Transformation Strain Research Articles

In-situ compensation for thermal strain by phase transformation strain in all-inorganic perovskite solar cells

The presence of defects and residual tensile strain in perovskites has a significant and deleterious impact on the efficiency and stability of all-inorganic perovskite solar cells (PSCs). Here, dimethyl sulfoxide (DMSO) with strong polarity and high boiling point is introduced into the PbBr2 precursor to delay the crystallization of PbBr2, leading to the formation of high-quality PbBr2 films by delaying the solution reaching supersaturation. The formation of PbBr2(DMSO) adduct facilitates the complete reaction of PbBr2 with CsBr via intramolecular exchange, thus preventing the formation of impurity phases such as CsPb2Br5. As a result, the defect density of CsPbBr3 films decreased by 33% due to the improved phase purity, thicker films, larger grain sizes, and reduced grain boundaries. The small porosity of PbBr2 film combined with a more complete phase-conversion process results in an expanded perovskite film that experiences compressive strain, effectively compensating for the tensile strain generated during annealing and thereby achieving the goal of strain relaxation. The optimized DMSO-based device, which exhibits compressive strain and minimal defects, achieves a remarkable efficiency of 10.11% with an open-circuit voltage of up to 1.611 V. Additionally, the unencapsulated PSC demonstrates excellent stability under 80% relative humidity and 80 °C over 30 days.

A gradient regularized model for shape memory alloys

In this work a transformation strain gradient enhancement is introduced into a phenomenological constitutive model for the pseudoelastic behavior of shape memory alloys. The constitutive model is able to capture several unique features of the constitutive response of these materials during the transformation between austenite and martensite during the pseudoelastic response. These features include the asymmetry in the initial transformation stresses in tension versus compression, the asymmetry in the transformation strains in tension and compression, and finally the asymmetry in the hardening behavior in tension and compression. In fact, experiments have shown that untrained NiTi exhibits hardening during its transformation in compression, but softening for tensile loading. It is this softening behavior that motivates the need for the introduction of the transformation strain gradient into the constitutive modeling. Transformation strain gradient effects are introduced via a phase variable that describes the extent of transformation. The free energy of the material then depends on gradients of the phase variable, which introduces a material length scale into the theory. The governing equation for the phase variable is developed from a microforce balance and continuum thermodynamics analysis. The model is implemented in the commercial finite element software Abaqus through user defined subroutines and several numerical simulations are performed to illustrate the model response and lack of numerical mesh-dependency of the results.

Isolation of a new Aspergillus iranicus strain from the soil of southwestern Algeria exhibiting potential palm date-cellulose degrading activity

Microorganisms with biotechnological properties are needed for transformation or degradation of biological products. Here we isolated a new Aspergillus strain (MS-34) from the soil of southwestern Algeria and found that it is related to Aspergillus iranicus. The MS-34 was examined for possible biotechnological properties and we found it to be highly efficacious at degrading palm date cellulose following fermentation. Pretreatment fermentation of palm in 50 % generated the highest (197.84 mg/L) carbohydrate concentration. Pretreatment with 20% KOH produced a maximum 112.97 IU/ml and 47.62 IU/ml for endoglucanase and exoglucanase activities respectively. Besides, glucose and proteins concentration in batch fermentations with 1% CMC medium fluctuated between 86.41 mg/l to 108.85 mg/l and the protein level was 0.713- 0.991 mg/ml. There were significant temperature and pH effects on the enzymatic activity and the optimal temperature conditions were found to be 50-60°C and pH 4.0. Since palm dates are one of the massive agricultural products of the Southeastern and Southwestern Algeria, use of this strain for transformation of palm tree waste will be an advanced biotechnology tool that will contribute to significantly reduce pollution linked to waste incineration. In addition, resulting fermentation products may be used as sources of energy.

