Prolonged Solution Treatment Research Articles

The effect of solution treatment time on the tensile deformation characteristics of naturally-aged A383 alloy die castings

A383 aluminum alloy high pressure die castings were solution treated at 490 °C for six durations ranging between 15 and 180 min, subsequently quenched in water and naturally aged for 4 days. The effect of solution treatment time on the evolution of microstructure and tensile properties were determined previously (G. Eisaabadi et al. Mater. Sci. Eng. A, 722, pp. 1–7, 2018.) In the current study, the tensile deformation characteristics of A383 alloy castings were determined by analyzing work hardening rate versus true stress in Kocks-Mecking plots. Results showed that (i) there was a sudden drop in work hardening rate immediately prior to fracture in all specimens, (ii) some specimens exhibited Portevin-Le Chatelier (PLC) effect, (iii) the stress where the PLC effect started increased with yield strength, (iv) the unitless Kocks-Mecking parameter, K. decreased with increasing solution treatment time, and there was a strong relationship between K and elongation (v) the main benefit of prolonged solution treatment of cast aluminum alloys is healing of structural defects, namely oxide bifilms. These results are discussed in detail in the paper.

Effect of Phase Transformation During Long-Term Solution Treatment on Microstructure, Mechanical Properties, and Bio-corrosion Behavior of Mg–5Zn–1.5Y Cast Alloy

The influence of long-term solution treatment for various intervals on the microstructure, mechanical properties, and corrosion resistance of the as-cast Mg–5Zn–1.5Y alloy was investigated. Variation of secondary phases was studied during solution treatment through thermal analysis test and thermodynamic calculations. Tensile and hardness tests, as well as polarization and immersion tests, were performed to evaluate the mechanical properties and corrosion behavior of the as-cast and heat-treated alloy, respectively. Results show that solution treatment transforms I-phase (Mg3Zn6Y) into W-phase (Mg3Zn3Y2), as well as dissolves it into the α-Mg matrix to some extent; therefore, the amount of W-phase increases. Moreover, prolonged solution treatment decreases the volume fraction of the phases. In the first stage of solution treatment for 14 h, the tensile properties significantly increase due to the incomplete phase transformation. Although long-term solution treatment sharply decreases the tensile and hardness properties of the alloy, it improves the corrosion resistance due to the transformation of I-phase into W-phase. In fact, it decreases corrosion potential and simultaneously dissolves intermetallic compounds into the α-Mg matrix, resulting in the reduction in galvanic microcells between the matrix and compounds. It is found that the optimum time for long-term solution treatment is 14 h, which improves both corrosion behavior and mechanical properties.

Selection of optimal solution heat treatment of the casting cylinder heads

The effect of solution treatment on mechanical properties (UTS, elongation, Brinell hardness) and microstructure (Si-morphology and Si-size) of an aluminium alloy (A356) used for casting cylinder heads was studied. The tests were carried out with specimens machined from the bulkheads of V8 engine blocks cast by the low pressure process. The samples were tested in as-cast and T6 heat treating conditions (solution heat treatment at 530°C with different time - 2, 3, 4, 5, 6, 7 hours, quenching in water at 20°C and precipitation hardened for 4 hour at 160°C). The results show that used heat treatment improves mechanical properties of the cylinder head casts. Tensile strength and hardness of specimens increase with solution treatment time. The hardness is a reflection of solution strengthening and silicon particle distribution in matrix. Solution temperature 530°C and 5 hours solution time is appropriate to obtain better morphology and distribution of Si particles in microstructure. Prolonged solution treatment (more than 5 hours) leads to a coarsening of the Si particles, while the numerical Si density decreases. As the particle density decreases, a fewer number of sites are available for crack nucleation, and hence, the fracture properties are improved. The data obtained from this study will be used to improve process control, and to help the selection of heat treatment of the casting for future products.

Tensile fracture behavior of AA7075 alloy produced by thixocasting

Tensile fracture behaviors of thixocast, artificially aged thixocast (thixocast+T6) and as-extruded AA7075 alloys were investigated. The microstructural and fractographic observations were carried out using optical microscope (OM) and scanning electron microscope (SEM). Experimental studies showed that as-extruded and thixocast+T6 specimens exhibited considerably more excellent mechanical properties than as-thixocast AA7075 specimen. T6 artificial heat treatment with prolonged solution treatment significantly improved the tensile properties of the thixocast AA7075 alloy. The tensile properties of as-extruded and thixocast+T6 specimens were close to each other. In as-thixocast specimen having remarkable micro-cracks, fracture was intergranular brittle type. The ductile fracture surfaces were observed in as-extruded and thixocast+T6 specimens. In as-thixocast specimens, decohesion started between the eutectic–matrix interfaces and propagated through grains. Micro-void coalescence was the dominant form of fracture in thixocast+T6 heat treated specimens. The micro-voids nucleation was initiated at the interface between the matrix and multinary eutectic structure.

