Phase Transformations

The present study is aimed to understand the influence of dynamic precipitation on the low cycle fatigue (LCF) behavior of an under-aged (UA) AA6063 Al–Mg–Si alloy. This was accomplished by the estimation of plastic strain energy density (PSED) at varied isolated cycles during LCF of the UA alloy with subsequent comparison of these results with those of peak-aged (PA) and over-aged (OA) ones. The LCF tests of the UA alloy were carried out in the range of strain amplitudes of 0.2–1.0 % together with the evolution of hardness and tensile properties. The UA alloy shows Masing behavior, evaluated in terms of the variation of Bauschinger strain with plastic strain amplitude, and exhibits continuous hardening till failure unlike the PA and OA alloys. Higher average PSED value for the UA alloy in comparison to that for the PA and the OA alloys indicates dynamic precipitation during cycling; the magnitudes of average PSED were calculated using a proposed method. In addition, pronounced increase in the post LCF hardness values substantiate the dynamic precipitation.

The mechanical properties and radiation tolerance of nanostructured ferritic alloys rely on a dense population of nanometer-scale Y–Ti oxides. The stability of these nano-oxides during extended service is critical in high temperature applications. Here, a model framework is developed for the thermodynamics and kinetics of Y–Ti oxide nucleation, growth and coarsening. The model, which is based upon available thermodynamic and kinetic data as well as key density functional theory calculations, shows that nano-oxide nucleation and growth are highly driven and that pipe diffusion is the dominant mode of their coarsening, in agreement with previous analyses of experimental high temperature data. The model predicts that the nano-oxides are thermally stable for 80 or more years below 1175 K. This analysis also provides insights into the effect of O and Ti on nano-oxide sizes, and on optimization of alloy microstructure.

The preparation of amorphous and tunnel structured-type MnO2 materials with the smallest particle size is reported here. The synthesis is based on the decomposition of solid KMnO4 powder in concentrated sulfuric acid in air at ambient temperature and aging for 18 days. Contrary to reports in the literature, the reaction did not explode possibly because it was done in low humidity and at 28 °C. Characterization of these materials was performed using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and N2 adsorption (BET) analysis. The XRD patterns showed that sublimation produced amorphous-type MnO2, while aging produced crystalline cryptomelane-type MnO2. The product yield for the latter was 16%. TEM studies confirmed a non-fibrous structure for the amorphous material and a fibrous structure for cryptomelane. The HRTEM images of amorphous MnO2 show a dynamic step-flow pattern near dislocations during growth and sublimation and also exhibit the classic rotating Archimedean spiral motion. For cryptomelane, the HRTEM images show that nanofibers have different orientations and discontinuity in the growth of the tunnel. Here we also observe some fibers, which are perpendicular to the other fibers that allowed us to calculate a diameter of ∼3 nm for the tunnel structure. The BET surface area of this nanomaterial was found to be 124 m2 g−1. The TGA measurements demonstrate that it possesses an excellent thermal stability up to 400 °C. We show here that two phases of MnO2 could be produced from the decomposition of KMnO4 in concentrated sulfuric acid at ambient temperature.

The present study elaborately discussed the effect of different modes of metal transfer (i.e., short circuit mode, spray mode and pulse mode) on grain structure and direction of grain growth in low nickel austenitic stainless steel weld metals. Electron backscattered diffraction (EBSD) analysis was used to study the grain growth direction and grain structure in weld metals. The changes in grain structure and grain growth direction were found to be essentially varied with the weld pool shape and acting forces induced by modes of metal transfer at a constant welding speed. Short circuit mode of metal transfer owing to higher Marangoni force (Ma) and low electromagnetic force (Rm) promotes the lowerweld pool volume(Γ) and higherweld pool maximumradius (rm). Short circuit mode also shows curved and tapered columnar grain structures and the grain growth preferentially occurred in <001> direction. In contrast, spray mode of metal transfer increases the Γ and reduces the rm values due to very high Rm and typically reveals straight and broad columnar grain structures with preferential growth direction in <111>. In the pulse mode of metal transfer relatively high Ma and Rm simultaneously increase the weld pool width and the primary penetration which might encourage relatively complex grain growth directions in theweld pool and cause a shift of major intensity from <001> to <111> direction. It can also be concluded that the fusion zone grain structure and direction of grain growth are solely dependent on modes of metal transfer and remain constant for a particular mode of metal transfer irrespective of filler wire used.

