Strengthening of AISI 2205 duplex stainless steel by strain ageing

In this study, static strain ageing behavior of commercially available and solution heat treated duplex stainless steel was investigated and the effect of static strain ageing on the mechanical properties was also determined in detail. Some of as-received duplex stainless steel test specimens were pre-strained in tension by 5% and then aged at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C for 30 min in furnace. Some of duplex stainless steel test specimens were solution heat treated at 1050 °C for 30 min, water quenched and then pre-strained for 5% in tension shortly after the solution heat treatment.In order to identify the effect of static strain ageing on the mechanical properties, the tensile strength, the change in the strength due to ageing (ΔY), elongation fracture and hardness were determined. The test results showed that the mechanical properties were affected by static strain ageing mechanism which was applied at different temperatures for same time interval.     

Heat treatment effect on thermo-mechanical fatigue and low cycle fatigue behaviors of A356.0 aluminum alloy

In the present paper, the heat treatment effect on A356.0, a cast aluminum alloy which has been widely used in diesel engine cylinder heads, is investigated under out-of-phase thermo-mechanical fatigue and low cycle fatigue (at different temperatures) loadings. A typical heat treatment is applied to the material including 8 h solution at 535 °C, water quench and 3 h ageing at 180 °C. The experimental fatigue results show that the heat treatment process has considerable influence on mechanical and low cycle fatigue behaviors, especially at room temperature, but its effect on thermo-mechanical fatigue lifetime is not significant. The improvement in the strength can be explained by the dislocation theory. Under thermo-mechanical fatigue loadings, the difference between the fatigue lifetime of A356.0 alloy and A356.0-T6 alloy decreases when the temperature range increases. In this condition, plastic strain increases severely during the fatigue cycles in A356.0-T6 alloy due to over-ageing phenomenon and therefore, the amount of cyclic softening in heat treated alloy is more.     

Effect of heat treatment on the tribological properties of Al–Cu–Mg/nanoSiC composites

In this study, the effect of heat treatment on the tribological properties of Al–Cu–Mg alloy reinforced with 4 wt.% SiC particles with 650 nm average particle size has been investigated. The age hardening process consists of solution treatment at 540 °C for 6 h, followed by water quenching and ageing at different temperatures of 175, 200 and 225 °C with soaking times of 3, 6 and 9 h. Hardness measurements were applied to monitor the precipitation effect and the aged samples were then subjected to wear tests under dry sliding conditions against steel and alumina counterfaces. The results showed that the reinforced material exhibits an enhanced ageing response compared to the unreinforced material in the same heat treatment conditions. The rate of ageing increases with increasing temperature; however, ageing at 200 and 225 °C for more than 6 h resulted in over-ageing. The best combinations for the enhanced tribological properties for the composite material were selected as 6 h ageing at 225 °C. The precipitation effect for this alloy can be enhanced by the small addition of SiC nanoparticles. Having a small amount of nanoSiC particles with fine precipitates inside the matrix further increases the hardness and wear properties.     

Glass transitions of hygrothermal aged pultruded glass fibre reinforced polymer rebar by dynamic mechanical thermal analysis

The changes that can occur in glass fibre reinforced polymer (GFRP) composites with ageing can affect its application, performance and lifetime. Hygrothermal ageing (i.e. accelerated ageing by moisture absorption and temperature change) is a very useful technique to evaluate durability as well as development of GFRP composites in a reasonable timeframe. Dynamic mechanical thermal analysis (DMTA) is essentially able to detect all changes in the state of molecular motion in polymer composites as temperature is scanned. In this work, pultruded GFRP rebars were accelerated aged in an alkaline aqueous environment at 60 °C for 1, 2, 3, 4 and 6 months to evaluate the changes in glass transition of viscoelastic GFRP rebars by DMTA. Five different glass transitions in an average temperate range from 110 to 165 °C were observed at storage modulus, loss modulus and damping factor traces of DMTA. It was also found that glass transition temperature (T_g) of the aged samples changed up to maximum 6 °C compared with that of controlled sample. This change in T_g with ageing time was believed to be due to moisture absorption by rebars.     

