Probing Defect Properties and Behavior under Mechanical Deformation and Extreme Conditions: On-Demand Oral Session: Defect Property, Characterization, and Evolution
Sponsored by: TMS Nanomechanical Materials Behavior Committee, TMS Nuclear Materials Committee, TMS Mechanical Bahavior of Materials Committee
Program Organizers: Zhe Fan, Lamar University; Tianyi Chen, Oregon State University; Shijun Zhao, City University of Hong Kong; Mitra Taheri, Johns Hopkins University; Yury Osetskiy, Oak Ridge National Laboratory
Friday 8:00 AM
October 22, 2021
Room: On-Demand Room 7
Location: MS&T On Demand
Invited
Defect Absorption at Grain Boundaries: A Grain Boundary Structure Perspective: David Srolovitz1; Jian Han1; Larissa Woryk2; Mitra Taheri3; Jaime Marian4; 1City University of Hong Kong; 2University of Pennsylvania; 3Johns Hopkins University; 4University of California, Los Angeles
The absorption of point defects and dislocation loops at a grain boundary (GB)inevitably modifies the GB structure. We present recent atomistic simulation and theoretical results on how grain boundaries accommodate defect absorption. We explore this through the concepts of metastable GB states and disconnections (defects on the GB plane that have both step and dislocation character). We show how absorption of point defects can lead to phase transitions in the GB and how continued absorption leads to periodic cycling between GB states. Next, we examine the evolution of dislocation loops following their absorption at GBs. In particular, we observe how lattice dislocations decompose into GB line defects and the faults between these. The line defects are analyzed in terms of disconnections and GB partial disconnections and the faults between these in terms of metastable GB states.
Invited
Automated Defect Detection in Electron Microscopy with Machine Learning: Dane Morgan1; Ryan Jacobs1; Mingren Shen1; Kevin Field2; 1University of Wisconsin-Madison; 2University of Michigan
Electron microscopy is widely used to explore defects in crystal structures, but human tracking of defects can be time-consuming, error prone, and unreliable, and is not scalable to large numbers of images or real-time analysis. In this work we discuss application of deep learning machine vision approaches to find the location and geometry of different defect clusters in irradiated steels. We assess both bounding box and pixel level segmentation approaches, namely the Faster Regional Convolutional Neural Network (RCNN) and Mask RCNN, respectively, and demonstrate similar performance. We also consider the You Only Look Once (YOLO) approach and demonstrate the capability for real-time analysis of electron microscopy videos, including tracking defect counts, growth, and diffusion. In all cases we are able to achieve performance comparable to human labeling, suggesting that these technologies can support a radical transformation to orders of magnitude gains in our ability to analyze defects.
Invited
Effects of Cr On 1/2<111> to <100> Loop Transformation in Concentrated Fe-Cr Alloys under Irradiation: Xian-Ming Bai1; 1Virginia Polytechnic Institute and State University
Fe-Cr based ferritic alloys are promising structural and cladding materials for next-generation reactors. Numerous experiments have shown that radiation can produce both 1/2<111> and <100> interstitial-type dislocation loops in these alloys. However, contradictory experimental evidence also exists regarding how Cr influences the relative abundance of the two loop types. In this work, molecular dynamics modeling is used to simulate the direct transformation from a 1/2<111> to a <100> loop based on the cascade overlapping mechanism in a series of Fe-Cr alloys and pure Fe. Our results show that the loop transformation probability decreases with the increasing Cr concentration. Further molecular statistics calculations show that the average formation energy of a <100> loop increases at a much faster rate than that of a ½<111> loop as Cr concentration increases, providing a reasonable explanation for why Cr suppresses the formation of <100> loops in Fe-Cr alloys as observed in many experiments.
Invited
A Statistical Approach for Atomistic Calculations of Vacancy Formation Energy and Chemical Potentials in Concentrated Solid-solution Alloys: Yongfeng Zhang1; Anus Manzoor2; Chao Jiang3; Dilpuneet Aidhy2; 1University of Wisconsin-Madison; 2University of Wyoming; 3Idaho National Laboratory
This talk presents a statistical approach for atomistic calculations of the vacancy formation energy in concentrated solid-solution alloys. Demonstrated using a random FeCrNi ternary alloy, a general formulation is given for applications in random, concentrated alloys with any numbers of components. The approach calculates the mean vacancy formation energy based on the total energies of a defect-free reference supercell and defected supercells each with a vacancy, without separate calculations for chemical potentials, avoiding the additional computation cost and the associated uncertainty. The chemical potential of each component can be back derived in a self-consistent manner to give the distribution of vacancy formation energy. It is also found that, with the same mean vacancy formation energy, a broader distribution may lead to a lower equilibrium vacancy concentration at a given temperature, indicating the critical importance of statistically obtaining the full distribution of vacancy formation energy.
