Microestrutural evolution in a CuZnAl shape memory alloy: kinetics and morphological aspects

Abstract

The microstructural evolution of the CuZnAl shape memory alloys was studied by indirect techniques relating to the atomic migration rate of grain boundaries. Addition elements were used in a Cu-15,5Zn-8,0Al alloy to provide a comparison with the same alloy without microelement additions. The alloys were melted in an induction furnace of 24 kVA. After casting, the bulk samples of the alloys were homogenized. Then they were solution treated and hot-rolled followed by water-quenching to initiate the recrystallization. Finally, annealing produced at different temperature ranges was made in different samples in order to establish a law for the grain growth. Following the heat treatments, all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the same empirical law of grain growth was found for the different alloys and the ln [D-Do] x 1/T diagrams were plotted in order to establish the kinetic behavior. Based on the estimated values of the activation energy, important conclusions were obtained concerning the addition elements. Keywords: microstructural evolution of the CuZnAl, effect of B and Fe on the CuZnAl alloys, kinetics and morphology of CuZnAl. alloys IntrodutionThe alloys of the CuZnAl system with shape memory effect present two problems that hinder their employment on an industrial scale: these are the natural ageing and the grain growth observed during thermomechanical processing. The first degrades the shape memory effect, while the second, observed during thermomechanical processing of the alloy, displaces the temperatures where the thermoelastic transformations are observed. The extent of the ageing and grain growth can be reduced by control of the rate of atomic diffusion of the solute elements from the matrix to the grain boundaries. This control can be obtained indirectly by increase of the activation energy of diffusion, which can be produced through the addition of microelements in the alloy.After academic euphoria concerning the phenomenological aspects of the shape memory effect, researche on the physical metallurgy turned over to the microstructural control of the alloy, in an attempt to eliminating such typical problems as ageing and grain growth. The traditional method for the stabilization of the microstructures was the addition of alloy microelements. For shape memory copper-based alloys, several microelements have been used. Wang. et al.1 verified the influence of zirconium, titanium, boron and iron on the refinement and the stabilization of grains in an alloy of the CuZnAl system. Morris et al. 2,3 verified the influence of simultaneous use of manganese and boron on the thermoelastic effects and the mechanical properties of a CuAlNi system.For any copper alloy system, the problem is the same: determining the amount required of the microelements to be added to the alloy system, while maintaining the ability for plastic forming required for the manufacturing process. In this work, therefore, the effects of the addition of B, Fe ...

INDENTATION RESISTANCE AND CORROSION BEHAVIOUR OF Fe-Mn MODIFIED Cu-Al ALLOYS IN SELECTED INDUSTRIAL AND BIOLOGICAL FLUIDS

“…This is a clear indication that Fe-Mn addition alters the grain morphology of the Cu-Al alloys. Similar structural modifications with tendencies towards grain refinement have been recorded by the use of some other microelements notably Boron, Titanium, and Titanium-Boron additions [13].…”

“…To some extent the use of microelements and thermomechanical processing has been adopted to address these challenges. The use of microelements such as Ti, B, Fe and thermomechanical treatment results in refinement of the grain structures of the Cu based alloys [13]. With the exception of Fe, some of the other microelements are quite scarce and expensive; and there has been interest to characterize the material behaviour of the alloys when other low-cost microelements are used.…”

“…In the present study, the influence of Fe-Mn addition on the indentation resistance and corrosion behaviour of Cu-Al based alloys in selected industrial and biological fluids is investigated. Fe-Mn was selected as microelement addition for the production of the Cu-Al based alloys because lean additions of both iron and manganese have very good solubility in Cu-Al based alloys [13]. This is also coupled with its relative cheap cost of purchase and lower melting point compared with the use of the pure forms of both iron and manganese.…”

MECHANICAL AND CORROSION BEHAVIOUR OF IRON MODIFIED Cu-Zn-Al ALLOYS

“…The Cu-25.5Zn-4.0Al and Cu-20Zn-4.0Al alloys (in wt %) were initially produced without iron addition. The same alloys were produced with 0.1wt% iron addition with the intention of minimizing grain growth and stabilizing the Cu-Zn-Al alloys microstructure [10]. Supplemental 2.5% Zinc was added during charge calculations to account for potential losses by evaporation during melting.…”

“…Most Cu based exhibit poor cold formability and tendency towards brittle fracture by virtue of their coarse grain structures. Also, they show a tendency to undergo natural ageing which affects its shape memory behaviour by raising the austenite/martensite transition temperature [10].The adoption of procedures to refine the grain structures of Cu-based SMAs by thermomechanical treatment and the use of microelements such as Boron, Iron, Titanium among others has been projected to be the best option in improving its properties [10]. To date the results available are still not exhaustive, and considering that Cu based SMAs remains the most attractive long term option for replacing Ni-Ti as a result of its relatively lower cost; research in Cu based SMAs will still continue to attract attention.…”