Precipitation in Al–Mg solid solution prepared by solidification under high pressure

The precipitation in Al–Mg solid solution containing 21.6 at.% Mg prepared by solidification under 2 GPa was investigated. The results show that the γ-Al₁₂Mg₁₇ phase is formed and the β′ phase cannot be observed in the solid solution during ageing process. The precipitation of γ and β phases takes place in a non-uniform manner during heating process, i.e. the γ and β phases are first formed in the interdendritic region, which is caused by the inhomogeneous distribution of Mg atoms in the solid solution solidified under high pressure. Peak splitting of X-ray diffraction patterns of Al(Mg) solid solution appears, and then disappears when the samples are aged at 423 K for different times, due to the non-uniform precipitation in Al–Mg solid solution. The direct transformation from the γ to β phase is observed after ageing at 423 K for 24 h. It is considered that the β phase is formed through a peritectoid reaction of α + γ → β which needs the diffusion of Mg atoms across the interface of α/γ phases.     

Colour and ultrasound propagation speed changes by different ageing of freezing/thawing and cooling/heating in granitic materials

In the present work we determined the chromatic coordinates (L*, a*, b*) and ultrasound propagation speeds on the three spatial planes (V_x, V_y, V_z) of three ornamental granites (Aqueduct of Segovia, Spain) before, during, and after being subjected to 70 cycles of two types of accelerated ageing (typical of cold regions): a) freezing/thawing and cooling/heating (T1), and b) freezing/thawing and cooling/heating + salt crystallization (T2). A multivariate technique (Canonical Biplot) was applied to the data obtained, with the observation of significant variations between the two types of accelerated artificial ageing as compared with those obtained in quarry rock in the three chromatic coordinates (L*, a*, b*). With regard to the ultrasound propagation speed, we only detected differences in the results of the T2 artificial ageing treatment with respect to those of quarry rock. This fact is confirmed by the estimated data of resistance to compression.     

Compression properties at different loading directions of as-extruded Mg–9RY–4Zn (RY: Y-rich misch metal) alloy with long period stacking ordered phase

The compression properties at different loading directions of as-extruded Mg–9RY–4Zn alloy with long period stacking ordered (LPSO) phase were investigated. The compressive yield strength (σ_0.2), ultimate compressive strength (σ) and elongation to failure (ε) are 272 MPa, 520 MPa and 19% at ED, 172 MPa, 412 MPa and 17% at TD, and 150 MPa, 370 MPa and 16% at 45° orientation, respectively. The excellent compression properties result from the 14H LPSO strips and random oriented DRX grains with 14H LPSO lamellae. Meanwhile, the as-extruded Mg–9RY–4Zn alloy exhibits obvious mechanical anisotropy. The strength at ED is much higher than that at 45° orientation. Specific to the present alloy, besides the weak basal texture, it is considered that the LPSO long strips with characteristic orientation play an important role in influencing the mechanical anisotropy.     

Improvement of mechanical properties of stainless maraging steel laser weldments by post-weld ageing treatments

The response of stainless maraging steel weldments to post-weld ageing treatment has been investigated. Post-weld ageing was performed at five different temperatures, viz., 420 °C, 460 °C, 500 °C, 540 °C, and 580 °C. Metallographic characterization of weldment revealed three zones, namely fusion zone, heat-affected zone (HAZ) and unaffected parent metal zone. Hardness and tensile properties were evaluated after ageing at different temperatures. Hardness in HAZ and fusion zone varied with ageing temperature differently from that of the parent metal; it became higher in HAZ and fusion zone than in parent metal zone above 420 °C. Among the applied ageing treatments, ageing at 460 °C achieved the highest tensile strength. A graph was constructed for determination of fracture location and post-weld heat treatment efficiency based on experimental results, using hardness ratio of HAZ to the treated parent material and hardness ratio of HAZ to the as-received parent material.     