Defect Properties and Deformation Mechanisms of Multi-component Intermetallics: Shijun Zhao1; 1City University of Hong Kong
Multi-component high-entropy alloys (HEAs) with compositionally disordered elemental arrangement have attracted extensive attention because of their excellent mechanical properties. However, these alloys usually exhibit low strength at elevated temperatures. Recent works suggest that HEAs strengthened by L12 ordered intermetallics of the Ni3Al type possess a superior synergy of strength and ductility. The performance of such ordered-intermetallics-strengthened HEAs depends critically on the composition and deformation mechanism of the strengthening component, which will inevitably suffer partial disorder in HEAs. Here we will present our results on defect properties and deformation mechanisms of a series of L12-type (Ni,Co,Fe)3(Al,Ti,Fe) and its derived subsystems. We show that the partial disorder has significant influence on the properties of these ordered intermetallics. These results pave the way for rationally designing ordered-precipitate-strengthened HEAs.
Invited
Irradiation-induced Self-organization of the Microstructure in Irradiated Alloys and Its Influence on Mechanical Properties: Pascal Bellon1; Qun Li1; Gabriel Bouobda Moladje1; Sung-Eun Kim1; Soumyajit Jana1; Robert Averback1; 1University of Illinois at Urbana-Champaign
Irradiation can induce the self-organization of phase-separating alloy systems into nanoscale compositional patterns (CP) owing to the competition between finite-range ballistic mixing with thermodynamically-driven decomposition. Here we extend the study to CP at grain boundaries (GBs), and consider its impact on mechanical properties. We propose phase-field models to investigate this coupled self-organization in phase-separating A-B alloys. In a first model, a multi-order parameter phase-field model is augmented by adding a finite-range gaussian ballistic mixing contribution. A second model considers GBs as arrays of dislocations, providing a more detailed information on solute segregation and precipitation at specific sites of the GBs. These models make it possible to identify irradiation conditions promoting coupled CP, thus simultaneously inside grains and at GBs. We compare these predictions to experimental results of precipitate evolutions in Al-Sc and Al-Sb, and assess the impact of solute re-distribution and precipitation on the hardness of these materials.
Invited
Machine Learning Driven In-situ TEM with Ion Irradiation: Meimei Li1; 1Argonne National Laboratory
Understanding and evaluation of a material's response to an irradiation environment has been a bottleneck in the development, qualification and deployment of new materials and manufacturing technologies in nuclear energy systems. The convergence of high-performance computing, machine learning, high-throughput experimentation, and in situ characterization has the potential to make the materials discovery and diagnosis many times faster than conventional approaches. This talk discusses our recent effort toward machine learning/artificial intelligence (ML/AI)-driven in situ TEM with ion irradiation to rapidly and efficiently assess the irradiation tolerance of a material in nuclear environments. This ML/AI-enabled irradiation diagnosis tool is envisioned to provide real-time microstructure imaging, real-time defect analysis, and real-time property prediction with increasing irradiation dose, and offer a high-fidelity forecast of a material's irradiation tolerance that could dramatically shorten the material discover and development cycles.
Invited
Lattice Distortion in NbTaTiV and NbTaTiVZr Refractory High-entropy Alloys: Chanho Lee1; Yi Chou2; George Kim3; Michael Gao4; Ke An5; Jamieson Brechtl5; Chuan Zhang6; Wei Chen3; Jonathan Poplawsky5; Gian Song7; Yi-Chia Chou2; Peter Liaw8; 1Los Alamos National Laboratory; 2National Chiao Tung University; 3Illinois Institute of Technology; 4National Energy Technology Laboratory/Leidos Research Support Team; 5Oak Ridge National Laboratory; 6Computherm LLC; 7Kongju National University; 8University of Tennessee
The lattice distortion is the core effect of HEAs to enhance the strength at room as well as high temperatures. The number of constituent elements with various atomic sizes have the identical possibility to occupy at atom positions of a crystal lattice, which induces a severe distortion of the crystalline lattice. Several studies have attempted to quantitatively measure the lattice distortion with increasing the number of alloying elements, using x-ray diffraction (XRD) and neutron diffraction (ND). However, only limited alloy systems have succeeded to demonstrate the correlation between lattice distortion and mechanical properties. In this study, we have systematically investigated the evolution of lattice distortions for Nb-Ta-Ti-V-Zr systems as function of number of additional elements and its effect on mechanical properties, using experimental [atom-probe-tomography (APT at CNMS), transmission-electron microscopy (TEM)] and modeling methods [density-functional-theory (DFT)]. It is found that lattice distortions have a critical role in improving yield strength.