Simulations of multivariant Si I to Si II phase transformation in polycrystalline silicon with finite-strain scale-free phase-field approach

Scale-free phase-field approach and corresponding finite element method simulations for multivariant martensitic phase transformation from cubic Si I to tetragonal Si II in a polycrystalline aggregate are presented. Important features of the model are large and very anisotropic transformation strain tensor ɛt={0.1753;0.1753;−0.447} and stress-tensor dependent athermal dissipative threshold for transformation, which produce essential challenges for computations. 3D polycrystals with stochastically oriented grains are subjected to uniaxial strain- and stress-controlled loadings under periodic boundary conditions and zero averaged lateral strains. Coupled evolution of discrete martensitic microstructure, volume fractions of martensitic variants and Si II, stress and transformation strain tensors, and texture are presented and analyzed. Macroscopic variables effectively representing multivariant transformational behavior are introduced. Macroscopic stress–strain and transformational behavior for 55 and 910 grains are close. Large transformation strains and grain boundaries lead to huge internal stresses of tens GPa, which affect microstructure evolution and macroscopic behavior. In contrast to a single crystal, the local mechanical instabilities due to phase transformation and negative local tangent modulus are stabilized at the macroscale by arresting/slowing the growth of Si II regions by the grain boundaries. This leads to increasing stress during transformation. The developed methodology can be used for studying similar phase transformations with large transformation strains and for further development by including plastic strain and strain-induced transformations.

Martensitic transformation induced planar deformation of AlN nanoprecipitates in high nitrogen stainless steels

Boosting the applications of high nitrogen steels requires delicate control of detrimental brittle AlN precipitates which often initiate cracking and thus degrade overall mechanical performance. Here we report that stacking faults in nano-sized AlN precipitates can be activated by martensitic transformation during quenching in a high nitrogen martensitic stainless bearing steel. Atomic-scale structure and strain analyses illuminate that the planar deformation mode, which is rarely seen in inorganic AlN ceramic precipitates, is stimulated by the martensitic transformation of the steel matrix to accommodate the transformation stresses and strains at the precipitate/matrix interfaces. The critical resolved shear stresses derived from the density functional theory study are consistent with the stress arising from the martensitic transformation obtained from the phenomenological theory of martensitic crystallography combined with phase field modeling, further validating the martensitic transformation induced plastic deformation in AlN nanoprecipitates. Our finding paves a new way to mitigate the harmful effect of AlN precipitates in high nitrogen steels and enables the production of high-quality high nitrogen steels with an upper limit of Al content raised to the normal steelmaking level, which offers new guidelines for effectively reducing the production costs of high nitrogen steels.

Silencing of the Laccase (lacc2) Gene from Pleurotus ostreatus Causes Important Effects on the Formation of Toxocyst-like Structures and Fruiting Body.

A wide variety of biological functions, including those involved in the morphogenesis process of basidiomycete fungi, have been attributed to laccase enzymes. In this work, RNA interference (RNAi) was used to evaluate the role of the laccase (lacc2) gene of Pleurotus ostreatus PoB. Previously, transformant strains of P. ostreatus were obtained and according to their level of silencing they were classified as light (T7), medium (T21) or severe (T26 and T27). The attenuation of the lacc2 gene in these transformants was determined by RT-PCR. Silencing of lacc2 resulted in a decrease in laccase activity between 30 and 55%, which depended on the level of laccase expression achieved. The silenced strains (T21, T26, and T27) displayed a delay in the development of mycelium on potato dextrose agar (PDA) medium, whereas in the cultures grown on wheat straw, we found that these strains were incapable of producing aerial mycelium, primordia, and fruiting bodies. Scanning electron microscopy (SEM) showed the presence of toxocyst-like structures. The highest abundance of these structures was observed in the wild-type (PoB) and T7 strains. However, the abundance of toxocysts decreased in the T21 and T26 strains, and in T27 they were not detected. These results suggest that the presence and abundance of toxocyst-like structures are directly related to the development of fruiting bodies. Furthermore, our data confirm that lacc2 is involved in the morphogenesis process of P. ostreatus.

Low hysteresis and high cyclic stability in a Ti50Ni45.2Cu1Fe3.8 shape memory alloy

Shape memory alloys, exhibiting shape memory effect and superelasticity, are the key components of actuators and sensors. However, the hysteresis and cyclic instability associated with the martensitic phase transformation, limit their use in the long-duration precise control. We report a quaternary TiNiCuFe shape memory alloy with ultra-low hysteresis and high cyclic stability. The discovery of this alloy is guided by investigating the synergistic effect of Fe and Cu on the martensitic phase transformation. The optimized alloy Ti50Ni45.2Cu1Fe3.8, shows a hysteresis measured by electric resistance, as low as 0.3 K (the hysteresis by differential scanning calorimetry is about 3.5 K), and a shift of phase transformation temperature about 0.05 K (after 60 cycles). Compared to the state-of-the-art TiNiCuPd alloy with a Cu content higher than 10 at %, the Cu content in our alloy is reduced to only 1 at %, thereby suppress the brittleness and improve the workability. In addition, the precious metal, Pd is replaced by the low-cost Fe, which may facilitate the wide applications of shape memory alloys. The origin of the optimized hysteresis and cyclic stability, is ascribed to the possible phase coexistence of B19 and R that is realized by manipulating the phase transformation paths with Fe and Cu. This singular microstructure may accommodate more phase transformation strain and consequently mitigate the incompatibility of the austenite/martensite interfaces.

Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices

We investigated the interplay between the thermomechanical and spin-crossover (SCO) properties of a series of stimuli-responsive polymer composites consisting of [Fe(NH2trz)3]SO4 and [Fe(Htrz)(trz)2]BF4 particles (trz = 1,2,4-triazolato) embedded in thermoplastic polyurethane, TPU, and poly(vinylidene fluoride–trifluoroethylene), P(VDF-TrFE), matrices. The effective thermoelastic coefficients and transformation stress and strain in the composites were assessed utilizing dynamical mechanical analysis (DMA), differential scanning calorimetry (DSC), thermal expansion, and thermal stress measurements. Remarkably, the composites display a characteristic elastic softening and increased mechanical damping around the spin transition temperature, which arise from the significant spin state–volume strain coupling in the particles and scale semiquantitatively with the pressure derivative of the low spin fraction (∂n∂P)T. Crucially, for a given particle volume fraction, the transformation strain (respectively stress) substantially increases (respectively decreases) in soft matrices, which was rationalized by micromechanical simulations. The results provide a fundamental understanding and a quantitative guideline for the design of SCO@polymer composites for applications in mechanical transducers.

Microstructural, crystallographic, and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

• Magnetic field-assisted directional solidification (MFADS) can control structure. • Macro-/micro-segregation and orientation can be adjusted by MFADS. • Microsegregation-induced microstructure influences mechanical properties. • Stress-induced crystallographic evolution is traced by in-situ EBSD. Abstract In this study, the effect of transverse magnetic field-assisted directional solidification (MFADS) on the microstructures in Ni-Mn-Ga alloys has been investigated. The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field, causing the emergence of martensite clusters in the austenite matrix. Moreover, the magnetic field alleviates the microsegregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of the above investigation, several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation. The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles. The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized. In addition, the microsegregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor. The stress-induced superelasticity is closely dependent on orientation, well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain. The crystallographic evolution has been characterized during in-situ stress-induced transformation. The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Crossover from mechanical stabilization of martensite to spanning avalanche-type reverse transformation of deformed Cu-Al-Ni shape memory crystals

A new interpretation of factors controlling the kinetics of reverse transformation of martensites stabilized by prestrain is suggested. We investigate the effect of amount of prestrain in the β1´martensite and in the β-phase on temperature and kinetics of the reverse martensitic transformation in Cu-Al-Ni shape memory alloy single crystals. Calorimetry and video recording of the jumping samples, provoked by the burst strain recovery during temperature-induced reverse transformation, are used in experiments. For crystals deformed both in the β´1 martensite and in the β-phase, we report the existence of a sharp transition (crossover) from the mechanical stabilization of martensite to the burst reverse martensitic transformation which proceeds as a single avalanche. The crossover occurs for prestrain values close to the maximum transformation strain. The specific energy of the jump during burst strain recovery depends on the maximum stress applied during prestraining the sample. The crossover from a broad transformation range to an infinite or spanning avalanche is predicted by 3D random field Ising model for disordered systems undergoing the first order phase transition. Based on this model, we explain the crossover in deformed Cu-Al-Ni crystals by a rapid decrease of structural disorder towards the critical one at the final stage of the formation of detwinned γ´1martensite. A simple solution is obtained for the critical transformation rate which produces jumping of the sample during burst reverse transformation after prestrain.

