product for F is less uniform and compact than T4 and T6, and it has been founded that GW73K-T6 had two different morphologies owing to the presence of β′. The results of polarization curves also confirm that proper heat treatment is beneficial to improve the corrosion resistance of GW73K alloy by transforming the microstructures.     

Influence of heat treatment on microstructure and passivity of Cu–30Zn–1Sn alloy in buffer solution containing chloride ions

Tin as an alloying element is of great interest in brasses for dezincification impediment. In this paper, Cu–30Zn–1Sn alloy was submitted to three different heat treatments, viz. A (heating up to 800 °C for 20 h, held at 200 °C for 20 h in salt bath and air cooled), B (heating up to 800 °C for 20 h and water quenched) and C (heating up to 600 °C for 20 h and water quenched). The influence of heat treatment on microstructure was evaluated by OM and SEM–EDS analysis. The corrosion resistance in buffer solution (pH 9), H₃BO₃/Na₂B₄O₇ ·10H₂O, with various concentrations of chloride ions was evaluated by potentiodynamic polarization curves and compared with multi-component Pourbaix diagrams. A correlation between the heat treatment, microstructure and passivity of the heat treated samples was observed. The results indicated that all heat treatment procedures led to formation of α, and γ-Sn-rich phases as microstructure constituents with a small fraction of β′ phase in A. Sn-rich phase appears in grain boundaries and its morphology was slightly changed due to heat treatment. Beneficial influence of low concentration chloride ions on passivity was associated with the formation of copper oxides/hydroxide and chloride complexes. Deterioration was observed at concentrations higher than 0·05 M NaCl due to accelerated dissolution of copper by formation of CuCl₂^-_{2}^{-}. As a result of dezincification process, preferential corrosion attack and copper redeposition on α phase (matrix) were observed. However, Sn-rich (γ₁) phase in grain boundaries was not attacked due to SnO₂ formation. In buffer solution, the higher passivity current density in A was related to the presence of small amount of β′ phase. On the other hand, in 1 M NaCl, lower critical current density for passivation in B and A (about two times lower than C) was attributed to the grain size effect.     

Measurement of surface tension and specific heat of Ni-18.8 at.% Si alloy melt by containerless processing

The surface tension and specific heat of superheated and undercooled Ni-18.8 at.% Si alloy melt have been measured by the oscillating drop method and the drop calorimetry technique in combination with electromagnetic levitation, respectively. The surface tension follows a linear relationship with temperature within the range of 1370–2100 K. The surface tension at the melting temperature and the temperature coefficient are determined to be 1.796 N/m and −3.858 × 10⁻⁴ N/m/K, respectively. The specific heat is determined to be 40.80 ± 1.435 J/mol/K over the temperature range 1296–2000 K. The maximum undercooling of 178 K is achieved in the experiments. Based on the measured data of surface tension and specific heat, the viscosity, solute diffusion coefficient, density and thermal diffusivity of liquid Ni-18.8 at.% Si alloy are calculated.     

Phase transformations in a welded near-α titanium alloy as a function of weld cooling rate and post-weld heat treatment conditions

In - titanium alloys, the high-temperature phase can decompose in several ways, depending on alloy composition and cooling rate. In the case of welded joints, cooling rates can vary widely as a function of heat input. In the current work, a dilute - Ti-Al–Mn alloy was welded over a range of heat inputs using electron beam and gas tungsten-arc welding processes. A major part of the rapidly cooled electron beam weld could be identified as lath-type or massive martensite. In the slower cooled gas tungsten-arc welds, the transformation product was a mixture of lamellar and phases formed entirely by diffusion. Post-weld heat treatment resulted, in all cases, in an - structure that coarsened with annealing temperature and time. Tensile elongation in the as-welded condition was poor on account of a large prior- grain size and an acicular microstructure. The ductility improved as the structure coarsened on heat treatment. Tensile fracture was always microscopically ductile, but the presence of a grain boundary layer tended to induce intergranular rupture, especially when a hard, intragranular matrix confined slip to occur in the grain boundary regions.     

Carburisation layer evolution of Fe–Cr–Ni alloy in furnace after long term service: experimental study and numerical prediction

Abstract

Owing to high temperature comprehensive properties, Fe–Cr–Ni alloys are designed to operate in corrosive gaseous environments of ethylene pyrolysis furnace. However, most premature failed tubes were caused by carburisation. In the present study, based on the Fick’s second law and equilibrium constant method, study on the carburised layer evolution of HP40Nb and KHR45A alloys by pack carburising experimental investigation and numerical simulation by MATLAB software were carried out. The results show that the experimental and simulated data agree with each other acceptably. The carburising layer rate of KHR45A alloy is much smaller than that of HP40Nb alloy due to higher contents of Cr and Ni element in the former. With increasing operating temperature ranging from 1000 to 1100°C, the maximum service lives of the two alloys sharply decrease.     

