Defects, particularly solidification cracking, remain persistent challenges in the laser powder bed fusion (LPBF) processing of AA6061 aluminum alloy. This study systematically investigates defect mitigation, microstructural evolution, and mechanical properties associated with high-temperature substrate preheating at 500 degrees C. Comprehensive microstructural analyses, including characterization of defects, grain structures, and precipitation behavior, were performed on samples in both as-built and T6 heat-treated states. Elevated preheating substantially reduced solidification cracking across a wide processing window, while demonstrating decreased crack sensitivity to laser parameters. Columnar cracks along the build direction were observed despite substrate preheating. Lack-of-fusion and keyhole porosity were effectively eliminated, though gas-induced microporosity persisted at higher powers. In-depth characterization of two distinct laser power and speed conditions confirmed the formation of micron-sized, non-coherent Mg2Si precipitates under heated substrate conditions, alongside alpha-AlFeCrMnSi intermetallic phases indicative of in-situ thermal effects during fabrication. Subsequent T6 heat treatment revealed the formation of fine, coherent needle-shaped beta '' strengthening precipitates. Despite substantial differences in processing parameters, comparable mechanical properties were measured in the as-built samples (similar to 52 MPa yield strength, similar to 130 MPa tensile strength), primarily due to reduced strain hardening effects and consistent precipitation characteristics. Meanwhile, the T6 heat treatment led to significant improvement in properties, enhancing yield strength by over 400%, aligning closely with the performance of conventional wrought AA6061-T6. These findings underscore that high-temperature substrate preheating offers an effective means to suppress cracks, control precipitation, and enhance mechanical performance in LPBF-processed AA6061.

Water-atomized (WA) steel powders are inevitably oxidized due to the chemical reaction between molten metal and water vapor. Minimization of surface oxides is essential for forming strong metallic bonds during sintering. However, the efficiency of H2-annealing is dependant on oxide chemistry, which is rarely made of pure/stoichiometric species. The objective here was to perform an in-depth characterization of the oxides found on WA steel particles to understand their behaviour during H2-annealing. It investigates the relationship between the chemical composition of pre-alloyed steels(Cr, Mn, Mo, Si), the chemical composition of the surface oxides and their modification due to H2-annealing. Particular attention is given to the impact of the difference between Si(wt.%) and Mn(wt.%) contents on powder oxidation. Characterization was performed using scanning and transmission electron microscopy coupled with energy dispersive X-ray spectroscopy. Characterization before and after annealing provided insights into the dynamics of oxides reduction according to powder chemistry.

Vibratory peening is a promising vibro-strengthening process combining shot peening and vibratory polishing into a single-step mechanical surface treatment. Vibratory peening induces surface compressive stresses and strain hardening, which improves the surface finish and fatigue life of components in aerospace and energy applications. This study comprehensively investigates the effect of five vibratory peening parameters (specimen surface preparation, rotating shaft eccentricity, frequency, specimen immersion depth, and processing time) on the surface properties of Ti-6Al-4V alloy, combining an experimental-statistical approach to establish process–property correlations. The results show improved surface properties, with surface roughness limited between 0.13 and 0.19 µm. Compressive residual stress distribution shows a maximum value of 812 MPa at a depth of 71 µm below the surface. The total depth of influence is found to be 230 µm. The rotating shafts eccentricity, frequency, and specimen immersion depth must be increased simultaneously to minimize the roughness reduction time, maximize the magnitude and depths of compressive residual stresses, as well as the plastic strain depth.