Role of decomposition of AlMgFeSi phase in mechanical properties and fracture behaviour of 357 alloys

The π-AlFeMgSi iron intermetallic phase in Al–Si–Mg alloys is known for its detrimental effect on ductility and strength. An attempt was made to correlate the impact and tensile properties of heat treated Al–7Si–0·55Mg–0·1Fe alloy samples with the changes that occur in the microconstituents such as the eutectic Si and π phase with prolonged solution treatment. Impact and tensile tests showed that the first 20 h of solution treatment is more sensitive to the changes occurring in the Si particle morphology than that in the π phase. Decomposition of the π phase significantly reduces the impact properties after prolonged solution times. Fracture behaviour is controlled mainly by the eutectic silicon morphology. The π phase particles act as crack initiation sites and facilitate crack propagation in as cast and heat treated alloys. Quality index values show that optimum solution time for Sr modified alloys is 12 h. Impact properties are more sensitive to changes in the π phase during solution treatment than tensile properties.

The negative effect of solution treatment on the age hardening of A356 alloy

The effect of solution treatment on the age hardening of an A356 (Al–7.0Si–0.3Mg) alloy was investigated by hardness measurements and electron microscopy. This study presents experimental evidences that a prolonged solution treatment has a detrimental influence on age hardening due to the porosity in the cast A356 alloy. The enrichment of Mg on the internal surface of the porosity was observed. Nanoscale particles of magnesium oxides exist in the samples that have been solution treated with an extended duration. Our results indicate that Mg atoms would diffuse toward the porosity pre-existed in the A356 alloy during solution treatment. This phenomenon is consistent with the degraded aging kinetics and hardening potential of the A356 alloys solution treated for a long time, since the reduction of Mg solutes in the Al matrix shall lead to a decrease of the volume fraction of the Mg–Si-containing strengthening precipitates, which are mainly the monoclinic β″ phase. Our findings emphasize the significance of controlling porosities for the A356 alloys and may provide useful information for designing thermal treatment parameters to reach the optimum balance between strength and ductility.

Dilatometry revealing Si precipitation in Al–Si-alloys

Abstract The precipitation kinetics of Si after different thermal treatments in AlSi-alloys have been studied by means of isothermal calorimetry, differential scanning calorimetry and dilatometry tests. The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with the increase and reduction in the coefficient of thermal expansion CTE(T), respectively. The deviation of the CTE(T) of the studied AlSi1.1 – 1.7 alloys with respect to pure Al correspond to the volume fraction of precipitating Si according to thermodynamic solubility limits. Two different kinetics in the growth of Si precipitates are distinguished: while Si precipitates in the range of 200 – 300 °C during heating of solution quenched samples supersaturated with vacancies, retardation into the temperature range of 300 – 420 °C is observed in samples slowly cooled after solution treatment. The presence of the eutectic Si increases the quench sensitivity of the precipitation kinetics of Si as observed in CTE(T) curves. The ripening of the eutectic Si during prolonged solution treatment retards the Si precipitation slightly.

An in situ composite of Al (graphite, Al4C3) produced by reciprocating extrusion

A reciprocating extrusion process was used to produce an in situ composite of Al (graphite, Al4C3). 6061 Al alloy powder was first dehydrated at 450°C in vacuum, and then mixed with graphite powder in various volume fractions up to 20%. The mixed powder was hot pressed as a billet and finally consolidated by a reciprocating extrusion machine. Each billet was extruded through the die 15 times. The results show that all the graphite particles were greatly refined and uniformly distributed in the matrix. The graphite in-situ reacted with the Al matrix to form fine dispersed Al4C3 particles both during reciprocating extrusion and subsequent solution treatment. The higher the volume fraction of graphite added to the matrix, and the longer the solution treatment, the harder the composite. The 6061Al-20% (graphite, Al4C3) composite that was solution-treated for 24 h, and then peak aged, could attain a hardness level of 7075-T6 alloy. As for the wear rate, this initially increased due to the presence of the graphite particles; but with higher graphite content and prolonged solution treatment the wear rate subsequently decreased. This phenomenon is attributed to the fact that although the graphite could soften the composites, the Al4C3 strengthened the composites during prolonged solution-treatment time. The wear test demonstrated that composite containing 20% (graphite, Al4C3) which has been solution-treated for 24 h can exhibit an excellent combination of coefficient frictional and wear rates, ∼50 and 70%, respectively, of those of 6061 alloy.