Invited
Synchrotron High-energy X-ray Studies of Nuclear Structural Materials: Deformation and Additive Manufacturing: Xuan Zhang1; Meimei Li1; Jonathan Almer1; Jun-Sang Park1; Peter Kenesei1; Andrew Chuang1; 1Argonne National Laboratory
Recent advances of synchrotron X-ray diffraction- and imaging-based techniques have provided us unprecedented time and spatial resolution in the study of nuclear structural materials in-situ and/or in 3D. Synchrotron high-energy X-rays are ideal tools for probing the evolution of deformation microstructures and for revealing the underlying deformation mechanisms in bulk irradiated materials in real-time and in realistic conditions. Such tools are also advantageous in the study of the laser additive manufacturing process and the additively manufactured materials. Research conducted at the Advanced Photon Source will be highlighted, including in situ deformation studies of ferritic and austenitic steels with wide-angle X-ray scattering, small-angle X-ray scattering, and in situ 3D characterizations by high-energy X-ray diffraction microscopy. In situ metal additive manufacturing studies with high-speed X-ray diffraction and imaging, and X-ray tomography of porosity distribution in additively printed materials will also be presented.
Invited
The Role of Interfaces in Mechanical Response and Radiation Resistance of Ceramics: Izabela Szlufarska1; Hongliang Zhang1; Jianqi Xi1; Xing Wang1; 1University of Wisconsin-Madison
Engineering of interfaces has been shown a promising way to control the response of materials to harsh environments. Here, I will discuss the role that interfaces play in response to radiation and/or mechanical stresses, focusing on high-temperature ceramics. I will demonstrate that in ceramics, the effects of interfaces are closely coupled to the complex energy landscape for defect reactions, leading to surprising new phenomena. For example, we discovered that even in ceramics that form line compounds, radiation can cause segregation of constituent elements to grain boundaries without precipitating new phases. Such radiation-induced segregation is expected to have significant impact on corrosion resistance. Secondly, although radiation generally causes degradation of materials properties, it can be also used as a tool to improve materials. In particular, I will demonstrate that radiation can be used as a tool for nano-engineering of interfaces in SiC-carbon nanotube composites, leading to significant improvements of fracture toughness.
Invited
Irradiation Defects and Strain-induced Martensitic Transformations: Janelle Wharry1; Chao Yang1; Yangyang Zhao1; Keyou Mao2; Yash Pachaury1; Anter El-Azab1; 1Purdue University; 2Oak Ridge National Laboratory
The objective of this talk is to understand how irradiation-induced defects modulate deformation-induced martensitic transformations. Martensitic phase transformations are low-temperature deformation mechanisms in which γ-fcc austenite transforms into α'-bcc martensite, either directly or indirectly through metastable ε-hcp martensite. Irradiation enhances the propensity for transformations to occur, although the reasons are not clearly understood. This experimental and theoretical work focuses on irradiation-induced cavities (e.g. voids, bubbles). Experiments are conducted on neutron irradiated 304L stainless steel and ion implanted Fe-xMn (x=30-50 wt%) containing systematically varied cavity populations. Micro-mechanical testing with site-specific transmission electron microscopy (TEM) reveal deformation mechanisms and their interaction with the microstructure. Cavity size generally governs whether the transformation occurs, while cavity number density controls whether the transformation pathway is direct (γ→α') or indirect (γ→ε→α'). Molecular dynamics (MD) simulations show that stress concentrations on cavity surfaces lead to bcc martensite nucleation through a strain-induced mechanism.