Effect of heat treatment and Fe content on the microstructure and mechanical properties of die-cast Al–Si–Cu alloys

The effect of solution and ageing heat treatment on the microstructure and mechanical properties of the die-cast Al–9 wt.%Si–3.5 wt.%Cu alloys containing 0.1–1.0 wt.% Fe was investigated. The results showed that the dendritic primary α-Al phase was varied from 20 to 100 μm in size and the globular α-Al grains were smaller than 10 μm in size. The Fe-rich intermetallics exhibited coarse compact or star-like shapes with the sizes from 10 to 20 μm and the fine compact particles at an average size of 0.75 μm. The solution treatment of the alloys could be achieved in a short period of time, typically 30 min at 510 °C, which dissolved the Cu-rich intermetallics into the primary α-Al phase and spheroidised the eutectic Si phase. During the subsequent ageing treatment, numerous fine precipitates of θ′ and Q′ phases were formed to provide effective strengthening to the α-Al phase, significantly improving the mechanical properties. Therefore, Fe content in the die-cast Al–Si–Cu alloys needs to be controlled at a low level in order to obtain the improved ductility and strength under solution and aged condition.     

Mechanical properties of aluminium–copper–lithium alloy AA2195 at cryogenic temperatures

Tensile testing was performed on a 4 mm thick sheet of the aluminum–lithium alloy AA2195 in T87 (solution treatment + water quenching + 7% cold work + peak aging) temper which was subjected to 7% cold working by combination of cold rolling and stretching, over a temperature range from ambient to liquid hydrogen (20 K) conditions. Properties were evaluated in longitudinal as well as transverse directions to characterize anisotropy with respect to strength and ductility. Strength and ductility were compared to the conventional aluminum alloy AA2219-T87, developed for similar cryogenic applications. Decreases in test temperature led to higher strengths with little or no change in ductility. As the temperature decreases, the differences between ultimate tensile strength as well as yield strength for two different combinations of cold roll and stretch studied in the present work, narrows down and become equal at 20 K.     

Natural convection flow from an isothermal horizontal circular cylinder in presence of heat generation

Natural convection laminar boundary layer flow from a horizontal circular cylinder with a uniform surface temperature at presence of heat generation has been investigated. The governing boundary layer equations are transformed into a non-dimensional form and the resulting nonlinear systems of partial differential equations are solved numerically applying two distinct methods namely (i) implicit finite difference method together with the Keller box scheme and (ii) series solution technique. The results of the surface shear stress in terms of the local skin friction and the surface rate of heat transfer in terms of the local Nusselt number for a selection of the heat generation parameter γ (= 0.0, 0.2, 0.5, 0.8, 1.0) are obtained and presented in both tabular and graphical formats. Without effect of the internal heat generation inside the fluid domain for which we take γ = 0.0, the present numerical results show an excellent agreement with those of Merkin [J.H. Merkin, Free convection boundary layer on an isothermal horizontal circular cylinders, in: ASME/AIChE, Heat Transfer Conference, St. Louis, MO, August 9–11, 1976]. The effects of γ on the fluid velocity, temperature distribution, streamlines and isotherms are examined.     

High-precision CTE measurement of hybrid C/SiC composite for cryogenic space telescopes

This paper presents highly precise measurements of thermal expansion of a “hybrid” carbon-fiber reinforced silicon carbide composite, HB-Cesic® – a trademark of ECM, in the temperature region of ～310–10 K. Whilst C/SiC composites have been considered to be promising for the mirrors and other structures of space-borne cryogenic telescopes, the anisotropic thermal expansion has been a potential disadvantage of this material. HB-Cesic® is a newly developed composite using a mixture of different types of chopped, short carbon-fiber, in which one of the important aims of the development was to reduce the anisotropy. The measurements indicate that the anisotropy was much reduced down to 4% as a result of hybridization. The thermal expansion data obtained are presented as functions of temperature using eighth-order polynomials separately for the horizontal (XY-) and vertical (Z-) directions of the fabrication process. The average CTEs and their dispersion (1σ) in the range 293–10 K derived from the data for the XY- and Z-directions were 0.805 ± 0.003 × 10^?6 K^?1 and 0.837 ± 0.001 × 10^?6 K^?1, respectively. The absolute accuracy and the reproducibility of the present measurements are suggested to be better than 0.01 × 10^?6 K^?1 and 0.001 × 10^?6 K^?1, respectively. The residual anisotropy of the thermal expansion was consistent with our previous speculation regarding carbon-fiber, in which the residual anisotropy tended to lie mainly in the horizontal plane.     

