Development of NiAI(B2)-Base Shape Memory Alloys
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temperatures. However, the main drawback to the practical applications of NiAl base SM alloys has been their inherent brittleness at room temperature. Hence considerable efforts have been devoted towards the improvement in the ductility of ft phase alloys as reviewed 1800
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Fig.1 Phase diagram of the Ni-Al system. 467 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
by Liu et al [4]. However, only single ft phase alloys of the Ni-Al-Fe system produced by melt-spinning techniques have been shown to have good ductility and also exhibit the SM effect[5-7]. No significant success in the attempts at improving the ductility of g phase by conventional methods in these systems has been reported so far. The present authors have recently shown that the modification of the microstructure of the single gt phase alloy by deliberately introducing the ductile disordered Y (Al) phase could result in a remarkable improvement in the hot-workability and room temperature ductility of these alloys [8-10]. Utilizing this technique of introducing a ductile phase into the brittle P matrix and using various combinations of thermal and mechanical treatments, a new type of ductile fl +7 two-phase SM alloys in the Ni-Al based alloy systems has been developed [11-13]. This paper describes the recent progress made in the developments pertaining to these Pt +y two-phase SM alloys. ALLOY DESIGN It has been clearly established [8,91, that the most promising method of improving the ductility of the /3 phase in these alloys is by the introduction of the ductile Y phase. Although the P / y phase equilibrium is a metastable one in the Ni-Al binary system as shown in Figure 1, a stable g / y equilibrium can be made to appear in appropriately chosen ternary systems. Figure 2 shows the four types of phase diagrams showing the relative dispositions of the phase fields in Ni-rich portions of Ni-AI-X ternary systems at elevated temperatures [14,15]. It can be seen that the Ply equilibrium becomes stable by the addition of alloying elements such as Fe,Cr,Co,Cu and Mn. From these the Ni-Al-Fe, Ni-Al-Mn and a Ni-Al-Fe-Mn systems were chosen for selecting Pt + y two-phase SM alloys because in these systems the P phase could be formed over wide ranges of composition and temperature, and could be manipulated to great advantage in the control of the thermoelastic transformation temperature of the ft phase. The phase equilibria and the iso-Ms temperature lines in the Ni-Al-Fe and Ni-Al-Mn systems are shown in Figure 3. It can be seen that these systems are appropriate choices for high temperature SM alloys operating above 100°C as will be discussed in a later section.
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