K. Ullakko

Giant Magnetostrictive Materials

The magnetostrictive materials exhibit a strain caused by the orientation of the magnetic moment when exposed to a magnetic field. A particular class of magnetostrictive materials is called magnetic shape memory (MSM) alloys or ferromagnetic shape memory alloy (FSMA) materials, which can change their shape remarkably when subjected to magnetic field. In the following, the MSM materials are introduced and their extraordinary structure, properties, and performance are described.

New developments in actuator materials as reflected in magnetically controlled shape memory alloys and high-strength shape memory steels

Shape memory alloys (SMA) are applied as actuator materials in smart structures and in fastening and pre-stressing devices. Shape memory alloys can be divided into three groups: one-way alloys, two-way alloys and magnetically controlled SMAs. The magnetically controlled SMAs recently suggested by one of the present authors are potential actuator materials for smart structures because they may provide rapid strokes with large amplitudes under precise control. The most extensively applied conventional SMAs are Ni-Ti and Cu- based alloys. Iron-based shape memory alloys, especially Fe-Mn-Si steels, are becoming more and more important in engineering applications due to their low price. The properties of Fe- Mn-Si steels have been improved by alloying, for example, with Cr, Ni and Co. Nitrogen alloying was shown to significantly improve shape memory, mechanical and corrosion properties of Fe-Mn-Si-based steels. Tensile strengths over 1500 MPa, recovery stresses of 300 MPa and recoverable strains of 4% have been attained. In fasteners made from these steels, stresses of 700 MPa were reached. The beneficial effect of nitrogen alloying on shape memory and mechanical properties is based on the decrease of stacking fault energy and increase of the strength of austenite caused by nitrogen atoms. Nitrogen alloyed Fe-Mn-Si- based steels are expected to be employed as actuator materials in pre-stressing and fastening applications in many fields of engineering. Nitrogen alloyed shape memory steels possess good manufacturing properties and weldability, and they are economical to process using conventional industrial methods.

Large-stroke and high-strength actuator materials for adaptive structures

The most important actuator materials in adaptive structures are shape memory alloys, piezoelectric and magnetostrictive materials and electrorheological fluids. However, no such material is available which would produce rapid and large strokes with high forces. Shape memory alloys exhibit large strokes and forces but their response is slow. Piezoelectric materials and magnetostrictive intermetallics are rapid, but the strokes are small. In the present study, employment of magnetic control of shape memory effect as a principle for rapid large stroke actuator materials is discussed. In such materials, detwinning is controlled by an external magnetic field. Twins in favorable orientation to the magnetic field grow at the expense of other twins and cause a shape change of the actuator. Strokes can be as high as those in shape memory alloys, but response times are short due to magnetic control. Another method which may be applied in actuators is inducing the martensitic transformation and controlling the growth of the martensite plates by magnetostrictive distortions of giant magnetostrictive particles embedded in the shape memory alloy matrix. Magnetostrictive inclusions can also be used as stress sensors in shape memory materials. In pre-stressing and fastening applications, materials which exhibit large strokes and high recovery stresses are required. Nitrogen alloyed shape memory steels, developed for actuators for those applications, is the second topic of this study. In nitrogen alloyed shape memory steels, yield strengths over 1100 MPa and tensile strengths even 1600 MPa were attained. Recoverable strains can be over 4% and recovery stresses 330 MPa. Stresses over 700 MPa were achieved in fasteners at room temperature. Nitrogen alloyed shape memory steels possess good corrosion properties, machinability and weldability (even the welds exhibit shape memory effect). They are economical to manufacture and use and they are expected to have applications in many fields of engineering.

Comparison between nitrogen alloyed Fe - Mn-based damping steels and ferritic damping steels

A variety of damping steels have been developed in recent years for construction purposes. Ferritic and martensitic high-damping steels are the most important groups of damping steels. In the former case, damping is based on magnetoelasticity and in the latter case damping is mainly based on dissipative motion of the interfaces between martensite and austenite phases. Damping properties of two steels of both groups were compared in this report. Compositions of the ferritic steels were Fe-12Cr-3Mo and Fe-2.5Al-0.5Si, and compositions of the martensitic steels were Fe-17.5Mn and Fe-13.7Mn-0.2N. Damping properties of all of those steels were good at room temperature. The highest damping capacity was in alloy Fe-13.7Mn-0.2N. This alloy is the most suitable of these steels to engineering applications because its mechanical properties are better than those of the other steels studied.

Properties of Fe-Al-Si high-damping steel

Several high-damping materials have been developed in recent years to decrease vibration and noise levels of machines and structures. The most important damping steels have been Fe- 12Cr-based alloys. These steels exhibit high damping capacity combined with rather good mechanical and corrosion properties. A new vibration damping Fe-2.5Al-0.5Si steel has been developed by NKK-Corporation in Japan, and it is produced under a trade name of NKK- SERENA. This steel is a potential multi-purpose damping steel, because it is more economical than the previous steels. Damping capacity of NKK-SERENA is very high in wide temperature and frequency ranges, and its mechanical properties are similar to those of common structural steels. In this study, mechanical, welding and corrosion properties, and the results of the microstructural characterization are presented.