(Al,Si)(3)(Zr,Ti)-D0(22)/D0(23) are phases that may form in aerospace and automotive aluminium alloys. The substitution of Zr/Ti in these solid solutions is widely reported in the literature; however, it remains relatively unexplored for Si. In this work, in situ precipitation of (Al,Si)(3)(Zr,Ti)-D0(22)/D0(23) intermetallics was performed using Al-Si-Zr-Ti alloys. The precipitation, sedimentation and concentration of numerous intermetallic particles were accomplished by filtrating the residual molten aluminium using a temperature/pressure-controlled vessel adapted with a PoDFA filter. A combination of SEM, TEM, XRD and EMP analysis allowed the identification of (Al,Si)(3)(Zr,Ti)-D0(22)/D0(23) intermetallics concentrated within..-FCC matrices of non-Si-doped (sample S2) and Si-doped (samples S4 and S6) alloys. EDS analysis confirmed that Zr and Ti substitute each other in the D0(22) and D0(23) phases, whereas Si substitutes in Al sites. Acceptance of Si inside the D0(23) phase was not expected according to FTlite (FactSage) and TCAL7 (Thermo-Calc) databases. Additionally, Si was found to enhance the formation of (Al,Si)(3)(Zr,Ti)-D0(22) intermetallics with high Zr-content, contrary to FactSage 7.3 predictions. TEM results showed intermetallic/FCC crystal coherency for samples S2 and S6, implying that these intermetallics acted as nucleation sites for the Al-phase due to their small lattice mismatch. Furthermore, Si site occupancy was calculated for both (Al,Si)(3)Ti-D0(22) and (Al,Si)(3)Zr-D0(23) phases via DFT, showing that sites 2b and 4e are the most favorable for Si occupation, respectively. Finally, a thermodynamic model is derived to describe Si substitution upon solidification. Experimental and numerical examinations indicate that Si substitution preferentially occurs in the D0(22) intermetallics compared to the D0(23) phase.

Inherent micro-cracking mechanisms in two contrasting high-gamma-prime (high-γ') Ni-based superalloys processed by laser powder bed fusion (LPBF) were investigated. RENÉ 65 (R65) has a 42% γ' volume fraction including Al, Ti, and Nb, while RENÉ 108 (R108) has a higher γ' volume fraction (63%) including Al, Ta, and lesser Ti. Quantitative analysis showed R108 exhibits 329% higher micro-cracking density than R65. All micro-cracks propagate along high angle boundaries (HABs) and exhibit interdendritic morphologies suggesting solidification cracking is the dominant micro-cracking mechanism. Moreover, secondary phases detrimental for solid-state cracking are not observed at the HABs. Atom probe tomography (APT) showed preferential segregation of Hf, Mo, C, B at the R108 HAB and Zr, Ti, Mo, C, B at the R65 HAB. The Kou solidification cracking criterion showed that Hf and Zr partitioning along the grain boundaries increases micro-cracking susceptibility. Gamma prime did not form during the LPBF process and titanium has a higher tendency than tantalum to partition in the last liquid to solidify.

Nanometric particles that are unintentionally released into the workplace are potentially toxic to workers. They can easily settle in the respiratory system and are distinctive because of their large specific surface area and high potential for causing pulmonary inflammation. The purpose of this study was to characterize unintentionally released nanoparticles (URNPs) found in six workplaces on the basis of a broad range of indicators. Concentrations were assessed according to numerical and mass metrics using an array of direct-reading instruments (DRIs). Integrated measurements were also taken, based on the type of contaminant specific to each workplace. These measurements included (i) respirable and submicron carbon (elemental and organic) fractions, as well as respirable combustible dust from diesel exhaust fumes (DEFs) found in an underground mine (M1), in a truck repair garage (M2) and in maintenance in an underground transit system (M3); (ii) gravimetric measurements and concentrations of 12 metals (aluminum, cadmium, chromium, cobalt, copper, iron, magnesium, manganese, nickel, lead, vanadium, zinc) in fumes and metallic dust released in a foundry (M4), as well as in a machine shop (welding, grinding and cutting) (M5); (iii) paraffin wax (C18-C36) from fumes released in a wax-moulding shop (M6). In parallel, measurements for the purpose of microscopic characterization were also taken in the six workplaces. For the measurements taken by DRIs, the daily numerical concentrations in the six workplaces ranged between 12,900 and 228,600 particles/cm³ and the mass concentrations between 0.01 and 3.22 mg/m³. In terms of number of particles, the underground mine was the environment with the highest concentrations, while the wax-moulding shop had the highest mass concentrations. In environments where DEFs were found, daily concentrations of elemental carbon (EC) ranging from 0.002 to 0.503 mg/m³ were measured with DRIs. For the integrated measurements, the concentrations of total carbon (TC) measured in this study were lower than the Quebec regulated level of 0.4 mg/m³ stipulated in the Regulation Respecting Occupational Health and Safety in Mines, with the exception of a level of 0.7 mg/m³ recorded in workplace M1. A comparison of the metal concentrations from workplaces M4 and M5 with the recommendations of the American Conference of Governmental Industrial Hygienists (ACGIH) found that all the concentrations were 10% below recommended levels, with the exception of one measurement of manganese (respirable fraction) in the foundry (M4). The paraffin wax concentrations measured were below the occupational exposure limit of 2 mg/m³ set by the ACGIH for the fumes this substance releases. Workers exposed to DEFs (M1, M2 and M3) are exposed to mostly nanometric-size airborne particles whose mass concentration is largely in the submicron fraction. In the presence of foundry fumes (M4), workers are exposed to airborne particles that are mostly nanometric in size and whose mass concentration is chiefly in the submicron fraction for chromium, cobalt, copper, iron, manganese, lead, vanadium and zinc. The workers in the machine shop (M5) are exposed to fumes and dust particles from machining, most of which are nanometric in size, but some of the processes they use generate larger, micrometric particles. The contribution of larger particles to the mass concentration is significant in this environment and, as a result, the mass concentration is to be found in the inhalable fraction, especially for chromium, copper, iron and nickel. Workers in the wax shop (M6) are exposed to fumes that are chiefly nanometric in size and whose mass concentration is mostly in the submicron fraction. Our innovative strategy enabled us to characterize the URNPs released in the different workplaces with respect to both numerical and mass concentrations. Microscopy studies on particle samples from the microscope grid taken with a Mini Particle Sampler® were used to characterize the particles collected based on their morphology and chemical composition.

