Thermoelastic martensite and shape memory effect in B2 Base Ni-Al-Fe alloy with enhanced ductility
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INTRODUCTION
THE NiAI(/3) phase with B2 structure containing 36 - 38 at. pct A1 has been known to undergo a thermoelastic martensitic transformation from B2 to L10(fl') structure and to exhibit shape memory effect (SME). tl,2] Numerous studies have been carried out on the various aspects of SME in this system, including crystallography and morphology of martensites, f3-~21transformation temperatures, ~2,7,~3-~51 premartensite phenomena, [~~ and deformation behavior, f~7] Figure 1 shows the phase boundaries [18]and the M, temperaturet2] variation with A1 content on the Ni-rich side of this system. It can be seen from Figure 1 that the Ms temperature is very sensitive to the A1 content, and therefore, the control of Ms temperature by conventional alloying technique is extremely difficult to achieve. Moreover, polycrystalline B2 base alloys are generally so brittle that they have not been fabricated by conventional rolling. Recently, some attempts have been made to improve the ductility of the B2 base alloys by rapid solidification t191 and powder metallurgy t2~ techniques. The present authors have found that the addition of Fe, Co, and Cr to the B2 base Ni-A1 alloys brings about a drastic improvement in hot workability and roomtemperature ductility due to the formation of a ductile face-centered cubic (fcc) 3' phase.[22] Extending this idea of ductile phase reinforcement, a new type of shape memory alloy in the Ni-A1 base systems has been developed in our group. The characteristic features of thermoelastic behavior of the martensite and SME in the Ni-A1-Fe ternary alloy with enhanced ductility are reported below.
1.5-mm thickness were obtained by hot forging followed by hot roiling as described in a previous article, t22] Some of the specimens were cold rolled to 0.5-mm thickness and then chemically polished to 0.15-ram thickness in a solution of 45 pct CH3COOH + 33 pct HNO3 + 11 pct HC1 + 11 pct H3PO4. Optical and scanning electron microscopy investigations were carried out on as-rolled and heat-treated specimens. Transmission electron microscopy (TEM) examination was carried out on thin foils prepared by jet polishing in a solution of 20 pct perchloric acid and rest methanol. X-ray diffraction of chemically polished surfaces of specimens using Cu-K~ radiation was performed to identify and characterize the phases present. The chemical compositions of/3 and y phases present in the specimens were determined by energy dispersion X-ray spectroscopy using a standard calibration method. Martensitic transformation temperatures were determined by the standard four-probe electrical resistance measurement method. The temperature variation of the specimens in this method was achieved by lowering (or raising) the four-probe assembly at a constant rate into an n-propyl alcohol bath surrounded by a liquid nitrogen bath. The shape recovery associated with the thermoelastic behavior of the specimens was determined with initial bending strips of 0.15 • 5 x 80-mm dimensions and measuring changes in curvature later. Specimens we
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