FENOMENA KEKERASAN, IMPAK, DAN STRUKTUR MIKRO BAJA 0.074 wt.% KARBON PASCA QUENCHING COOLANT

Dalam penelitian ini dilakukan proses pemanasan material baja dengan kadar karbon 0.074 wt. % hingga temperatur 1000 °C. Baja tersebut dilakukan variasi penahanan dalam furnace selama 10, 20 dan 30 menit. Kemudian masing-masing baja yang telah di tahan lalu di quenching dengan media coolant. Setelah dilakukan quenching kemudian diukur nilai kekerasannya dengan alat uji vickers. Setelah selesai uji keras kemudian diuji dengan impak charphy untuk mengetahui nilai penyerapan energinya. Setelah pengujian mekanis semua selesai kemudian dilanjutkan dengan pengamatan foto mikro dengan mikroskop optik. Dari hasil pengujian maupun pengamatan dapat diketahui semakin lama waktu holding time maka kekerasan semakin turun. Selanjutnya, semakin lama waktu holding time maka nilai impak semakin naik. Sehingga dapat diketahui bahwa nilai kekerasan dan impak saling berbanding terbalik dari variasi holding time yang sudah dilakukan. Terakhir, pada pengamatan struktur mikro spesimen, fasa yang terbentuk adalah ferit dan perlit.

Whole Blood Holding Time Prior to Plasma Processing Alters microRNA Expression Profile

MicroRNAs (miRNAs) can exhibit aberrant expression under different physiological and pathological conditions. Therefore, differentially expressed circulating miRNAs have been a focus of biomarker discovery research. However, the use of circulating miRNAs comes with challenges which may hinder the reliability for their clinical application. These include varied sample collection protocols, storage times/conditions, sample processing and analysis methods. This study focused on examining the effect of whole blood holding time on the stability of plasma miRNA expression profiles. Whole blood samples were collected from healthy pregnant women and were held at 4°C for 30 min, 2 h, 6 h or 24 h prior to processing for plasma isolation. Plasma RNA was extracted and the expression of 179 miRNAs were analyzed. Unsupervised principal component analysis demonstrated that whole blood holding time was a major source of variation in miRNA expression profiles with 53 of 179 miRNAs showing significant changes in expression. Levels of specific miRNAs previously reported to be associated with pregnancy-associated complications such as hsa-miR-150-5p, hsa-miR-191-5p, and hsa-miR-29a-3p, as well as commonly used endogenous miRNA controls, hsa-miR-16-5p, hsa-miR-25-3p, and hsa-miR-223-3p were significantly altered with increase in blood holding time. Current protocols for plasma-based miRNA profiling for diagnostics describe major differences in whole blood holding periods ranging from immediately after collection to 26 h after. Our results demonstrate holding time can have dramatic effects on analytical reliability and reproducibility. This highlights the importance of standardization of blood holding time prior to processing for plasma in order to minimize introduction of non-biological variance in miRNA profiles.

Effect of Holding Time on Populations of Microparticles in Spheroidal Graphite Irons

Non-metallic microparticles in spheroidal graphite irons are a product of the inoculation and the Mg-treatment of the liquid melt. Besides the influence on the mechanical properties of these iron–carbon–silicon alloys, they are also responsible for the nucleation and the morphology of the graphite phase. The present investigation is undertaken to study holding time effects of a (Ba, Ca, Al)–ferrosilicon (called Ba-inoculant) and (Ca, Al)–ferrosilicon (called Ca-inoculant) inoculants on the overall distribution of microparticles. Using the 2D to 3D conversions method, which is typically used for graphite nodules, the non-metallic microparticles’ statistical parameters, such as size distributions and number densities, are quantified. The total number of particles is similar after Mg-treatment and inoculation for Ca-inoculant but not for Ba-inoculated samples, which lose approximately 25 pct of microparticles after 1 minute of holding time. Iron treated with the Ca-inoculant loses about 37 pct of its nodules after 5 minutes, while the Ba-inoculated melts maintain their performance even after 10 minutes. Based on extrapolating the trend of the undercooling, Ba-inoculated samples would reach the uninoculated undercooling values in 48 minutes, while Ca-inoculated samples in only 11 minutes. By evaluating the size distributions of the non-metallic microparticles, the Ostwald ripening hypothesis or particle aggregation can be verified. The results suggest that sulfides are more critical for graphite nucleation since they can be correlated with the graphite number densities. However, due to the small difference in the microparticle population of the uninoculated sample with Ca-inoculated samples, other aspects of the fading mechanism need to be considered, such as transient metastable states, since the central hypothesis of loss of inclusions cannot alone explain the decrease in the nucleation frequency of graphite.