In this paper, the microstructure of a gas-atomized steel powder pre-alloyed with aluminum (11.4 Cr, 10.0 Co, 5.7 Al, 5.1 Ni and Fe in wt pct) is characterized. The as-atomized powder contains NiAl-rich precipitates and most of the particles are covered with a thin (< 10 nm) Fe-rich oxide layer alloyed with aluminium. During the heating ramp of the sintering treatment, aluminum diffuses at the surface of the powder to grow an alumina layer which prevents sintering. When adding copper to the powder mix, however, liquid copper diffuses at the interface between steel and alumina, detaches alumina filaments from the steel and reshapes them into rounded particles. The evolution of the microstructure and the hardness are described as a function of tempering temperatures and time. Finally, the possibility of 3D printing this powder using the selective laser melting technique is investigated.

Abstract This paper presents extensive electrical, microstructural and chemical characterizations of HTS coated conductor samples in which an oxide layer (CeO x ) has been added between the superconductor (GdBaCuO) and the silver (Ag) layers, in an attempt to increase the interfacial resistance between these two conductive layers. This increase of interfacial resistance is required to realize the current flow diverter (CFD) architecture in HTS tapes. All samples in this paper have been characterized before and after performing an annealing in oxygen atmosphere. The purpose of the annealing was to reduce the interfacial resistance generated by the CeO x layer, in order to achieve a proper value for the CFD architecture. Samples with different thicknesses of CeO x , namely 0, 10, 35 and 100 nm, have been produced and characterized. The critical current and the critical temperature have been measured to determine the quality of the superconducting layers, while cross-section transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) have been used to monitor the impact of the annealing. The results show a clear degradation of the superconducting layer for samples with a thick layer ( ≥ 35  nm) of CeO x after annealing at 450  o C. According to the EDS results, a reduction of the amount of barium is observed in the superconducting layer, which could explain the observed reduction of the critical current.

