EFFECTS OF ALLOYING ELEMENTS ON THE PROPERTIES OF ALUMINIUM AND ITS ALLOYS

Abstract

The present investigation forms a part of a series of invest igat ions - that have been izad:rtak.on in this ea-artmentt on a study of aluminium and its alloys. The various physical and mechanical properties studied include metallo graphic structure, lattice parameter, density, coefficient of thermal expansion, hardness, tensile strength,percentage elongation and machinability. Most of the earlier investigations of the lattice parameter were carried out at ordinary tempera-ture and very little work of the study of lattice para-meter at elevated temperatures has been reported. In the present investigation, an attempt has been made to determine as accurately as possible the changes in lattice spacing with' temperature and composition under the equilibrium conditions of alum in ium , aluminium-copper and aluminium-silver alloy systems by using high temperature Unicam X-ray diffraction camera. The knife edge of the camera was calibrated by taking silver standards. The camera was also calibrated for mean specimen temperature corresponding to thermo-couple temperature by taking photographs of silver specimens at different temperatures. Nelson Riley's extrapolation method was adopted for the precise determination of lattice parameter. The equations representing the relation between lattice parameter and temperature iii have been determined by using a digital computer. The values of lattice parameter as determined from the equations are in close agreement with the experimentally determined values. The density and coefficient of thermal expansion were found out in single phase regions. In the case of pure aluminum, the coefficient of ther-mal expansion as determined with reference to lattice parameter at 00 C decreases slightly with the increase in temperature. In the case of aluminium-copper system, the values at 525° and 548° C as determined with refer-ence to lattice parameter at the same temperature, were found to remain almost unchanged upto 4% copper addition. Beyond this the values of coefficient of thermal expansion decrease at an appreciable rate' as the copper concentra tlon increases. The coefficient of thermal expansion for the alloy containing 7.5% silver is lower and for the alloy containing 19.9% silver is higher than that of pure .luminium at a given temperature and determined in the single phase region. The value of lattice parameter for stloys containing 7.5% and 19.9% silver decreases and increases respectively with rise in temperat`ru.re. The solid solubility line as determined by high temperature X-ray diffraction method between 45° and 548° C was found in agreement with that reported by earlidr workers baskd on other methods. The maximum solid solubility at the eutectic temperature was found out to be 5.? weight iv percent ccafirming the results obtained by Raynor. However, the value does not agree with that reported by Ellwood and Silcock which is 5.85 wei-,ht percent.