Microstructure and mechanical properties of friction spot welding aluminium–lithium 2A97 alloy

In this study, we investigated the microstructure and mechanical properties in different regions of the friction spot welded 2A97 aluminium–lithium alloy subjected to different heat treatment processes. The 2.0 mm thick hot-rolled sheet of 2A97 alloy was successfully welded using friction spot welding method with optimised welding parameters. Afterwards, the as-welded 2A97 joints experienced two subsequent heat treatment procedures: solution and ageing; directly ageing. The corresponding microstructure and mechanical properties of the heat-treated specimens were studied by means of optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), hardness test and tensile test. The results show that the mechanical properties of the 2A97 joints before and after heat treatment were significantly modified, which was mainly related to fine-grained microstructure, size and type of precipitates, and dislocation density. Compared to the base material and the material that only experienced direct ageing, the whole friction spot welded 2A97 joint after solution and ageing treatment delivered better mechanical properties.     

Effect of fabrication conditions and Cr,Zr contents on the grain structure of 7075 and 6061 aluminum alloys

The correlation between compression conditions at temperatures in the range of 573–773 K with the strain rate range of 0.002–2 s^?1 and grain size after solution heat treatment of 7075 alloy was investigated, as contrasted with 6061 alloy. The grain coarsening occurred under specific Zener-Hollomon (Z) parameters of 5 × 10¹⁰–10¹² s^?1 for 7075 alloy, 10⁸–2 × 10¹² s^?1 for 6061 alloy, respectively. These phenomena could be explained by crystalline orientation analysis and stored deformation strain evaluation. The site of subgrains with less than 15° misorientation and stored strain after compression increased, but the site of recrystallized grains after solution heat treatment increased with Z parameter. Small Z parameter condition could get low stored strain with fine grain which is stable during SHT. Effect of Cr and Zr on the grain structure of 7075 alloy was also investigated. Cr or Zr addition could inhibit the grain coarsening. The role of Zr addition was confirmed to pinning effect of Al₃Zr dispersoids to subgrain boundaries.     

Evaluation of thermal conductivity in air permeable concrete for dynamic breathing wall construction

Air permeable concrete (APC) is potentially useful as a dynamic insulator. The dynamic function is achieved by passing air through the material in the direction of heat flow to facilitate heat recovery. An APC sample of 200 mm length with 60% cement filling of large voids (between 0.5 and 5 mm), was tested between 5 and 10 Pa differential pressures; permeabilities were 0.28–0.32 m²/Pa h, confirming its suitability as a dynamic insulator. To characterise properties it is necessary to determine the static thermal conductivity, i.e., no air flow. A one-dimensional heat flow model for predicting the effective thermal conductivity (λ_e) of APC is developed using as variables the fractions of voids, aggregate and cement paste comprising the material. Measured values of λ_e were 0.7–1.4 W/m K. A theoretical model predicts and further improves the performance and formulation of APC. The water/cement ratio (w/c) also controls the λ_e. Increasing w/c increases the volume of micropores, adding resistance to heat flow.     

Study of decomposition of ferrite in a duplex stainless steel cold worked and aged at 450–500 °C

The influence of cold-deformation on ferrite decomposition in duplex stainless steel during heat treatment at 450–500 °C was investigated using micro-hardness measurements and transmission electron microscopy. It was found that cold-deformation can change the mechanism of the α → α + α′ phase separation in the ferrite from nucleation and growth to spinodal decomposition. This finding is discussed in terms of the influence of an increased dislocation density on coherency strains.     