Questions still exist regarding which indicator better estimates worker's exposure to diesel particulate matter (DPM) and, especially for ultrafine particles (UFP), how exposure levels and the characteristics of the particles vary in workplaces with different exposure conditions. This study aimed to quantify and characterize DPM exposures in three workplaces with different exposure levels: an underground mine, a subway tunnel, and a truck repair workshop. The same sampling strategy was used and included measurements of the particle number concentration (PNC), mass concentration, size distribution, transmission electron microscopy (TEM), and the characterization of carbonaceous fractions. The highest geometric means (GMs) of PNC and elemental carbon (EC) were measured in the mine [134 000 (geometric standard deviation, GSD = 1.5) particles cm-3 and 125 (GSD = 2.1) µg m-3], followed by the tunnel [32 800 (GSD = 1.7) particles cm-3 and 24.7 (GSD = 2.4) µg m-3], and the truck workshop [22 700 (GSD = 1.3) particles cm-3 and 2.7 (GSD = 2.4) µg m-3]. This gradient of exposure was also observed for total carbon (TC) and particulate matter. The TC/EC ratio was 1.4 in the mine, 2.5 in the tunnel and 8.7 in the workshop, indicating important organic carbon interference in the non-mining workplaces. EC and PNC were strongly correlated in the tunnel (r = 0.85; P < 0.01) and the workshop (r = 0.91; P < 0.001), but a moderate correlation was observed in the mine (r = 0.57; P < 0.05). Results from TEM showed individual carbon spheres between 10 and 56.5 nm organized in agglomerates, while results from the size distribution profiles showed bimodal distributions with a larger accumulation mode in the mine (93 nm) compared with the tunnel (39 nm) and the truck workshop (34 nm). In conclusion, the composition of the carbonaceous fraction varies according to the workplace, and can interfere with DPM estimation when TC is used as indicator. Also, the dominance of particles <100 nm in all workplaces, the high levels of PNC measured and the good correlation with EC suggest that UFP exposures should receive more attention on occupational routine measurements and regulations.

Binary solvent additive engineering is an effective strategy to optimize photoactive films for high‐efficiency organic solar cells, however, the effect of single components on device performance and the combination principle of binary solvent additives remain unclear. Herein, synchrotron‐based grazing incident X‐ray diffraction, Derjaguin–Muller–Toporov modulus imaging, and plasmon energy shift imaging acquired by scanning transmission electron microscopy to investigate the effect of new binary solvent additive of 1,8‐diiodooctane (DIO) and less‐toxic and p‐anisaldehyde (AA) on device performance of solar cells based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3‴‐di(2‐octyldodecyl)2,2′;5′,2″;5″,2‴‐quaterthio‐phen‐5,5‴‐diyl)] (PffBT4T‐2OD) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) are used. It is found that AA mainly favors polymer order and high crystallinity of PffBT4T‐2OD. Differently, DIO mainly enables PC61BM diffusing into PffBT4T‐2OD polymer matrix, leading to enlarged donor–acceptor (D–A) interface. As expected, by combining AA and DIO, the composite film provides large D–A interface and more balanced charge carrier transport. Consequently, their beneficial synergistic effect results in enhanced short circuit current and fill factor, and thereby increased power conversion efficiency of 10.64%, improved by 16% with respect to the control device. Herein, a general mechanism of enhancing device performance via the combination of solvent additives with different contributions to photoactive film is unveiled.

The performance of molecule-based thin-film devices such as organic light-emitting diodes, photovoltaic cells, and thin-film transistors depends on the electronic properties of the individual molecular components, as well as on their association to form complex morphologies. Transmission electron microscopy (TEM) can be used to image the morphologies and help reveal how the devices work and can be improved. We have examined the suitability of various ways to prepare samples of thin molecular films for imaging by TEM. Specifically, we have used focused ion beams to mill cross sections of complete devices that have been glued together with epoxy adhesives. In addition, thin films of the type used as active layers in molecule-based devices can be deposited on disks of NaCl, which can then be dissolved in water to release free-standing films that can be imaged by TEM, without loss of nanostructural details. Films of this type can also be deposited on Si wafers, which can then be fractured to expose sections of film that overhang edges of fragments and can be imaged conveniently by TEM. This allows TEM to be used as a quick method for screening samples and monitoring the purification of active materials.

The effect of copper precipitation on the mechanical properties of Fe-Cu-C alloys prepared for Powder Metallurgy and used to manufacture connecting rods for the automotive industry through powder forging was evaluated at engine operating temperatures, ranging between 100 degrees C and 150 degrees C. Tensile tests were conducted at room temperature as well as at 120 degrees C and 150 degrees C on specimens machined from connecting rods. The test results clearly indicated an improvement in the strength of Fe-Cu-C alloys at 120 degrees C and 150 degrees C. Scanning and transmission electron microscopies were employed to investigate the strengthening mechanism causing the improvement. The microscopy investigations pointed to stress-induced second phase precipitation strengthening in super-saturated Fe-Cu-C alloys even at these relatively low temperature levels as copious Cu nano precipitates were observed in the specimens submitted to tensile testing at 120 degrees C and components submitted to engine dynamometer testing. Clear evidence of interactions between dislocations and copper precipitates was found in both tensile specimens and components.

