Top Metal Magazine “Acta Materialia”: Fatigue Crack Growth Behavior of Shape Memory Alloys

Shape memory alloys (SMAs) have a characteristic deformation response to thermomechanical stimuli. Thermomechanical stimuli originate from high temperature, displacement, solid-to-solid transformation, etc. (high-temperature high-order phase is called austenite, and low-temperature low-order phase is called martensite). Repeated cyclic phase transitions lead to a gradual increase in dislocations, so the untransformed areas will reduce the functionality of the SMA (called functional fatigue) and produce microcracks, which will eventually lead to physical failure when the number is large enough. Obviously, understanding the fatigue life behavior of these alloys, solving the problem of expensive component scrap, and reducing the material development and product design cycle will all generate huge economic pressure.

Thermo-mechanical fatigue has not been explored to a large extent, especially the lack of research on fatigue crack propagation under thermo-mechanical cycles. In the early implementation of SMA in biomedicine, the focus of fatigue research was the total life of “defect-free” samples under cyclic mechanical loads. In applications with small SMA geometry, fatigue crack growth has little effect on life, so the research focuses on preventing crack initiation rather than controlling its growth; in driving, vibration reduction and energy absorption applications, it is necessary to obtain power quickly. SMA components are usually large enough to maintain significant crack propagation before failure. Therefore, to meet the necessary reliability and safety requirements, it is necessary to fully understand and quantify the fatigue crack growth behavior through the damage tolerance method. The application of damage tolerance methods that rely on the concept of fracture mechanics in SMA is not simple. Compared with traditional structural metals, the existence of reversible phase transition and thermo-mechanical coupling poses new challenges to effectively describe the fatigue and overload fracture of SMA.

Researchers from Texas A&M University in the United States conducted pure mechanical and driven fatigue crack growth experiments in Ni50.3Ti29.7Hf20 superalloy for the first time, and proposed an integral-based Paris-type power law expression that can be used for Fit the fatigue crack growth rate under a single parameter. It is inferred from this that the empirical relationship with crack growth rate can be fitted between different loading conditions and geometric configurations, which can be used as a potential unified descriptor of deformation crack growth in SMAs. The related paper was published in Acta Materialia with the title “A unified description of mechanical and actuation fatigue crack growth in shape memory alloys”.

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The study found that when Ni50.3Ti29.7Hf20 alloy is subjected to uniaxial tensile test at 180℃, the austenite is mainly elastically deformed under low stress level during the loading process, and the Young’s modulus is about 90GPa. When the stress reaches about 300MPa At the beginning of the positive phase transformation, austenite transforms into stress-induced martensite; when unloading, stress-induced martensite mainly undergoes elastic deformation, with a Young’s modulus of about 60 GPa, and then transforms back to austenite. Through integration, the fatigue crack growth rate of structural materials has been fitted to the Paris-type power law expression.
Fig.1 BSE image of Ni50.3Ti29.7Hf20 high temperature shape memory alloy and size distribution of oxide particles
Figure 2 TEM image of Ni50.3Ti29.7Hf20 high temperature shape memory alloy after heat treatment at 550℃×3h
Fig. 3 The relationship between J and da/dN of mechanical fatigue crack growth of NiTiHf DCT specimen at 180℃

In the experiments in this article, it is proved that this formula can fit the fatigue crack growth rate data from all experiments and can use the same set of parameters. The power law exponent m is about 2.2. Fatigue fracture analysis shows that both mechanical crack propagation and driving crack propagation are quasi-cleavage fractures, and the frequent presence of surface hafnium oxide has aggravated the crack propagation resistance. The obtained results show that a single empirical power law expression can achieve the required similarity in a wide range of loading conditions and geometric configurations, thereby providing a unified description of the thermo-mechanical fatigue of shape memory alloys, thereby estimating the driving force.
Fig. 4 SEM image of the fracture of NiTiHf DCT specimen after 180℃ mechanical fatigue crack growth experiment
Figure 5 Fracture SEM image of NiTiHf DCT specimen after driving fatigue crack growth experiment under constant bias load of 250 N

In summary, this paper conducts pure mechanical and driving fatigue crack growth experiments on nickel-rich NiTiHf high temperature shape memory alloys for the first time. Based on cyclic integration, a Paris-type power-law crack growth expression is proposed to fit the fatigue crack growth rate of each experiment under a single parameter

Post time: Sep-07-2021