Invited
Effects of Electronic Structures on Defect and Mechanical Properties of BCC Multicomponent Alloys: Yong-Jie Hu1; Liang Qi2; 1Drexel university; 2University of Michigan
Mechanical properties of BCC transition metals and alloys are significantly affected by their partially filled d-band electrons. Here I present several examples of understanding and quantifying these electronic effects on the defect behavior and mechanical properties based on first-principles calculations. The first example shows a general linear correlation that can be found between two descriptors of local electronic structures at defects in pure metals and the solute-defect interaction energies in binary alloys of refractory metals with transition-metal substitutional solutes. In addition, with these local electronic descriptors and a simple bond-counting model, we developed regression models to accurately and efficiently predict the unstable stacking fault energy and surface energy for BCC refractory multicomponent alloys. Using the regression models, we performed a systematic screening of planar fault energies and their ratio in the high-order multicomponent systems to search for alloy candidates that may have enhanced strength-ductile synergies.
Invited
The Impact of Elastic Anisotropy on Hydride Morphology in Zirconium: Pierre-Clement Simon1; Michael Tonks2; Arthur Motta1; Long-Qing Chen1; Mark Daymond3; 1Pennsylvania State University; 2University of Florida; 3Queen's University
In this work, we present new insights on the role of elastic anisotropy on hydride morphology in zirconium from a quantitative phase field model. Hydrogen in nuclear reactor fuel cladding can precipitate as a brittle hydride phase. When the nanoscale hydrides stack into circumferentially-oriented mesoscale hydrides, their impact is small. However, if they stack radially, which can occur under an applied load, they provide a preferred fracture path. Our phase field model predicts the hydride morphology observed experimentally and identifies the mechanisms responsible for nanoscale hydride stacking. The model accurately predicts experimentally-observed elongated nanoscale hydride shape and the stacking of hydrides along the basal plane of the hexagonal zirconium matrix. It also shows that an applied stress does not alter the hydride stacking within a grain. In polycrystalline structures, preferential precipitation in grains with circumferentially-aligned basal poles under applied stress appears to cause radial hydride orientation.
Invited
Impact of Carbon Nanotube Defects on Fracture Mechanisms in Ceramic Nanocomposites: Yingchao Yang1; Brian Sheldon2; Izabela Szlufarska3; Jun Lou4; 1University of Maine; 2Brown University; 3University of Wisconsin; 4Rice University
It is widely believed that defects in the CNTs will alter key properties that ultimately dictate composite toughening. With ceramic nanocomposites with a polymer derived ceramic (PDC) matrix, reinforced with multiwalled CNTs, we will present fracture measurements on full composite films, combined with careful testing of individual CNTs embedded in the same PDC matrix. The defect levels in these CNTs were controlled with high energy carbon ions. By using this approach, it was possible to directly correlate defect levels with the interfacial shear strength (IFSS) of the CNT/PDC interface, the CNT fracture strengths, and the pull-out lengths observed after fracture of the full composites. The radiation induced defects led to substantial increases in the IFSS, and only a marginal decrease is observed in the measured fracture strength. Based on this, the shorter pull-out lengths that occurred with higher defect levels were primarily attributed to stronger bonding at the CNT/PDC interface.
Invited
Studying Radiation Effects in Nuclear Fuels via Advanced Characterization and Modeling: Lingfeng He1; Kaustubh Bawane1; Tiankai Yao1; Pengyuan Xiu1; Marat Khafizov2; Miaomiao Jin3; Chao Jiang1; Cody Dennett1; Zilong Hua1; Anter El-Azab4; David Hurley1; Jian Gan1; 1Idaho National Laboratory; 2The Ohio State University; 3Pennsylvania State University; 4Purdue University
In radiation environments, the degradation of mechanical properties, corrosion/oxidation resistance, and thermal conductivity of materials has been associated with defect generation and composition redistribution in the microstructure. Radiation creates point defects (vacancies, interstitials) and also extended defects, such as dislocation loops and cavities. In some cases, element segregation, phase transition and chemical interactions could also happen. Ceramic nuclear fuels have been widely used in light water reactors and proposed as candidates for advanced reactors. In this work, we study the defect and phase evolution at early stage in oxide and nitride nuclear fuels using a combination of in situ/ex situ ion irradiation, advanced characterization and modeling. In addition, the effects of defects on thermal transport in nuclear fuels are probed using modulated thermoreflectance techniques. Investigating these early-stage microstructural changes is of significance in understanding the performance degradation of nuclear fuels in reactor environments.
