JOURNAL OF MATERIALS SCIENCE 28(1993) 1509- 1514 Training effect in Fc Mn-Si shape-memory alloys Y. WATANABE*, Y. MORI, A. SATO Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan The training effect in Fe-Mn-Si shape-memory alloys has been examined by length change and electrical resistivity measurements. After 13 deformation-heating cycles, it was found that the major recovery took place at a temperature lower by 30 K than the first cycle. Simple thermal cycling also lowered the starting temperature of the reverse transformation and increased the finishing temperature. At the same time, the martensitic transformation temperature was found to increase significantly, for example by 35 K, at the 14th thermal cycle. The characteristics of shape-memory effect affected by development of the homogeneous and fine deformation structure by the thermal cycling are discussed in the light of the training effect. 1. Introduction It is known that an Fe Mn-Si alloy containing suit- able amount of manganese and silicon exhibits a good shape-memory effect (SME) of a one-way type, gov- erned by the f c c (7)~ h c p (~) transformation [1-5]. For example, the alloy containing 31 wt % Mn and 6.5 wt% Si exhibits an SME greater than 99% upon deformation at any temperature between 4.2 and 300 K and subsequent heating above 500 K [2]. The origin of the SME is solely attributable to the genera- tion of a particular type of Shockley partial disloc- ations, as has been demonstrated in the earlier works [1, 5]. The more popular and historical shape-memory alloys are Ni-Ti and copper-based alloys: These al- loys, which have an ordered structure, show thermo- elastic properties. It is also known that the SME characteristics of copper-based alloys are strongly affected by the prior deformation and heat treatment [6, 7]. For example, the starting temperature of the martensitic transformation, Ms, increases with the number of cycles and concurrently improves the SME property [7]. This is called a "training effect". In a recent study [8], the "training effect" has been noticed in the SME of a polycrystalline Fe-Mn-Si alloy by repeated deformation and heating cycles; similarly, in other iron-based alloys [9] governed by the 7 ~ a martensitic transformation. The purpose of the present study was to determine whether such an effect also appears in a single crystal. It was expected that clearer information would be obtained by the use of a single crystal in discussing the origin of the training effect. 2. Experimental procedure 2.1. Deformation-heating cycles A single-crystal rod of Fe-31 wt % Mn 6 wt % Si alloy was grown by the Bridgman method in an argon atmosphere. After homogenization treatment at 1453 K for 100 h, tensile specimens of a rectangular shape shown in Fig. 1 were cut by a wheel cutter. These specimens were annealed at 1273 K for 1 h and subsequently quenched into silicone oil at 473 K to avoid the occurrence of martensitic transformation during quenching. The composition of the single crys- tal specimens thus prepared varied from batch to batch by ~ 1 wt % for the manganese content and ~0.5 wt % for silicon, depending on the location of the single-crystal rod. After removing the surface ox- ide layers by electrolytic polishing, the rods were further annealed at 473 K for 20 rain in order to eliminate c-martensite introduced during specimen preparation. The crystallographic geometry of the specimen used in the cyclic deformation heating tests is shown in Fig. l a. The [414] tensile direction was chosen be- cause the Schmid factor is 0.500 for the primary [1 2 1] (1T 1) shear system and 0.385 for the secondary one. The surface plane (1 8 1) was chosen because the width of the specimen does not change'for this geometry in the tensile test. Seven point markings were made using a microVickers hardness tester for length change measurements as shown in Fig. lb. Tensile strain of 1%-6% was given at room temperature at a strain rate of 3 x 10 .4 s 1 with an Instron-type testing machine. After tensile deformation, the specimen was annealed at 673 K for 20 min in order to complete the *Present address: Department of Metallurgical Engineering, Hokkaido University, Kita-ku, Sapporo, Hokkaido 060, Japan. 0022-2461 9 1993 Chapman & Hall 1509