Les particules nanométriques émises non intentionnellement (PNNI) en milieux de travail présentent un potentiel de toxicité pour les travailleurs. Elles montrent une importante capacité de se déposer dans le système respiratoire et se distinguent par leur grande surface spécifique et un potentiel élevé d’inflammation pulmonaire. Cette étude vise à caractériser les PNNI émises dans six milieux de travail à travers un large éventail d’indicateurs. Les concentrations ont été évaluées selon les métriques numériques et massiques à l’aide d’une panoplie d’instruments à lecture directe (ILD). Des mesures intégrées ont aussi été effectuées en tenant compte du type de contaminant spécifique à chaque milieu. Ces mesures incluent : (i) les mesures de carbone (élémentaire et organique) des fractions sous-microniques et respirables ainsi que des poussières combustibles respirables pour les émanations de moteur diesel (ÉMD) trouvées dans une mine souterraine (M1), dans un atelier de réparation de camions (M2) et dans le contexte d’activités d’entretien dans un réseau souterrain de transport (M3); (ii) les mesures gravimétriques et les concentrations de 12 métaux (aluminium, cadmium, chrome, cobalt, cuivre, fer, magnésium, manganèse, nickel, plomb, vanadium, zinc) dans les fumées et poussières métalliques émises dans une fonderie (M4) ainsi que dans un atelier d’usinage (soudage, meulage et coupage) (M5); (iii) la mesure de cire de paraffine (C18-C36) des fumées émises dans un atelier de moulage à la cire (M6). En parallèle, des mesures de caractérisation en microscopie ont été effectuées dans les six milieux. Pour les mesures obtenues par ILD, les concentrations numériques journalières dans les six milieux variaient de 12 900 à 228 600 particules/cm³ et les concentrations massiques de 0,01 à 3,22 mg/m³. En nombre de particules, le milieu des mines souterraines était celui avec les plus fortes concentrations alors que l’atelier de moulage à la cire présentait les concentrations massiques les plus élevées. Dans les milieux où des ÉMD étaient présentes, des concentrations journalières de carbone élémentaire (CE) de 0,002 à 0,503 mg/m³ ont été rapportées avec les ILD. Pour les mesures intégrées, les concentrations de carbone total (CT) mesurées dans cette étude sont inférieures à la valeur réglementaire québécoise de 0,4 mg/m³ du Règlement sur la santé et la sécurité du travail dans les mines sauf une valeur de 0,7 mg/m³ mesurée dans le milieu M1. Lorsque l’on compare les concentrations de métaux des milieux M4 et M5 aux recommandations de l’American Conference of Governmental Industrial Hygienists (ACGIH), on constate que toutes les concentrations sont inférieures à 10 % des valeurs recommandées, sauf pour une mesure de manganèse (fraction respirable) dans la fonderie (M4). Les concentrations de cire de paraffine mesurées sont inférieures à la valeur limite d’exposition professionnelle de 2 mg/m³ proposée par l’ACGIH pour les fumées que dégage cette substance. Les travailleurs exposés à des ÉMD (M1, M2 et M3) sont exposés à des particules aéroportées majoritairement de taille nanométrique et dont la concentration massique se trouve largement dans la fraction sous-micronique. En présence des fumées de fonderie (M4), les travailleurs sont exposés à des particules aéroportées majoritairement de taille nanométrique et dont la concentration massique se trouve principalement dans la fraction sous-micronique pour le chrome, le cobalt, le cuivre, le fer, le manganèse, le plomb, le vanadium et le zinc. Les travailleurs de l’atelier d’usinage (M5) sont exposés à des fumées et poussières d’usinage qui sont majoritairement de taille nanométrique, mais ils recourent à certains procédés qui génèrent des particules plus grossières dont la taille est de l’ordre du micromètre. La contribution des particules plus grossières à la concentration massique est importante dans ce milieu et, par conséquent, la concentration massique se trouve dans la fraction inhalable notamment pour le chrome, le cuivre, le fer et le nickel. Les travailleurs de l’atelier de cire (M6) sont exposés à des fumées qui sont principalement de taille nanométrique et dont la concentration massique se trouve en majorité dans la fraction sous-micronique. La stratégie innovante mise en place a permis de caractériser les PNNI émises dans les différents milieux de travail sur le plan des concentrations numériques et des concentrations massiques. Les études de microscopie, subséquentes aux prélèvements de particules sur les grilles de microscopie avec les échantillonneurs MPS (Mini Particle Sampler®), ont permis de caractériser les particules collectées selon leur morphologie et leur composition chimique.