Influences of heat treatment on microstructural evolution and tensile behavior of squeeze-cast Mg–Gd–Y–Zr alloy

The effects of solution heat treatment and aging on the microstructural evolution and mechanical behavior of a squeeze-cast (SC) Mg–10Gd–3Y–0.5Zr (GW103K) alloy, processed using various applied pressures (e.g., 0.1, 40, 80 and 160 MPa) were systematically investigated. Our results show that, after solution heat treatment, secondary phases and pressure-induced dislocations are dissolved in the matrix of the squeeze-cast alloys. Moreover, subsequent aging heat treatment leads to an increased age-hardening response relative to that in squeeze-cast GW103K and this trend increases with increasing applied pressure. The room temperature tensile test results show that the yield strength (YS) for the squeeze-cast alloy in the as-cast, the as-T4 heat-treated and the as-T6 heat-treated states increases with increasing applied pressure, from 0.1 to 80 MPa, and remains relatively constant when the applied pressure is increased to 160 MPa, whereas the ultimate tensile strength (UTS) and elongation-to-failure (E _f) increases continuously with increasing applied pressure. The measured increases in YS and UTS (or E _f), are discussed in terms of the mechanisms that govern the evolution of microstructure in squeeze-cast GW103K, paying particular attention to gain size and porosity.     

Microstructure evolution in abrasion-induced surface layer on an Al–Zn–Mg–Cu alloy

An altered surface layer forms on an Al–Zn–Mg–Cu alloy during surface preparation by abrasion with grinding paper. Strain-induced dissolution of η′/η precipitates and formation of nano-sized subgrains were observed in the surface layer with thickness of several hundred nanometers. The segregation of solute elements along dislocations and subgrain boundaries and the precipitation of Al₂Cu phase at the sub-boundaries and the free surface were related to enhanced diffusion accelerated by deformation-induced vacancies, dislocations and subgrain boundaries. The microstructure evolution in this layer is mainly attributed to the shear strain and is modified by temperature rise during surface abrasion. The unique surface microstructural changes produced by abrasion might alter the surface properties.     

Influence of fluoride treatment on surface properties,biodegradation and cytocompatibility of Mg–Nd–Zn–Zr alloy

Fluoride treatment is a commonly used technique or pre-treatment to optimize the degradation kinetic and improve the biocompatibility of magnesium-based implant. The influence of changed surface properties and degradation kinetics on subsequent protein adsorption and cytocompatibility is critical to understand the biocompatibility of the implant. In this study, a patent magnesium alloy Mg–Nd–Zn–Zr alloy (JDBM) designed for cardiovascular stent application was treated by immersion in hydrofluoric acid. A 1.5 μm thick MgF₂ layer was prepared. The surface roughness was increased slightly while the surface zeta potential was changed to a much more negative value after the treatment. Static contact angle test was performed, showing an increase in hydrophilicity and surface energy after the treatment. The MgF₂ layer slowed down in vitro degradation rate, but lost the protection effect after 10 days. The treatment enhanced human albumin adsorption while no difference of human fibrinogen adsorption amount was observed. Direct cell adhesion test showed many more live HUVECs retained than bare magnesium alloy. Both treated and untreated JDBM showed no adverse effect on HUVEC viability and spreading morphology. The relationship between changed surface characteristics, degradation rate and protein adsorption, cytocompatibility was also discussed.     

Effect of heat treatment on microstructure and microhardness evolution in a Ti–6Al–4V alloy processed by high-pressure torsion

A Ti–6Al–4V alloy was heat-treated to give two types of microstructures with different volume fraction of equiaxed α phase and lamellar (α + β) microstructure. Disks were cut from the heat-treated rods and processed by quasi-constrained high-pressure torsion (HPT) at room temperature with an applied pressure of 6.0 GPa and torsional straining from 1/4 to 20 turns. The results show that there is a gradual evolution of homogeneity in microhardness and grain size with increasing numbers of revolutions in HPT such that the microhardness values attain a maximum constant value across the disk after processing by HPT for 10 turns and the measured equilibrium grain sizes after 20 turns are ~130 nm in Ti64-1 and ~70 in Ti64-2. The results show also that a larger fraction of lamellar (α + β) in the microstructure of Ti–6Al–4V leads to a higher hardenability after processing by HPT.