Transformation strain and volume effect associated with martensitic transformation in polycrystalline Ni49+x+yMn23-xGa28-y Heusler alloys

Combined thermal strain and magnetization measurements were performed in polycrystalline alloys Ni49.31Mn22.76Ga27.93(Ni49), Ni53.00Mn21.04Ga25.96(Ni53) and Ni55.62Mn17.52Ga26.86(Ni55.5) to understand the influence of magnetic field (B) on transformation strain (Δλ) and relative volume change (ω) at martensitic transformation (MT). It is found that during cooling and heating both Ni49 and Ni53 samples undergo a MT while the Ni55.5 sample experiences a coupled magnetostructural transformation (MST). Such a MST can enhance the Δλ and ω whereas thermal hysteresis weakens them, which results in a non-monotonic variation in Δλ: 0.211% (Ni49) → 0.166% (Ni53) → 0.185% (Ni55.5) under B = 0 T. Similar tendency was observed in ω. Besides, the values of Δλ and ω increase with increasing field, partially due to the contribution of magnetic field-induced strain. The maximum value ω = 0.674% was obtained in Ni49 sample under 3 T, indicating that an extra volume space of ∼1% should be needed to avoid the destruction of refrigeration chamber. These findings provide key information for future use of Ni–Mn-Ga alloys in refrigeration engineering field.

Spatial and temporal evolution of localized deformation in NiTi tubes in a constant stress thermal cycle: Experiments and analysis

A novel experimental setup is presented that allows for precise control of thermal and mechanical loads, and simultaneous monitoring of the temperature and the full-field deformation of small SMA structures that undergo phase transformations. The facility is used to conduct two experiments in which NiTi tubes are taken through a temperature cycle under constant load that leads to phase transformations in the form of helical localization bands that propagate along the specimen. The latent heat of transformation causes a complex interaction with the prescribed load and thermal environment. By changing the rate of the airflow through the environmental chamber it is revealed that the velocities of the transformation fronts depend on the rate at which heat is removed/added by the controlled environment. The experiments are simulated using a new fully coupled thermomechanical extension of the constitutive framework developed by this research group. Key features of the framework include the modeling of the reversible A⇄M transformation through a single surface in the deviatoric stress-temperature space that obeys kinematic hardening; with the transformation strain and entropy as the internal variables governed by an associative flow rule; and the inhomogeneous deformation exhibited in tension being modeled as softening. The tube is analyzed in a finite element coupled static displacement transient temperature analysis, and taken through the cool/heat cycle of the experiment. The temperature-strain response is accurately reproduced with the two transformations initiating at essentially the same temperatures as in the experiment and propagating in similar localized banded manners at similar speeds. Reproduction of the complex behavior observed in the experiments requires the calibration of the constitutive model, its discretization, and the modeling of the structure and its boundary conditions to work together to near perfection. The simulation also demonstrated that the heat exchange between the structure and the environment, in the present analysis governed by only by convection, requires further enhancement.

Soft Actuators Based on Spin‐Crossover Particles Embedded in Thermoplastic Polyurethane

Molecular spin‐crossover (SCO) complexes are phase‐change materials that develop large spontaneous strains across the thermally induced phase transition, which can be advantageously used in designing soft actuators. Herein, a bilayer bending cantilever, made of thermoplastic polyurethane (TPU) with embedded [Fe(NH2trz)3](SO4) SCO particles (25 wt%), is presented. The proposed actuator is fabricated by blade casting an SCO@TPU layer on a conducting Ag@TPU film to convert electrothermal input into mechanical response. Experiments are conducted to characterize the curvature of bilayer beams, which is then further analyzed using the Euler–Bernoulli beam theory. The beam curvature change, free transformation strain, and effective work density associated with the SCO are 0.11 mm−1, 1.6%, and 1.25 mJ cm−3, respectively. Further, the open‐ and closed‐loop response of the actuator is investigated using a custom‐built setup. The open‐loop identification suggests that the actuator gain increases monotonously when the control current increases. This natural adaptive character can explain the drastically diminished response time in closed‐loop proportional–integral–derivative control experiments (2–3 s). Finally, tracking experiments are carried out to evaluate the robustness of the actuator with and without payloads. The results for 30 240 endurance cycles reveal a mean positioning error of 0.8%.

