The creep behaviour of pressure diecast zinc-aluminium based alloys

The creep behaviour of three pressure diecast commercial zinc-aluminium based alloys: Mazak 3, corresponding to BS 1004A, and the new alloys ZA.8 and ZA.27 with a series of alloys with compositions ranging from 0% to 30% aluminium was investigated. The total creep elongation of commercial alloys was shown to be well correlated using an empirical equation. Based on this a parametrical relationship was derived which allowed the total creep extension to be related to the applied stress, the temperature and the time of test, so that a quantitative assessment of creep of the alloys could be made under different conditions. Deviation from the normal creep kinetics occurred in alloys ZA.8 and ZA.27 at very low stresses, 150°C, due to structural coarsening combined with partial transformation of ε -phase into T' phase. The extent of primary creep was found to increase with aluminium content, but secondary creep rates decreased in the order Mazak 3, ZA.8 and ZA.27. Thus, based on the above equation, ZA.8 was found to have a substantially better total creep resistance than ZA.27, which in turn was marginally better than Mazak 3 for strains higher than 0.5%, but inferior for smaller strains, due to its higher primary creep extension. The superior creep resistance of ZA.8 was found to be due to the presence of strictly-orientated, thin plate-like precipitates of ε(CuZn4) phase in the zinc matrix of the eutectic and the lamellarly decomposed β phase, in which the precipitation morphology and orientation of ε in the zinc matrix was determined. Over broad ranges of temperature and stresses, the stress exponents and activation energies for creep were found to be consistent with some proposed creep rate mechanisms; i.e. viscous glide for Mazak 3, dislocation climb over second phase particles for ZA.8 and dislocation climb for ZA.27, controlled by diffusion in the zinc-rich phase. The morphology of aluminium and copper-rich precipitates formed from the solid solution of zinc was clearly revealed. The former were found to further increase the creep rate of inherently low creep resistant zinc, but the latter contributed significantly to the creep resistance. Excess copper in the composition, however, was not beneficial in improving the creep resistance. Decomposition of β in copper-containing alloys was found to be through a metastable Zn-Al phase which is strongly stabilised by copper, and the final products of the decomposition had a profound effect on the creep strength of the alloys. The poor creep resistance of alloy ZA.27 was due to the presence of particulate products derived from decomposed β-phase and a large volume of fine, equiaxed products of continuously decomposed α-dendrites.

The fatigue properties of pressure diecast zinc-aluminium based alloys

The fatigue behaviour of the cold chamber pressure-die-cast alloys: Mazak3, ZA8, ZA27, M3K, ZA8K, ZA27K, K1, K2 and K3 was investigated at temperature of 20°C. The alloys M3K, ZA8K and ZA27K were also examined at temperatures of 50 and 100°C. The ratio between fatigue strength and tensile strength was established at 20°C at 107 cycles. The fatigue life prediction of the alloys M3K, ZA8K and ZA27K was formulated at 20, 50 and 100°C. The prediction formulae were found to be reasonably accurate. All of the experimental alloys were heterogeneous and contained large but varying amounts of pores. These pores were a major contribution and dominated the alloys fatigue failure. Their effect, however, on tensile failure was negligible. The ZA27K possessed the highest tensile strength but the lowest fatigue strength. The relationship between the fracture topography and the microstructure was also determined by the use of a mixed signal of a secondary electron and a back-scattered electron on the SEM. The tensile strength of the experimental alloys was directly proportional to the aluminium content within the alloys. The effect of copper content was also investigated within the alloys K1, K2, ZA8K and K3 which contained 0%, 0.5%, 1.0% and 2.0% respectively. It was determined that the fatigue and tensile strengths improved with higher copper contents. Upon ageing the alloys Mazak3, ZA8 and ZA27 at an ambient temperature for 5 years, copper was also found to influence and maintain the metastable Zn-Al (αm) phase. The copper free Mazak3 upon ageing lost this metastable phase. The 1.0% copper ZA8 alloy had lost almost 50% of its metastable phase. Finally the 2.0% copper ZA27 had merely lost 10% of its metastable phase. The cph zinc contained a limited number of slip systems, therefore twinning deformation was unavoidable in both fatigue and tensile testing.

Center of shelf attention:understanding the role of visual attention on product choice

Two eye-tracking studies and an offline experiment explored the effect of central shelf location on attention and choice. Investigation of the attention process revealed that the central gaze cascade effect, progressively increasing attention focused on the central option predicted choice.

Recollections of pressure to eat during childhood, but not picky eating, predict young adult eating behavior

Picky eating is a childhood behavior that vexes many parents and is a symptom in the newer diagnosis of Avoidant/Restrictive Food Intake Disorder (ARFID) in adults. Pressure to eat, a parental controlling feeding practice aimed at encouraging a child to eat more, is associated with picky eating and a number of other childhood eating concerns. Low intuitive eating, an insensitivity to internal hunger and satiety cues, is also associated with a number of problem eating behaviors in adulthood. Whether picky eating and pressure to eat are predictive of young adult eating behavior is relatively unstudied. Current adult intuitive eating and disordered eating behaviors were self-reported by 170 college students, along with childhood picky eating and pressure through retrospective self- and parent reports. Hierarchical regression analyses revealed that childhood parental pressure to eat, but not picky eating, predicted intuitive eating and disordered eating symptoms in college students. These findings suggest that parental pressure in childhood is associated with problematic eating patterns in young adulthood. Additional research is needed to understand the extent to which parental pressure is a reaction to or perhaps compounds the development of problematic eating behavior.

Perceptions of parental pressure to eat and eating behaviours in preadolescents:the mediating role of anxiety

Previous research suggests that parental controlling feeding practices are associated with children's overeating and undereating behaviours. However, there is limited research addressing the link between children's mental health symptoms (specifically anxiety and depression) and their reports of eating behaviours, despite knowledge that these psychopathologies often co-exist. The current study aimed to identify the relationships between preadolescents' perceptions of their parents' feeding practices with reports of their own anxiety, depression and eating behaviours. Three hundred and fifty-six children (mean age 8.75 years) completed questionnaires measuring their dietary restraint, emotional eating and external eating, as well as their perceptions of their parents' use of pressure to eat and restriction of food. Children also completed measures of general anxiety, social anxiety and depression symptomology. Results indicated that preadolescents' eating behaviours were associated with their perceptions of the controlling feeding practices their parents used with them. Preadolescents' dietary restraint, emotional eating and external eating behaviours were positively associated with their reports of general and social anxiety, and depression symptomology. In addition, perceptions of parental pressure to eat were positively related to preadolescents' anxiety and depression levels. Child anxiety (general and social) was found to mediate the relationship between perceptions of parental pressure to eat and preadolescents' eating behaviours (dietary restraint, emotional eating and external eating). The results suggest that greater anxiety in preadolescents may explain why children who perceive greater pressure to eat by their parents are more likely to exhibit maladaptive eating behaviours. © 2014 Elsevier Ltd.