Design and Mechanical Behavior of Martensitic Alloys

1. Introduction

In the world of materials, few phenomena are as intriguing and simultaneously as useful as martensitic phase transformations. They make shape memory materials morph into new geometries when instructed, they let transformation-induced plasticity (TRIP) alloys absorb large amounts of mechanical energy without rupturing, and they produce some of the hardest and most resistant materials that technology has to offer. Martensitic transformations have revolutionized materials technology and continue to do so by enabling:

• miniaturized, compact actuators and sensors for aerospace, automotive, and minimally invasive surgery;
• lightweight yet highly resilient and tough structures for automotive and transportation;
• the production of extremely strong steels.

The Special Issue “Design and Mechanical Behavior of Martensitic Alloys” delves into martensitic alloys and offers a mixed, illuminating snapshot of recent developments, current challenges, and future opportunities of this broad and interdisciplinary topic.

2. Overview of Published Contributions

The present rapid advancement of artificial intelligence and machine learning (ML) opens new possibilities for predicting the properties and behaviors of engineering materials, including martensitic materials. In a timely review, contribution 1 therefore introduces the new paradigm of data-driven science in the context of the design and development of martensitic alloys. The authors first examine ML tools and techniques for the prediction of martensitic transformation temperatures and then discuss improvements needed to arrive at accurate and precise models capable of handling broad ranges of alloying elements and concentrations. Specifically in the context of multi-principal element (MPE) shape memory alloys (SMAs), ML approaches offer to efficiently sweep the vast search space, provided sufficient amounts of valid, correct training data are available. Furthermore, as highlighted by the authors, physics-informed multi-objective ML models hold substantial promise for the simultaneous optimization of transformation temperatures and thermal hysteresis for novel SMAs with broad operation ranges and enhanced functional stability.

Current shape memory technology strongly relies on alloys, which exhibit operation temperatures between 0 °C and ≥100 °C. In contrast to these common SMAs, SMAs that undergo thermoelastic phase transitions below 0 °C are underexplored. However, such low-temperature SMAs are important for cryogenic equipment and extreme environments like space. Contribution 2 provides a comprehensive overview of SMAs for low-temperature operation, spanning from the first report of a cryogenic SMA in 1974 to the most recent developments. The authors categorize low-temperature SMAs based on their martensitic transformation temperatures, highlighting Cu-based systems with ultra-low transformation temperatures. To facilitate characterizing low-temperature behavior and improving insight into materials performance, cryogenic testing systems need to be developed in parallel. Furthermore, the impact of alloy composition on transformation temperatures poses a significant challenge for material synthesis and upscaling.

The stress-induced martensite transformation in NiTi SMAs typically proceeds at constant external load, leading to a load plateau extending for 5–8% of strain. The absence of a unique correspondence between the load and inelastic deformation on the plateau prevents the precise control of total strain via an external load. To overcome this limitation, the authors of contribution 3 created graded NiTi samples with varying cross-sections along the sample length. In this way, they achieve sequences of plateaus by introducing steps in the sample width, on the one hand. On the other hand, linear force–strain responses are obtained by continuously grading the sample width. The authors then developed a heat treatment to enhance cyclic stability by combining recrystallization annealing and low-temperature aging, promoting the formation of Ni₄Ti₃ nanoprecipitates. Contribution 3 is an important step forward in achieving better control of the superelastic response and functional stability of NiTi SMAs.

Contribution 4 investigates a novel non-equiatomic VCrFeCoNi MPE alloy under various deformation rates and temperatures. Depending on temperature, this alloy exhibits different deformation mechanisms, including TRIP due to the martensitic phase transformation of face-centered cubic into body-centered cubic; deformation twinning (TWIP); and crystallographic slip. The authors observed TRIP activity at cryogenic temperatures subject to the rate of deformation. Slow deformation results in extensive martensitic transformation, leading to high ultimate tensile strength and ductility. On the other hand, fast deformation hinders the martensitic transformation and reduces the hardening capacity. The authors found that at room temperature, TWIP dominates, particularly at intermediate deformation rates, while TRIP only occurs for slow deformation. At 300 °C, TWIP or TRIP are absent, and slip dominates plasticity. This study highlights the complex interactions between deformation rate, temperature, TRIP, and TWIP, emphasizing the strong impact of both deformation mechanisms on the mechanical behavior.

Contribution 5 investigates the effects of various heat treatments at elevated and cryogenic temperatures on the mechanical performance and corrosion resistance of maraging M789 steel prepared by directed energy deposition (DED). The authors examined the microstructures and hardness in both as-printed and heat-treated states and found that the high cooling rate of the DED process leads to α’ martensite in the as-built state. Quenching into liquid nitrogen reduces the content of retained austenite by transforming austenite to martensite, resulting in the formation of homogeneously dispersed nano-carbide precipitates during subsequent aging. Such treatment significantly increases the hardness. Moreover, optimization of heat treatments can slow down corrosion, as the authors found. For instance, direct aging of as-built samples results in the best resistance against corrosion in a NaCl solution.

To conclude, martensitic transformations remain a central theme in material science and engineering, as vividly exemplified by the articles in this Special Issue. I am confident that they make for interesting and instructive reading. With ongoing research and development of new alloys, novel characterization and numerical techniques, martensitic transformations will continue to play a key role in the creation of present and future innovative materials.

List of Contributions

• Subedi, U.; Poudel, S.; Gyanwali, K.; Amorim Coutinho, Y.; Matula, G.; Kunwar, A. State-of-the-Art Review on the Aspects of Martensitic Alloys Studied via Machine Learning. Metals 2022, 12, 1884.
• Nespoli, A.; Ninarello, D.; Fanciulli, C. A Review on Shape Memory Alloys with Martensitic Transition at Cryogenic Temperatures. Metals 2023, 13, 1311.
• Chen, W.; Xi, R.; Jiang, H.; Li, X.; Dong, G.; Wang, X. Superelasticity of Geometrically Graded NiTi Shape Memory Alloys. Metals 2023, 13, 1518.
• El Batal, O.; Abuzaid, W.; Egilmez, M.; Alkhader, M.; Patriarca, L.; Casati, R. Deformation Rate and Temperature Sensitivity in TWIP/TRIP VCrFeCoNi Multi-Principal Element Alloy. Metals 2022, 12, 1510.
• Han, S.-C.; Chaudry, U.M.; Cenalmor, S.B.; Yeon, S.M.; Yoon, J.; Lee, H.; Kim, K.; Jun, T.-S. Effect of Heat Treatment on Corrosion and Mechanical Properties of M789 Alloy Fabricated Using DED. Metals 2023, 13, 1214.