Aqueous environmental crack propagation in high-strength beta titanium alloys

The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mVSCE, as indicated by a reduced threshold for subcritical crack growth (K TH ), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast, the initiation fracture toughness (K ICi ) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σYS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is eliminated by cathodic polarization (to −1000 mVSCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant, but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture.

Heat-treatment parameters for a 359/Al 2O 3/10p composite modified with 0.07 wt% strontium

In melt-produced composites, the properties of particle-reinforced aluminum metal-matrix composites (MMCs) can be compromised by the interfacial reactions that may occur between the molten matrix and the reinforcement depending on the melt conditions. The reinforcement can also considerably influence the response of the matrix alloy to solution heat treatment and age-hardening. A study of the melting and heat-treatment parameters and their effect on the tensile properties and fracture behaviour of 359/Al 2O 3/10p composite modified with 0.07 wt% (700 ppm) strontium (Sr) was conducted, with the aim of assessing the properties of particulate alumina (Al 2O 3) as reinforcement compared to silicon carbide (SiC), as well as the effect of a high Sr content. It was found that the latter significantly improved the fluidity/castability of the composite alloy. Although overmodification occurred, no associated Sr-containing intermetallics were observed, owing to the high silicon content of the matrix. During solidification, mechanical trapping of the alumina particles was observed for particle sizes much larger than that of the α-aluminum dendrites, as well as nucleation of eutectic silicon on their surfaces. Spinel particles were also observed to form at the alumina-particle/matrix interface, their formation progressing during solution heat treatment. Prolonged solution treatment at high temperatures resulted in a significant lowering of the strength upon subsequent ageing. Isothermal/isochronal ageing of the composite showed that the alumina particles only accelerate the kinetics but do not alter the precipitation behaviour of the matrix. Composite fracture can occur within the aluminum matrix, along the matrix/reinforcement interface, as well as by shearing of the alumina particles.

Effect of heat treatment on the microstructure, tensile properties, and fracture behavior of permanent mold Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C) and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties, and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300 MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323 MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC particles. A detailed discussion of the fracture mechanism is given.

Sodium Ion-Selective Chalcogenide Glass Electrodes

Abstract Analytical characteristics and sensing mechanism of sodium ion-selective electrodes based on NaCl-Ga2S3-GeS2 glasses have been investigated. Chalcogenide glass electrodes containing 10 mol.% NaCl in the membrane showed near-Nernstian response in the concentration range from 10-3 to 1 M sodium nitrate solution. These sensors were superior to the conventional pNa oxide glass electrodes in selectivity in the presence of hydrogen ions and in Na+ ion sensitivity in fluoride media. Prolonged solution treatment for several days reduces, however, the detection limit of the sensors and the slope of the electrode response. Ionic processes at the membrane surface have been investigated using XPS technique and 22Na tracer measurements. It was shown that sodium ion-exchange governed Na+ ion response of chalcogenide

The effect of carbon on magnetic properties of an Fe-Cr-Co permanent magnet alloy

The effect of carbon on magnetic properties of an Fe- 24Cr-12Co-1Si-1Ti alloy with carbon content from 0.012 to 0.291 percent (by weight) was studied. It has been found that for a carbon content larger than 0.05 percent there is an abrupt degradation in magnetic properties for alloys treated according to a normal procedure to develop the magnetically optimum state. The reasons have been thoroughly explored by optical metallography, X-ray diffractometry, transmission electron microscopy, carbide extraction, and analysis and hardness measurements. Carbon stabilization of the γ phase, the amount of which increases with carbon content, is the main deteriorating factor found in this alloy. Abnormal spinodal decomposition between α phase and non-α-phase regions and Cr-rich carbide precipitated in or near α-phase impair magnetic properties further. It has been shown that prolonged solution treatment at temperatures near the solidus can compensate part of the carbon effect at the expense of (B-H)m value. Hence, for practical purposes, the 0.05 percent C is set to be the upper limit for ensuring (B-H)m value greater than 5.5 MGOe for this alloy, and the 0.05 percent C limitation is suggested to be common to all Fe-Cr-Co permanent magnet alloys.