Strength, ductility and zero thermal expansion in multicomponent TRIP-Invar alloys

Abstract

Multi-principal-element alloys (MPEAs) have emerged as a transformative class of metallic materials, surpassing conventional alloys due to their “four core effects”. The inherent compositional complexity and programmable multifunctionality of MPEAs collectively drive their emergence as a vanguard in materials innovation. By synergistically modulating metastable engineering and magneto-volume effects, we developed a MPEA (Fe,Co,Cr)100−xNix with an ultralow coefficient of thermal expansion (αl = 1.00 × 10−6 K−1, 100–400 K) and exceptional mechanical properties (tensile strength: 560 MPa, the elongation to failure: 53%). This alloy exhibits both significant transformations induced plasticity (TRIP) and zero thermal expansion effects (Invar) at room temperature, classified as a recently proposed TRIP-Invar alloy. In situ magnetic analysis reveals that ferromagnetic order mediates pronounced magnetic compensation of intrinsic lattice contraction during cooling through spin-state transitions, thereby generating zero thermal expansion behavior. In situ neutron diffraction reveals that the good strength–plasticity trade-off arises from a deformation-triggered martensitic transformation, which enhances strain hardening through dislocation multiplication and grain boundary reinforcement. This work proposes a materials design strategy for next-generation structural-functional integrated materials, advancing the fundamental understanding of thermal expansion-mechanical property optimization in MPEAs.

1. Introduction

Multi-principal-element alloys (MPEAs), characterized by their entropy-dominated phase stability, exhibit the ability to form complex solid solutions comprising multiple principal elements, thereby enabling access to a vast compositional space near the center of the phase diagram. This distinctive compositional design flexibility facilitates the synergistic optimization of material properties that are traditionally considered mutually exclusive. Initial investigations predominantly concentrated on the mechanical properties of MPEAs, including strength, toughness, fatigue resistance, and ductility. Recently, research endeavors have extended to the exploration of functional MPEAs, spanning diverse domains such as electronics, electrochemistry, magnetism, and catalysis. The optimal design paradigm for MPEAs aims to preserve their superior mechanical performance while achieving effective integration of functional attributes.

Zero thermal expansion (ZTE) alloys are of critical importance in precision instruments, aerospace engineering, and liquefied natural gas storage. Since the pioneering discovery of Fe65Ni35 (Invar alloy) by Nobel laureate Guillaume in the 1920s, this system has served as the benchmark for ZTE materials. However, ZTE alloys designed for practical applications must simultaneously fulfill stringent requirements, including exceptional functional properties as well as robust mechanical performance and processability. Regrettably, conventional ZTE alloys often exhibit an inherent trade-off between mechanical properties and thermal expansion behaviors. Invar-based alloys, due to their high sensitivity to compositional and microstructural variations, are inherently limited in their response to conventional strengthening approaches. Moreover, composite materials and dual-phase alloys frequently suffer from inadequate tensile plasticity, which significantly restricts their processability and broad applicability.

This study proposes an innovative MPEA design strategy that overcomes the inherent trade-off between ZTE behavior and mechanical performance. Departing from conventional microalloying approaches typically employed in Invar alloys, our strategy utilizes a foundational system comprising fourth-period transition elements (Cr, Fe, Co, Ni), which are characterized by their unfilled d-electron orbitals and rich magnetic properties. Systematic investigations reveal that maintaining Cr, Fe, and Co at specific atomic ratios (Cr : Fe : Co = 10:39:48) while modulating the Ni content enables the concurrent regulation of thermal expansion and mechanical properties.