Plasticity induced transformation in a metastable β Ti-1023 alloy by controlled heat treatments

Abstract

Investigations into the possibility of improving the strength–ductility relation in a metastable β-titanium alloy (Ti–10V–2Fe–3Al) through plasticity induced transformation (PiTTi) have been carried out. Various heat treatments in the β and/or α+β condition were performed to study their influence on both the microstructure and solute partitioning, which eventually control the PiTTi effect. Stress induced martensite formation promoting such effect has been observed upon compression testing for β and β+(α+β) microstructures. The stress–strain curves exhibiting stress induced martensite show a ～20% increase in strength, while still retaining a reasonable ductility level. Microstructural parameters such as grain size and solute concentration (especially V) in β have been related to the alloy's ability to exhibit PiTTi.     

'Quench sensitivity' of 7075 aluminium alloy plates

Abstract

7075 aluminium alloy is widely used especially in those applications for which high mechanical performances are required. In the technical literature it is well known that the corrosion resistance and the mechanical properties of this material strongly depend on the cooling rate during quenching. This phenomenon is known as 'quench sensitivity'. The main aim of this paper is to investigate the influence of the cooling rate during quenching of samples taken from plates by varying the parameters of the heat treatment and the rolling direction (L, LT and ST). All the samples were heat treated in laboratory equipment to reach T6, T76 and T73 tempers. The samples were prisms (13 × 13 × 100 mm) and were quenched in water; the cooling rate was imposed by changing the water temperature or changing the polymer amount in a water agitated bath at 20°C. In each of the experimented condition, the cooling rate was measured by a thermocouple placed in the sample. Moreover, a finite element method (FEM) simulation was carried out in order to estimate the heat transfer coefficient during the cooling in all the experimented conditions. Tensile and intergranular corrosion tests were performed to point out the influence of the investigated cooling rates.     

Friction stir welding of aluminium 7136-T76511 extrusions

Abstract

This research programme evaluates the as welded properties of Al 7136-T76511 extrusions joined through friction stir welding (FSW). Microstructural characterisation and mechanical testing were performed on the baseline material and on panels friction stir welded at 250 and 350 rev min^–1 (all other weld parameters held constant). Transmission electron microscopy revealed the microstructural features in each of the unique weld regions and demonstrated that the precipitate density and morphology in these regions correlates with the temperature profile produced by the FSW process. A thermal model of FSW is developed that utilises an energy based scaling factor to account for tool slip. The slip factor is derived from an empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures and profiles over a range of energy levels. The mechanical behaviour after welding is correlated to the temperature distribution predicted by the model and to the observed microstructural characteristics. As welded mechanical properties of the alloy trended positively with the energy per unit length of weld, i.e. the highest joint efficiency was achieved at the highest welding temperature.     

Microstructural evolution of AS21X HPDC alloy during thermal treatment

Abstract

The microstructure of a High Pressure Die Cast Magnesium (HPDC) AS21X alloy was investigated after various heat treatments. The material, supplied in the as cast state, consisted of Mg-α grains separated by intermetallic particles such as Mg₁₇Al₁₂, Mg₂Si and Al–Mn. The alloy was subjected to solution treatment at 415° C for times ranging from 0.5 to 48 h and to aging to assess grain growth stability and precipitation hardening. Light microscopy showed that Mg-α grains increase slightly in size whereas intermetallic particles do not disappear but assume a more rounded shape. Static precipitation and/or dissolution were followed by electrical conductivity, hardness measurements and X-ray diffractometry. Tensile properties at room temperature were evaluated on both the as cast and solution treated samples. Density was used as an indicator of porosity to explain the scatter in elongation to fracture data. Study of the fracture surfaces revealed the morphology of porosity and the otherwise ductile fracture failure mechanism.     