A compositionally-graded steel composed of martensite with 0.4%C on the centre and bainite with 0.1%C on the surface was manufactured by partial decarburization. It is reported that the as quenched material can be cold rolled up to an equivalent strain of 1.5 without cracks. The mechanical properties of the cold-rolled material exhibits up to 2.5 GPa strength and ductility. A simple mechanical model is developed to predict the stress state after rolling of the graded structure explaining the good ductility of the present high strength materials.

Most advanced high-strength steel products contain complex phases, including ferrite, bainite and martensite, which form successively during elaboration. It is essential to understand the effect of prior ferrite transformation on the subsequent bainite and martensite transformation kinetics to achieve precise control of the final microstructure. Nevertheless, the effect of the interface between the prior formed ferrite and the residual austenite (α/γ), together with the related chemical heterogeneity at the interface, on the subsequent phase transformations has been studied only rarely, and remains unclear. This study pays particular attention to the effect of the α/γ interface and its related concentration gradients on bainite and martensite transformation. It is shown that the interface and its related concentration gradients can play a very significant role on the subsequent bainite or martensite transformation kinetics: it retards bainite transformation whereas it accelerates martensite transformation. It is revealed from microprobe wavelength-dispersive spectrometry analysis and model calculations that there are both manganese and carbon gradients in front of the α/γ interface at the end of the ferrite transformation holding. The subsequent bainite transformation kinetics is controlled by the competition between the acceleration effect of the interface boundary itself and the retardation effect of the higher alloying concentration near the interface. Martensite transformation should initiate at the pre-existing dislocations in the center of the residual austenite grains, where the C and Mn contents are the lowest. A simple martensite transformation kinetics model taking into account C heterogeneity is proposed that can describe well the martensite transformation kinetics following the prior ferrite transformation.

Ashby's performance indexes are a fundamental tool for material selection especially for structures lightening. Unfortunately the indexes are available only for simple mechanical solicitation as pure bending or pure tension. For real applications, it is required to have a performance index for combined solicitations. This publication proposes an approach to develop this kind of extended performance index and shows the exploitation in order to compare materials performance in more realistic situations.

Constant demand to optimize production of plastic components puts pressure on mold steels manufacturers to supply pre-hardened blocks with larger cross-sections. As the size of mold blocks increases, it becomes more difficult to maintain the same microstructure throughout their volume. Thus, the size of mold steel blocks is limited by the cooling rate reachable at their center. This situation has created the need for the development of new alloys with improved hardenability. Additions of vanadium up to 0.35 wt-% have been made to P20 + 0.5Ni mold steel. The addition of vanadium significantly increased hardenability, allowing the production of pre-hardened blocks with a cross section 88% larger than the same alloy without vanadium. Such additions of vanadium also increased the impact toughness by 120%, raised the yield strength by 17% and improved hardness after tempering by 24%. The effect of boron on the hardenability of the P20 + 0.5Ni steel was also investigated. It resulted in a significant increase in hardenability and, in theory, could permit the production of pre-hardened blocks with a cross-section 400% larger than the original material. Finally, our results show that 0.35 wt-% of Ni can be substituted by 0.15 wt-% of vanadium in a P20 + 0.5Ni steel and still maintain the same hardenability.

With a square wave electrochemical reduction method, Pd single-crystal nanothorns (Pd NTs) are prepared at room temperature, without template or surfactant. By varying the potentials applied, we can control the length and accumulation state of the Pd NTs. TEM study shows that the growth of the thorn occurs along the (2 (2) over bar0) plane. The nanothorn is made by a succession of epitaxic dodecahedrons of decreasing sizes aligned in the direction of the (111) plane. The surface chemical state of the Pd NTs is studied with XPS. Pd NTs have higher catalytic activity than commercial Pd black for the oxidation of formic acid.