Invited
Irradiation Response of FCC and BCC Compositionally Complex Alloys Using In-situ and Ex-situ Irradiations: Adrien Couet1; Calvin Parkin1; Michael Moorehead1; Lin Shao2; Frank Garner1; Lingfeng He3; Pengyuan Xiu3; Wei-Ying Chen4; Meimei Li4; Kumar Sridharan1; 1University of Wisconsin-Madison; 2Texas A&M University; 3Idaho National Laboratory; 4Argonne National Laboratory
In-core structural materials for advanced reactors are expected to exhibit corrosion and radiation tolerance superior to that of currently licensed stainless steels and ferritic-martensitic steels. Specifically, sodium-cooled fast reactor cladding material will be subjected to several hundreds of displacements per atom (dpa) over the operating lifetime. Compositionally Complex Alloys (CCAs) present a novel radiation resistant matrix design parameter to potentially complement usual microstructurally-based sink strength alloy design used in optimized steels. CCAs consisting of four or more principle alloying elements in single-phase solid solution are theorized to resist radiation effects thanks to unique energy and mass transport properties. In situ transmission electron microscope and ex-situ heavy-ion irradiation experiments were performed on CrFeMnNi and NbTaTiV CCA families using multiple ion beam facilities at various dpas and temperatures. The void swelling, dislocation loop density and chemical redistribution of CCAs are discussed and compared to less compositionally complex alloys.
Effects of Annealing and Ion Irradiation on Helium Implanted NiCo and NiFe Concentrated Solid-solution Alloys: Zhe Fan1; Xing Wang1; Di Chen2; Yongqiang Wang2; Yuri Osetsky1; Hongbin Bei1; William Weber3; Yanwen Zhang1; 1Oak Ridge National Laboratory; 2Los Alamos National Laboratory; 3The University of Tennessee
Helium cavities in irradiated concentrated solid-solution alloys can be tailored by alloying elements, however, how these helium cavities further evolve is rarely investigated. Here, we compared effects of annealing and ion irradiation on helium-implanted Ni40Fe60 and Ni50Co50. After 200 keV He+ implantation at room temperature, smaller helium cavities and “black spot” defects formed in Ni40Fe60 than Ni50Co50. The helium-implanted alloys were further irradiated by 21 MeV Ni7+ ions at 500 oC. After irradiation, helium cavity distribution remained homogeneous in Ni50Co50, but became inhomogeneous in Ni40Fe60 where helium cavity size was the smallest in peak damage region with extended dislocation networks, and helium cavity size was larger outside the region with few dislocations. A similar inhomogenous distribution was also observed in helium-implanted Ni40Fe60 annealed at 500 oC. The underlying mechanisms for cavity evolution are discussed. This work is supported by EDDE, an EFRC funded by U.S. DOE-BES.
Cancelled
Deformation Microstructure of Ferritic/Martensitic Steels after Spallation Neutron Irradiation: Kun Wang1; 1Alfred University
Three different Ferritic/martensitic steels were tensile tested to understand the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after irradiation in the Swiss spallation neutron source. The irradiation conditions were in the range: 10.7 – 20.4 dpa with 850-1750 appm He at 160-300 °C. After tensile tests, TEM observation was employed to investigate the deformation microstructures. Defect-free channels with {110} and {112} slip planes were found in the deformed area of some specimens, indicating plastic flow localization. Regarding the brittle samples, the TEM-lamella were extracted directly below intergranular/cleavage fracture surfaces by the focused ion beam. Strikingly, deformation twinning was observed as the main feature in three irradiated specimens at the high dose. Twins with {112} planes were observed in all of these samples. As deformation twinning was first observed in irradiated FM steels, the novel features of deformation twinning are summarized in this presentation.
Elemental Partitioning Behavior among Precipitates in Alumina-forming-austenitic Stainless Steel: Qing-Qiang Ren1; David Hoelzer1; Michael Lance1; Yukinori Yamamoto1; Michael Brady1; Jonathan Poplawsky1; 1Oak Ridge National Laboratory
The microstructural stability of strengthening precipitates is critical for alumina-forming-austenitic (AFA) stainless steels for high temperature applications. The precipitates size kinetics subject to thermally aging and creep test at 750 oC was studied by SEM and (S)TEM, the results of which are related to the creep and mechanical properties. To better understand the precipitate stability, atom probe tomography (APT) was used to measure elemental partitioning among various precipitate interfaces in thermally aged and creep ruptured AFA samples. The corresponding energy reduction associated with the partitioning is calculated from the Gibbsian excess of alloying element additions. These results serve as a quantitative relation between alloying element additions and microstructural stability, which can guide future alloy design strategies. APT was performed at ORNL’s CNMS, which is a US DOE office of science user facility. The work is funded by U.S. DOE Office of Fossil Energy eXtremeMAT program.