Physico-mechanical properties of particleboards bonded with pine and wattle tannin-based adhesives

This study investigated the physical and mechanical properties of particleboards made using two types of tannin-based adhesives, wattle and pine, with three hardeners, paraformaldehyde, hexamethylenetetramine (hexamine) and TN (tris(hydroxyl)nitromethane), by measuring the physical (thickness swelling, linear expansion and water absorption) and mechanical properties (bending strength and internal bond strength). The performance of the particleboards made using tannin-based adhesives was influenced by physical conditions such as press time and temperature as well as by chemical conditions, such as the chemical structure of the tannin and the hardener. Wattle tannin-based adhesive being a thermoset, the wattle tannin-based particleboards were more influenced by physical conditions, while the pine tannin-based particleboards were influenced by the chemical structure of the pine tannin nuclei, which include phloroglucinolic A-rings. The reactivity of the hardener toward the tannin was in the order: paraformaldehyde > hexamine > TN for wattle tannin, while for pine tannin the order was hexamine > paraformaldehyde > TN.     

Effect of nanosized reinforcement particles on mechanical properties of high density polyethylene–hydroxyapatite composites

Abstract

The effect of nanoscaled hydroxyapatite (HA) filler particles on the mechanical properties of the high density polyethylene–hydroxyapatite (HDPE–HA) composite samples has been investigated. Nanosized HA particles with an average size in the range of 40–50 nm were synthesised by mechanical milling method. The composite samples with various amounts of nanoscaled HA particles were produced by mixing the ceramic and high density polyethylene particles using a single screw extrusion system. The results of the mechanical testing on the composite samples showed an increase in the fracture strength and the young's modulus values with increasing volume fraction of HA content in the composite samples. At the same time, there were decreases in both the fracture strain and toughness values with increasing volume fraction of the ceramic filler particles. In addition the comparison of the results obtained in this study with the mechanical properties of the commercially available composite samples (HAPEX) shows that similar mechanical properties can be reached at a much lower ceramic content, if nanoscaled HA particles are used in the fabrication of these composite biomaterials.     

Mechanical properties and wear behaviour of PM aluminium composite reinforced with (Fe3Al) particles

Abstract

Mechanical properties and wear behaviour of an aluminium matrix composite reinforced with Fe₃Al intermetallics have been studied. A 2014 alloy manufactured through mechanical alloying was used as the matrix. Three different Fe₃Al intermetallics have been used as reinforcement, also manufactured through mechanical alloying. The difference between them was the different mechanical alloying times (5 and 20 h were used) and the possibility of carrying out a heat treatment at 1000°C (on the 20 h milled intermetallic) before admixing to the aluminium matrix. The processing of these composite materials included mixing and cold compacting (conventional powder metallurgy) followed by hot extrusion (without caning and degassing). The effect of a T6 heat treatment was also evaluated. The influence of intermetallic additions on the mechanical properties (hardness and tensile strength) and wear behaviour (pin on disk test) was established. All intermetallics showed a good link with the matrix, and high reactivity with it during the heat treatment, as the microstructural study supports.     

Synthesis of hydrophobic aerogels for transparent window insulation applications

Abstract

The present paper deals with the synthesis of hydrophobic aerogels using methyltrimethoxysilane (MTMS) as a hydrophobic agent for transparent window insulation applications. The molar ratio of methanol (solvent), water, and ammonia (catalyst) to the tetramethoxysilane (precursor) MeOH/H₂O/NH₄OH/TMOS was fixed at 12/4&/3·6×10^-3/1 throughout the experiments and the MTMS/TMOS molar ratio M was varied from 0 to 1·55. After gelation, the alcogels were dried supercritically using the high temperature alcohol method. It has been found that lower (<0·26) M values resulted in highly transparent (optical transmission >90% for a 10 mm thick sample at 800 mm wavelength) and negligible volume shrinkage (<2%) but less hydrophobic aerogels whereas higher (>1·03) M values resulted in semitransparent (<20% optical transmission of 800 nm for a 1 cm thick sample) aerogels with >10% volume shrinkage but excellent hydrophobicity. A good compromise of acceptable optical transmittance (～85% optical transmission at 800 nm for a 1 cm thick sample), hydrophobicity with 42 kg m^-3 bulk density, and negligible volume shrinkage were obtained at M≈0·70. Hydrophobicity of the aerogels was tested by measuring the contact angle between a water droplet and the aerogel surface. The aerogels were characterised by infrared spectroscopy, bulk density, optical transmittance, and thermal conductivity measurements.