JOURNAL OF MATERIALS SCIENCE LETTERS 12 (1993) 326-328 A magnetic-functionally graded material manufactured with deformation-induced martensitic transformation Y. WATANABE*,Y. NAKAMURA, Y. FUKUI, K. NAKANISHI Department of Mechanical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40Korimoto Kagoshima 890, Japan Functionally graded materials (FGMs) are some of the most promising materials, whose compositions and microstructures are varied continuously from place to place [1-3]. FGMs have been developed to decrease the thermal stress induced by the large temperature difference within the thickness of thermal barrier materials. FGMs are applicable not only to the mechanical field, but also to the electronic, chemical, optical, nuclear, biomedical and other fields [1, 2]. For instance, a magnetic sensor for position measurement may be developed if the magnetic property of the material used for the sensor is varied continuously from place to place. However, most of the previous studies on FGMs have dealt with the mechanical function, and little attention has been focused on other functions and their applications. FGMs can be classified into composite and mono- lithic materials. Although most of the current FGMs are made of composite materials, they are not considered to be preferable from the viewpoint of material recycling, because it is difficult to separate the dispersion phases from the composite matrices. In contrast, monolithic FGMs have the possibilities of being recycled or the recovery of their functions using very easy methods such as melting or other heat treatments. In particular, metallic FGMs are considered to be readily obtainable, since various conditions of plastic work and heat treatment result in different microstructures depending on the condi- tions. Unfortunately, however, very few works have been concerned with monolithic FGMs. It is known that the paramagnetic phase in austenitic stainless steels, such as an Fe-18Cr-8Ni, transforms into ferromagnetic o:'-martensite phase when they are plastically deformed in a particular low-temperature region [4, 5]. The amount of defor- mation-induced martensite increases at a larger strain and at a lower deformation temperature. Thus, the saturation magnetization of the deformed austenitic stainless steel increases with increasing strain at a constant deformation temperature. Therefore, gradually inhomogeneous deformation is considered to bring about the change of the satura- tion magnetization depending on the local strain. If the relationship between the amount of defor- mation-induced martensite and the amount of plastic *Present address: Department of Metallurgical Engineering, Faculty of Engineering, Hokkaido University, N-13, W-8, Kitaku Sapporo 060, Japan 326 deformation is known, it would be easy to design the profile of the saturation magnetization by changing the local strain. In addition, the deformation- induced martensite transforms into austenite when the deformed stainless steels are heated to tem- peratures in the austenitic region. This is one of the simplest methods of obtaining virgin materials for FGMs. The aims of this study were to manufacture the magnetic FGM by applying gradually inhomo- geneous deformation to SUS304 austenitic stainless steel (Fe-18Cr-8Ni) and to examine the relation- ship between the saturation magnetization and the local strain. Three kinds of tensile specimens, called types I, II and III, were machined from a 1 ram-thick plate of as-received SUS304 stainless steel. The chemical composition of the stainless steel studied was (wt %): Cr 18.06, Ni 8.47, C 0.04, Si 0.52, Mn 1.27, P0.026, S 0.005, Cu0.06, Mo 0.06, N0.049 and O 0.0039. The shape and dimensions of the speci- mens are shown in Fig. 1. Type I specimens were standard testpieces with a uniform cross-sectional area in its reduced gauge section (Fig. la). In this type of testpiece a uniform plastic strain is expected to be introduced, resulting in the corresponding saturation magnetization constantly distributed along the tensile axis. In type II and III specimens the cross-sectional areas of the reduced gauge sections decrease linearly from the left shoulders of Fig. lb and c (labelled A) in the direction of the tensile axis. The inclination angles, defined as the angles of the specimen edges from the tensile axis, were 3° for type II and 1 ° for type III. In this study it was necessary to observe, the distribution of the plastic strain along the tensile axis precisely. The plastic strains were measured by the point-marking method. Markings with a 1 mm in- terval in the direction of the tensile axis were made by a micro-Vickers hardness tester before the tensile tests. Tensile tests were conducted at room tem- perature at a crosshead speed of 0.5 mm min -1 using an Instron-type testing machine. After the tensile deformation the relative displacements between markings were measured under a precision machin- ary microscope, and the corresponding local strains in the direction of the tensile axis were calculated. Then deformed specimens were cut into pieces of width 1 mm by a low-speed cutter perpendicular to the tensile axis. The saturation magnetization of each piece was measured by magnetic balance at room temperature. 0261-8028 © 1993 Chapman & Hail