An outstanding synergy of high strength and ductility/toughness by combining gradient dislocation structure and DIM transformation

Constructing heterostructures is a feasible approach to achieving high strength and ductility/toughness in metals. Here, various gradient dislocation structures (GDS) with different dislocation density distributions were engineered in commercial 304 stainless steel by cyclic–torsion (CT) treatments. With increasing the twist angle of CT treatments, the evident enhancement of strength is along with a slow reduction of ductility following a positive deviation from the inverse linear rule, showing an exceptional strength–ductility synergy. This condition is validated by the simultaneous enhancement of yield strength and static toughness. The enhanced strength was attributed to the introduction of gradient distributed dislocations and the hetero–deformation induced (HDI) stress (300–500 MPa). The retention of high ductility and toughness was provided by the continuous work-hardening from the core to the surface, resulting from the HDI work-hardening and the promoted deformation induced martensite (DIM) transformation (γ → α′) and dynamical strain partitioning effect by the gradient dislocation structure. Moreover, the detwinning mechanism rather than twinning was active for the GDS samples in the tension and for the coarse–grained sample in both CT and tension, and the effect of detwinning on the mechanical properties was also discussed.

A three-dimensional constitutive model for shape memory alloy considering transformation-induced plasticity, two-way shape memory effect, plastic yield and tension-compression asymmetry

In this paper, a three-dimensional phenomenological constitutive model for shape memory alloys (SMA) is proposed with consideration given to various thermomechanical mechanisms. This model includes, for the first time in literature, the major mechanisms influencing the behaviors of SMA actuators in a unified thermodynamics framework: transformation-induced plasticity (TRIP), two-way shape memory effect (TWSME), plastic yield as well as tension-compression asymmetry. Furthermore, a new method to address the characteristics of transformation strain varying with stress levels was proposed. Moreover, a new transformation hardening function was proposed which can give a more accurate prediction of the transformation process. The three-dimensional constitutive model is solved using return mapping algorithm for an increment in a material point in the form of strain and temperature increment. The complete algorithm is proposed and the user-defined material subroutine (UMATs) for finite element problem is established. To validate the proposed SMA constitutive model, the parameters of the SMA material are determined through uniaxial tensile tests. The SMA tubes of the same material are used to conduct both experiments and simulation, with tension-compression asymmetry considered. Training experiments and simulations have been carried out on the SMA tubes, with evident TWSME observed. The typical loading conditions to trigger simultaneous transformation and plastic yield were designed, with experiments and simulations performed. According to the results, the SMA constitutive model is effective in capturing the thermomechanical response of SMA structures under various loading conditions.

Elastic Properties of the Iron(II)–Triazole Spin Crossover Complexes [Fe(Htrz)2trz]BF4 and [Fe(NH2trz)3]SO4

Two benchmark Fe(II)-triazole spin-crossover (SCO) compounds, [Fe(Htrz)2trz]BF4 (1) and [Fe(NH2trz)3]SO4 (2), have been investigated using variable temperature and pressure powder X-ray diffraction, dynamical mechanical analysis, and differential scanning calorimetry techniques. We assessed the transformation strain, the compressibility, and their anisotropy in both compounds. The measured bulk moduli are relatively high (20 and 13 GPa in 1 and 2, respectively), but against expectations, the Fe–triazole chain direction is the most compressible crystal axis in 1. We revealed considerable elastic softening and associated mechanical damping at the SCO in both compounds, which we interpreted, using the Landau theory, as a consequence of the coupling between the spin state and the lattice strain. The magnitudes of the elastic potential energy (∼15%), stored elastic work (<2–3%), and frictional work (<3–4%) with respect to the latent heat of the spin transition (26.8 and 19.5 kJ/mol in 1 and 2, respectively) have been also assessed. The knowledge of these properties is important both for a better understanding of the SCO behavior of this important family of complexes and for guiding their integration into mechanical transducers and other technological applications.

Thermal Energy Harvesting in a Composite Beam Using Piezoelectric, One-Way and Two-Way Shape Memory Alloys

Shape memory alloys (SMA) exhibit the shape memory effect (SME), allowing the alloy which is upon a cyclic thermal loading to return to its original shape after heating beyond the transformation temperature. Two-way SMAs deform during heating and cooling and are suitable for thermal energy harvesting applications. Also, piezoelectric materials (PM) can convert mechanical strain into electrical voltage or current. In this work, a thermal energy harvesting structure is introduced and analyzed. The structure is a composite cantilever beam, comprising two layers, i.e., one-way or two-way SMA and PM. The beam is subjected to different fluctuating temperature ranges, and then we obtain results about energy harvesting performance such as phase transformation strain and output voltage. After developing the nonlinear constitutive equations for one-way and two-way SMEs, the effect of pre-strain and two types of SMAs, i.e., one-way and two-way SMAs are compared with each other under a particular thermal loading. Moreover, the effect of the SMA thickness layer on the output voltage and power has been investigated. Regarding the results, we conclude that for larger pre-strains and maximum temperature of the cycle, greater piezoelectric power is induced. But for a small pre-strain (0.2%), no phase transformation and no output power are observed. Furthermore, it is concluded that in a thermal loading by using two-way SMAs, an electrical potential can be derived without any pre-strain, and increasing the thickness of SMA can rise output power remarkably.