Since the Bronze Age, humans have been altering the properties of materials by adding alloying elements. For example, a few percent by weight of copper was added to silver to produce sterling silver for coinage a thousand years ago, because pure silver was too soft. Examples from the modern era include steels that consist primarily of iron, to which elements such as carbon and chromium are added for strength and corrosion resistance, respectively, and copper alloyed with beryllium to make it strong and non-sparking for use in explosive environments. With few exceptions, the basic alloying strategy of adding relatively small amounts of secondary elements to a primary element has remained unchanged over millennia. It is even reflected in the way alloys are named after their principal constituent: ferrous alloys, aluminium alloys, titanium alloys, nickel alloys and so on. However, such a primary-element approach drastically limits the total number of possible element combinations and, therefore, alloys, most of which have been identified and exploited. New approaches are needed if the compositional space to explore is to be significantly enlarged. One such approach is based on mixing together multiple principal elements in relatively high (often equi-atomic) concentrations. This approach stands in sharp contrast to the traditional practice and has, therefore, attracted much attention. The related surge in research activity, especially during the past 5 years, can be traced back to the publication of two seminal papers 1,2 in 2004. Two groups independently proposed the study of a new class of alloys containing multiple elements in near-equiatomic concentrations. It was subsequently pointed out that conventional alloys tend to cluster around the corners or edges of phase diagrams, where the number of possible element combinations is limited, and that vastly more numerous combinations are available near the centres of phase diagrams, especially in quaternary, quinary and higher-order systems 3. Owing to their sheer numbers, little is known about concentrated, multi-component alloys but, by the same token, because there are so many possible combinations, the concept offers promise to discover interesting new alloys with useful properties in their midst. Jien-Wei Yeh and co-workers 1 provided an additional intriguing rationale for investigating these alloys: they hypothesized that the presence of multiple (five or more) elements in near-equiatomic proportions would increase the configurational entropy of mixing by an amount sufficient to overcome the enthalpies of compound formation, thereby deterring the formation of potentially harmful intermetallics. This was a counter-intuitive notion because the conventional view-likely based on binary phase diagrams in which solid solutions are typically found at the ends and compounds near the centres-was that the greater the number of elements in concentrated alloys, the higher the probability that some of the elements would react to form compounds. But Yeh and colleagues reasoned that, as the number of elements in an alloy increased, the entropic contribution to the total free energy would overcome the enthalpic contribution and, thereby, stabilize solid solutions (Box 1; Fig. 1). They coined a catchy new name, high-entropy alloys (HEAs), for this Abstract | Alloying has long been used to confer desirable properties to materials. Typically , it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking.

Microstructure can vary significantly through thickness after ultrafast cooling of rolled steel plates, impacting their mechanical properties. This study examined the microstructure, microstructural banding at centerline, and mechanical properties through thickness for different ultrafast cooling conditions and rolling temperatures. One set of steels (UC1 and UC2) were ultrafast-cooled (UFC) at 40 K/s after finish rolling at 1223 K and 1193 K (950 °C and 910 °C), respectively, while the second set (LC) was cooled by laminar cooling at 17 K/s after finish rolling at 1238 K (965 °C). UFC produced microstructural variation through thickness; highly dislocated lath-type bainitic ferrite was formed near the surface, whereas the primary microstructure was acicular ferrite and irregular polygonal ferrite in the interior of UC1 and UC2 steels, respectively. However, UFC has the advantage of suppression of microstructural banding in centerline segregation regions. The ferrite grain size in both UFC-cooled steels was refined to ~5 lm, increasing strength and toughness. The optimum combination of properties was obtained in UC2 steel with appropriate low finish rolling temperature, being attributed to the distinct microstructure resulting from work-hardened austenite before UFC.

Many historic cities display the expression of individual and cultural identity as a common phenomenon in house designs; this paper inspects it in context of Lahore city. Study is focused on architectural transformation of house designs based on lifestyles of different time periods in Lahore. The objective is to highlight the key modifications in designing of houses over time period of half century. It is believed that home is the most endearing structure to be built given the poignant connection of dwellers to their living spaces. In Lahore, houses show diverse architectural profile as a result of socioeconomic changes and global influences; these demand to investigate the present prototype of traditional versus modern house design in Lahore. A case study of traditional as well as modern house was conducted using field inspection and typological analysis. Findings of such design focused research provide helpful insights for practitioners and academicians of contemporary architecture in Lahore and other likewise cities, about major spatial and design alterations in order to achieve a balanced approach in residential architecture.