Caracterização microestrutural e mecânica de ligas diluídas de alumínio-nióbio

Dissertação (mestrado)—Universidade de Brasília, Faculdade UnB Gama, Programa de Pós-Graduação em Integridade de Materiais da Engenharia, 2017.Neste presente trabalho foram fabricadas em laboratório duas ligas binarias diluídas do sistema Al-Nb, sendo a primeira com 0,8%Nb e a segunda com 1,2%Nb. Essas ligas foram solidificadas de forma que o fluxo de calor durante a solidificação ocorresse de forma unidirecional e transiente. Para tanto, foi utilizado um dispositivo de solidificação unidirecional vertical ascendente, constituído por um forno com controle eletrônico de temperatura, uma lingoteira de aço inoxidável com chapa molde de aço carbono 1020, instrumentada com termopares posicionados em função da chapa molde e um sistema de arrefecimento capaz de manter a chapa molde em uma temperatura constante de 25o C. Inicialmente foram obtidas as curvas experimentais e determinadas as variáveis térmicas de solidificação, tais como: velocidade de deslocamento da isoterma liquidus e a taxa de resfriamento para ambas as ligas estudadas. As microestruturas foram medidas com o auxilio de técnicas metalográficas e de microscopia óptica e correlacionadas com as variáveis térmicas obtendo-se assim as leis experimentais de crescimento dendrítico primário e secundário. Foram confeccionados corpos de prova para a obtenção do modulo elástico dinâmico, do fator de amortecimento e da dureza em função da variação das microestruturas. Os resultados indicam que as microestruturas mais refinadas tiveram maiores valores de modulo elástico e de dureza.In this work two dilute binary alloys of the Al-Nb system were fabricated in the laboratory, a first one with 0.8% Nb and the second with 1.2% Nb. These alloys were solidified so that the heat flux during solidification occurred unidirectionally and transiently. For this purpose, an upward vertical unidirectional solidification device was used, consisting of an oven with electronic temperature control, a stainless steel ingot mold with 1020 carbon steel mold plate, instrument with thermocouples positioned as a plate and a cooling system Capacity to maintain a plate at a constant temperature of 25o C. Initially they were obtained as experimental curves and determined as solidification thermal variables, such as: velocity of displacement of the liquid isotherm and cooling rate for both alloys studied. The microstructures are measured with the aid of metallographic and optical microscopy techniques and correlated with thermal variables, thus obtaining experimental laws of primary and secondary dendritic growth. Test specimens were prepared to obtain the dynamic elastic modulus, the damping factor and the hardness as a function of the microstructures variation. The results indicate that as more refined microstructures had higher values of elastic modulus and hardness

Upward unsteady-state solidification of silute Al–Nb alloys : microstructure characterization, microhardness, dynamic modulus of elasticity, damping, and XRD analyses

Aluminium alloys form many important structural components, and the addition of alloying elements contributes to the improvement of properties and characteristics. The objective of this work is to study the influence of thermal variables on the microstructure, present phases, microhardness, dynamic modulus of elasticity, and damping frequency in unidirectional solidification experiments, which were performed in situ during the manufacturing of Al–0.8 Nb and Al–1.2 Nb (wt.%) alloys. Experimental laws for the primary ( 1) and secondary ( 2) dendritic spacings for each alloy were given as a function of thermal variables. For Al–0.8%wt Nb, 1 = 600.1(˙T)1.85 and 2 = 186.1(VL)3.62; and for Al–1.2%wt Nb, 1 = 133.6(˙T)1.85 and 2 = 55.6(VL)3.62. Moreover, experimental growth laws that correlate the dendritic spacings are proposed. An increase in dendritic spacing influences the solidification kinetics observed, indicating that metal/mold interface distance or an increase in Nb content lowers the liquidus isotherm velocity (VL) and the cooling rate (˙T). There is also a small increase in the microhardness, dynamic modulus of elasticity, and damping frequency in relation to the composition of the alloy and the microstructure