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Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one-way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni 50 Ti 50 and ternary Ni 40 Ti 50 Cu 10 SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape-constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring-making are documented. The mechanical and functional characteristics of the wires are investigated in the stress-strain-temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported.

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In the present work the toughness of powder metallurgical tool steels and Metal Matrix Composites (MMC) is being investigated, specifically the influences of heat treatment and the addition of hard phases on the fracture toughness KIc. The experimental materials are cold work tool steel X230CrVMo13-4 as well as hot work tool steel X40CrMoV5-1 with coarse hard phases of the type titanium carbide (TiC), and fused tungsten carbide (FTC) in contents of 0, 10 and 30 vol.-%. The materials were produced by hot isostatic pressing and subsequently heat treated by quenching and tempering in two essentially different treatments. The fracture toughness KIc was measured at precracked three point bending specimens. Fractographical investigations should characterize the fracture formation, progression and mechanism and give information about the influences of microstructure components on the fracture toughness. The experiments showed that generally an addition of coarse hard phases leads to a large decrease in fracture toughness. However, in case of powder metallurgically produced hot work tool steel X40CrMoV5-1, an increase in fracture toughness was achieved by adding 10 vol.-% of fused tungsten carbide. Even an addition of 30 vol.-% FTC exhibits KIc values in the region of a pure cold work matrix. Furthermore it is shown that retained austenite positively affects the fracture toughness

KurzfassungMit dem Ziel, die Lebensdauer von Werkzeugen oder Bauteilen zu verlängern, gewann die Cryobehandlung von Werkzeugstählen in den letzten Jahrzehnten zunehmend an Bedeutung. Dabei handelt es sich um einen einmaligen Zusatzprozess zur konventionellen Wärmebehandlung, bei dem das Werkzeug bis weit unterhalb von Raumtemperatur (−150 °C bis −196 °C) gekühlt wird. In zahlreichen Untersuchungen wird berichtet, dass eine deutliche Steigerung der Werkzeugstandzeit bzw. der Bauteillebensdauer durch diese Art der Behandlung erzielt werden kann. Trotz vieler Veröffentlichungen zu diesem Thema sind gerade die metallkundlichen Vorgänge während dieser Behandlung und die mikrostrukturellen Veränderungen noch nicht vollständig verstanden.In dieser Studie werden die Auswirkungen einer Cryobehandlung insbesondere auf die tribologischen Eigenschaften des Kaltarbeitsstahls 1.2379 untersucht. Neben Laborversuchen wurden zusätzlich Standzeitversuche unter praxisnahen Bedingungen durchgeführt.

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