High-temperature electrical resistivity and heat capacity studies on nano-crystalline geikielite

We report here for the first time the temperature dependence of the electrical resistivity and heat capacity of nano-crystalline MgTiO₃ geikielite of up to 1000 K. The temperature dependence of heat capacity of nano-crystalline geikielite expressed as C_p = 46.44(5) + 0.0502(2)T − 4.56 × 10⁶T² + 1.423 × 10³T^− 0.5 − 8.672 × 10^− 6T^− 2, where C_p = is specific heat expressed in J/mol. K and T is the temperature in K. Both the electrical resistivity and heat capacity behaviour show that the geikielite (both the natural and synthetic nano-crystalline samples) are stable and remains electrically insulating up to 1000 K.     

Non-isothermal flow of a power law fluid past a rectangular obstacle (of aspect ratio 1 × 2) in a channel: Drag and heat transfer

A two-dimensional numerical study has been carried out to investigate the drag and Nusselt number characteristics under forced convection conditions between a streaming power law liquid and a rectangle (with its longer side aligned with the direction of flow) placed symmetrically between two solid walls. In particular, the values of the individual and total drag coefficients, and of the Nusselt number are obtained as functions of the flow behaviour index (1.4 ⩾ n ⩾ 0.5), of Reynolds number (5 ⩽ Re ⩽ 40) and of the Peclet number (5 ⩽ Pe ⩽ 400) for a fixed value of the blockage ratio (1/8). Within these ranges of kinematic and rheological conditions, the drag and Nusselt number show only fair to moderate deviation from the corresponding Newtonian values at the same values of the Reynolds and Peclet numbers. Qualitatively speaking, the shear-thinning behaviour (n < 1) augments the drag and heat transfer while the shear-thickening behaviour (n > 1) causes both the drag and heat transfer to drop below the corresponding Newtonian values. The power-law fluid behaviour does not seem to alter the streamline, isovorticity and isotherm plots in a significant manner, except for the fact that the shear-thinning behaviour not only delays the formation of a visible wake but the resulting wake is also somewhat shorter than that in a Newtonian fluid. The shear thickening, on the other hand, has exactly the opposite influence on wake formation.     

First-principles calculations of mechanical and thermodynamic properties of YAlO3

First-principles calculations are performed to investigate the crystal structure, electronic properties, the elastic properties, hardness and thermodynamic properties of YAlO₃. The calculated ground-state quantities such as lattice parameter, bulk modulus and its pressure derivative, the band structure and densities of states were in favorable agreement with previous works and the existing experimental data. The elastic constants C_ij, the aggregate elastic moduli (B, G, E), the Poisson’s ratio, and the elastic anisotropy have been investigated. YAlO₃ exhibits a slight elastic anisotropy according to the universal elastic anisotropy index A^U = 0.24. The estimated hardness for YAlO₃ is consistent with the experimental value, and Al–O bond in AlO₆ octahedra plays an important role in the high hardness. The Y–O bonds in YO₁₂ polyhedra exhibit different characteristic. Using the quasi-harmonic Debye model considering the phonon effects, the temperature and pressure dependencies of bulk modulus, heat capacity and thermal expansion coefficient are investigated systematically in the ranges of 0–20 GPa and 0–1300 K.     

Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment

The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl₂ and Al₅Mg₈Cu₂Si₆) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7 wt% Si–3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3 wt% Mg with 150 ppm Sr, viz. for the Y4S alloy. The corresponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), and 1.50% (%El), compared to 145 MPa (YS), 232 MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520 °C solution temperature, incipient melting of Al₅Mg₈Cu₂Si₆ phase and undissolved block-like Al₂Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al₂Cu, incipient melting of Al₅Mg₈Cu₂Si₆ phase and Al₂Cu phase, as well as the Si particle characteristics.