Survey of Defect Absorption Effects in Grain Boundaries: Larissa Woryk1; David Srolovitz2; Jian Han2; 1University of Pennsylvania; 2City University of Hong Kong
Defect absorption at grain boundaries requires their accommodation within the boundary, which can occur through various mechanisms. We present a variety of cases of dislocations and dislocation loops to highlight similarities and differences in grain boundary response to absorption. We examine interstitial and vacancy loops of various sizes and allowed Burgers vectors (within the same family) and straight dislocations with various allowed Burgers vectors and line directions in multiple grain boundaries, as Burgers vectors within the same family each have a different orientation relative to the grain boundary, and different grain boundaries have different allowed disconnections and accessible metastable states. We characterize the absorption response in terms of disconnections and metastable grain boundary states produced by the decomposition of the initial defect Burgers vectors.
Enhanced Load Transfer and Ductility in Al-9Ce Alloy through Heterogeneous Lamellar Microstructure Design by Cold Rolling and Annealing: Chi Zhang1; 1Shanghai Jiao Tong University
Al-Ce alloy has attracted much attention due to its outstanding heat resistance and castability. To further improve the performance of Al-Ce alloy, a two-phase heterogeneous lamellar structure was designed with oriented Al11Ce3 particles, fine grains, and coarse grains in layers in Al-Ce hypoeutectic alloy through cold rolling and annealing. Higher strength and better ductility were obtained compared with the as-cast alloy with the same composition. Mechanism of strengthening and ductility were analyzed by experimental findings and theoretical calculations. The results revealed that fine-grained eutectic aluminum in bimodal structure and the load transfer of oriented Al11Ce3 particles contribute much to the alloy strength. The coarse Al grains in the bimodal structure and the lamellar structure lead to good ductility. The idea of strengthening and toughening Al-Ce alloy is provided, and the process discovered here is amenable to large-scale industrial production at low cost.
Influence of Microstructural Variation on Spall Failure of Al7085: Dung-Yi Wu1; Chengyun Miao1; Christopher DiMarco1; K.T. Ramesh1; Todd C. Hufnagel1; 1Johns Hopkins University
Spallation in ductile metals is a complex, multi-stage and dynamic process involving the nucleation of voids under hydrostatic tensile loading, followed by growth and coalescence of these voids, eventually leading to a final fracture. The influence of microstructure on these processes can be obscured in conventional large-scale spall experiments in which the response is averaged over a large volume of material. In contrast, laser-driven microflyer (LDM) experiments probe smaller volumes, making them more sensitive to local microstructural variations. Using the LDM method on a commercial 7085 aluminum alloy, we observe that increasing the aluminum grain size (via solutionizing heat treatment) causes an increase in the variance of the measured spall strength, even though the average spall strength is largely unchanged. We discuss the relationship between the measured microstructures and the distribution of the spall strength, as well as the morphology of the spall surface.
Switching the Fracture Toughness of Single Crystal ZnS by Light Irradiation: Tingting Zhu1; Kuan Ding1; Anahid Amiri1; Yu Oshima2; Enrico Bruder1; Robert Stark1; Karsten Durst1; Katsuyuki Matsunaga2; Atsutomo Nakamura2; Xufei Fang1; 1Technische Universität Darmstadt; 2Nagoya University
An enormous change of the dislocation-mediated plasticity has been found in bulk semiconductor that exhibits the photoplastic effect. In this talk, we report that the UV (365 nm) light irradiation during mechanical testing dramatically decreases the fracture toughness of ZnS. The crack tip toughness on a (001) single-crystal ZnS, as measured by the near-tip crack opening displacement method, is increased by ~45% in complete darkness compared to that in UV light. The increase of fracture toughness is attributed to a significant increase of the dislocation mobility in darkness, as explained by the crack tip dislocation shielding model. Our finding suggests a route towards controlling the fracture toughness of photoplastic semiconductors by tuning the light irradiation.