Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin.

CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters½ (MPa·m½) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.

Neutron diffraction analysis of stress and strain partitioning in a two-phase microstructure with parallel-aligned phases.

By time-of-flight (TOF) neutron diffraction experiments, the influence of segregation-induced microstructure bands of austenite (γ) and martensite (α' ) phases on the partitioning of stress and strain between these phases was investigated. Initially, tensile specimens of a Co-added stainless steel were heat treated by quenching and partitioning (Q&P) processing. Tensile specimens were subsequently loaded at 350 °C parallel to the length of the bands within the apparent elastic limit of the phase mixture. Lattice parameters in both axial and transverse directions were simultaneously measured for both phases. The observation of a lattice expansion for the γ phase in the transverse direction indicated a constraint on the free transverse straining of γ arising from the banded microstructure. The lateral contraction of α' imposed an interphase tensile microstress in the transverse direction of the γ phase. The multiaxial stress state developed in the γ phase resulted in a large deviation from the level of plastic strain expected for uniaxial loading of single phase γ. Since segregation-induced banded microstructures commonly occur in many engineering alloys, the analysis of stress and strain partitioning with the present Q&P steel can be used to interpret the observations made for further engineering alloys with two-phase microstructures.

Author Correction: Neutron diffraction analysis of stress and strain partitioning in a two-phase microstructure with parallel-aligned phases.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Characterisation of nanovoiding in dental porcelain using small angle neutron scattering and transmission electron microscopy.

OBJECTIVES: Recent studies of the yttria partially stabilised zirconia-porcelain interface have revealed the presence of near-interface porcelain nanovoiding which reduces toughness and leads to component failure. One potential explanation for these nanoscale features is thermal creep which is induced by the combination of the residual stresses at the interface and sintering temperatures applied during manufacture. The present study provides improved understanding of this important phenomenon. METHODS: Transmission electron microscopy and small angle neutron scattering were applied to a sample which was crept at 750°C and 100MPa (sample C), a second which was exposed to an identical heat treatment schedule in the absence of applied stress (sample H), and a reference sample in the as-machined state (sample A). RESULTS: The complementary insights provided by the two techniques were in good agreement and log-normal void size distributions were found in all samples. The void number density was found to be 1.61µm-2, 25.4µm-2 and 98.6µm-2 in samples A, H and C respectively. The average void diameter in sample A (27.1nm) was found to be more than twice as large as in samples H (10.2nm) and C (11.6nm). The crept data showed the highest skewness parameter (2.35), indicating stress-induced growth of larger voids and void coalescence that has not been previously observed. SIGNIFICANCE: The improved insight presented in this study can be integrated into existing models of dental prostheses in order to optimise manufacturing routes and thereby reduce the significant detrimental impact of this nanostructural phenomenon.

Flexible sample environment for high resolution neutron imaging at high temperatures in controlled atmosphere.

High material penetration by neutrons allows for experiments using sophisticated sample environments providing complex conditions. Thus, neutron imaging holds potential for performing in situ nondestructive measurements on large samples or even full technological systems, which are not possible with any other technique. This paper presents a new sample environment for in situ high resolution neutron imaging experiments at temperatures from room temperature up to 1100 °C and/or using controllable flow of reactive atmospheres. The design also offers the possibility to directly combine imaging with diffraction measurements. Design, special features, and specification of the furnace are described. In addition, examples of experiments successfully performed at various neutron facilities with the furnace, as well as examples of possible applications are presented. This covers a broad field of research from fundamental to technological investigations of various types of materials and components.