Homogenization of Precipitation Hardening Nickel Based Superalloys

Allvac 718 Plus and Haynes 282 are relatively new precipitation hardening nickel based superalloys with good high temperature mechanical properties. In addition, the weldability of these superalloys enhances easy fabrication. The combination of high temperature capabilities and superior weldability is unmatched by other precipitation hardening superalloys and linked to the amount of the γ’ hardening precipitates in the materials. Hence, it is these properties that make Allvac 718 Plus and Haynes 282 desirable in the manufacture of hot sections of aero engine components. Studies show that cast products are less weldable than wrought products. Segregation of elements in the cast results in inhomogeneous composition which consequently diminishes weldability. Segregation during solidification of the cast products results in dendritic microstructure with the segregating elements occupying interdendritic regions. These segregating elements are trapped in secondary phases present alongside γ matrix. Studies show that in Allvac 718Plus, the segregating phase is Laves while in Haynes 282 the segregating phase is not yet fully determined. Thus, the present study investigated the effects of homogenization heat treatments in eliminating segregation in cast Allvac 718 Plus and Haynes 282. Paramount to the study was the effect of different homogenization temperatures and dwell time in the removal of the segregating phases. Experimental methods used to both qualify and quantify the segregating phases included SEM, EDX analysis, manual point count and macro Vickers hardness tests. Main results show that there is a reduction in the segregating phases in both materials as homogenization proceeds hence a disappearance of the dendritic structure. In Allvac 718 Plus, plate like structures is observed to be closely associated with the Laves phase at low temperatures and dwell times. In addition, Nb is found to be segregating in the interdendritic areas. The expected trend of increase in Laves as a result of the dissolution of the plate like structures at the initial stage of homogenization is only detectable for few cases. In Haynes 282, white and grey phases are clearly distinguished and Mo is observed to be segregating in interdendritic areas.

Imágenes de Hispanoamérica : Un análisis crítico de material didáctico de ELE

This paper investigates how Latin America and its cultures are represented in textbooks on Spanish as a foreign language. The study aims at investigating how much attention and of what type is dedicated to Latin America in the investigated material, whether the textbooks contribute to giving a varied and nuanced image of the Spanish-American cultures and how this relates to the educational goal of promoting an intercultural competence.A qualitative method of analysis has been applied in order to carry out the analysis of three textbooks for intermediate levels of language studies: Caminando 3, Alegria and De acuerdo.The results of the investigation show that the investigated textbooks mostly present a simplified, ethnocentric, homogenized and sometimes postcolonial image of the Spanish-American cultures. Texts where the culture constitutes the context and not the subject can promote a process of identification and consequently an intercultural competence.The study’s main conclusions show that Spanish-American cultures are underrepresented in the investigated material and that a non-native perspective dominates in the majority of the texts. This combined with the lack of variety and profundity, may have consequences for the promotion of an intercultural competence and for teachers’ work with textbooks and cultural content.