Ion-beam radiation-induced Eshelby transformations: The mean and variance in hydrostatic and shear residual stresses

Ion beam plays a pivotal role in ion implantations and the fabrication of nanostructures. However, there lacks a quantitative model to describe the residual stresses associated with the ion-beam radiation. Radiation-induced residual stress/transformation strain have been mostly recognized in the hydrostatic sub strain space. Here, we use molecular dynamics (MD) simulations to show that the response of a material to irradiation is generally anisotropic that depends on the ion-beam direction, and should be described using tensorial quantities. We demonstrate that accelerator-based ion beam irradiation, combined with the intrinsic lattice anisotropy and externally induced anisotropy (such as anisotropic mechanical loadings), causes radiation-actuated shear transformation strains in addition to hydrostatic expansion. We map out these complex correlations for several materials. Radiation-induced defects are shown to be responsible for residual shear stresses in the manner of Eshelby inclusion transformation. We propose such tensorial response model should be considered for accurate nanoscale fabrication using ion-beam irradiation.

Full Field Model Describing Phase Front Propagation, Transformation Strains, Chemical Partitioning, and Diffusion in Solid–Solid Phase Transformations

AbstractA novel mathematical formulation is presented for describing growth of phase in solid‐to‐solid phase transformations and it is applied for describing austenite to ferrite transformation. The formulation includes the effects of transformation eigenstrains, the local strains, as well as partitioning and diffusion. In the current approach the phase front is modeled as diffuse field, and its propagation is shown to be described by the advection equation, which reduces to the level‐set equation when the transformation proceeds only to the interface normal direction. The propagation is considered as thermally activated process in the same way as in chemical reaction kinetics. In addition, connection to the Allen–Cahn equation is made. Numerical tests are conducted to check the mathematical model validity and to compare the current approach to sharp interface partitioning and diffusion model. The model operation is tested in isotropic 2D plane strain condition for austenite to ferrite transformation, where the transformation produces isotropic expansion, and also for austenite to bainite transformation, where the transformation causes invariant plane strain condition. Growth into surrounding isotropic austenite, as well as growth of the phase which has nucleated on a grain boundary are tested for both ferrite and bainite formation.

Expression of glycerol-3-phosphate acyltransferase increases non-polar lipid accumulation in Nannochloropsis oceanica

Microalgae are considered a suitable production platform for high-value lipids and oleochemicals. Several species including Nannochloropsis oceanica produce large amounts of essential omega-3 polyunsaturated fatty acids (PUFAs) which are integral components of food and feed and have been associated with health-promoting effects. N. oceanica can further accumulate high contents of non-polar lipids with chemical properties that render them a potential replacement for plant oils such as palm oil. However, biomass and lipid productivities obtained with microalgae need to be improved to reach commercial feasibility. Genetic engineering can improve biomass and lipid productivities, for instance by increasing carbon flux to lipids. Here, we report the overexpression of glycerol-3-phosphate acyltransferase (GPAT) in N. oceanica during favorable growth conditions as a strategy to increase non-polar lipid content. Transformants overproducing either an endogenous (NoGPAT) or a heterologous (Acutodesmus obliquus GPAT) GPAT enzyme targeted to the endoplasmic reticulum had up to 42% and 51% increased non-polar lipid contents, respectively, compared to the wild type. Biomass productivities of transformant strains were not substantially impaired, resulting in lipid productivities that were increased by up to 37% and 42% for NoGPAT and AoGPAT transformants, respectively. When exposed to nutrient stress, transformants and wild type had similar lipid contents, suggesting that GPAT enzyme exerts strong flux control on lipid synthesis in N. oceanica under favorable growth conditions. NoGPAT transformants further accumulated PUFAs in non-polar lipids, reaching a total of 6.8% PUFAs per biomass, an increase of 24% relative to the wild type. Overall, our results indicate that GPAT is an interesting target for engineering of lipid metabolism in microalgae, in order to improve non-polar lipid and PUFAs accumulation in microalgae.