Joint Analysis and Reliability Test of Epoxy-Based Nano Silver Paste Under Different Pressure-Less Sintering Processes

Recent years, the sintered silver paste was introduced and further developed for power electronics packaging due to low processing temperature and high working temperature. The pressure-less sintering technology reduces the stress damage caused by the pressure to the chip, improves reliability, and is widely applied in manufacturing. Currently, most existed studies are focused on alcohol-based sintered silver pastes while resins have been demonstrated to improve the bonding properties of solder joints. Hence, the performance and sintering mechanisms with epoxy-based silver paste need to be further explored. In this work, a methodology for multi-factor investigation is settled on the epoxy-based silver paste to reveal the relationship between the strength and the different influence factors. We firstly analyzed the characteristics of commercialized epoxy-based silver paste samples, including silver content, silver particle size, organic paste composition, sample viscosity, and thermal conductivity. Samples were then prepared for shear tests and microstructure analysis under different pressure-less sintering temperatures, holding time, substrate surface, and chip size. Full factor analysis results were further discussed in detail for correlation. The influence factors were ranked from strong to weak as follows: sintering temperature, substrate surface, chip size, and holding time. Finally, a thermal cycling test was carried out for reliability analysis. Epoxy residues are one of the possible reasons which result in shear strength decreasing exponentially.

Effect of Organizational Evolution on the Stress Corrosion Cracking of the Cr-Co-Ni-Mo Series of Ultra-High Strength Stainless Steel

Different microstructures were obtained under various thermal conditions by adjusting the heat treatment parameters of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel. The effect of organizational evolution on the stress corrosion cracking (SCC) of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel was investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and other test methods in combination with slow strain rate tensile tests (SSRTs). The results show that the Mo- and Cr-rich clusters and precipitation of the Laves phase reduce the corrosion resistance, while increasing the austenite content can improve the corrosion resistance. The Cr-Co-Ni-Mo series of ultra-high strength stainless steel has a high SCC resistance after quenching at 1080 °C and undergoing deep cooling (DC) treatment at −73 °C. With increasing holding time, the strength of the underaged and peak-aged specimens increases, but the passivation and SCC resistance decreases. At the overaged temperature, the specimen has good SCC resistance after a short holding time, which is attributed to its higher austenite content and lower dislocation density. As a stable hydrogen trap in steel, austenite effectively improves the SCC resistance of steel. However, under the coupled action of hydrogen and stress, martensitic transformation occurs due to the decrease in the lamination energy of austenite, and the weak martensitic interface becomes the preferred location for crack initiation and propagation.

Analysis of the Corrosion Behavior of the TiNi Alloy in the Coarse-Grained State

In this work was investigate the corrosion behavior of the TiNi alloy in a coarse-grained state in inorganic field with different concentration and holding time. An increase in the concentration of the solution leads to a significant acceleration of corrosion processes in the Ti49.1Ni50.9 alloy with a high Ni content, including until the samples are completely dissolved. It was revealed that solutions of 1 M sulfuric and hydrochloric acids after a month's exposure did not change in color and no precipitations were found, while solutions of 5 M hydrochloric and sulfuric acids acquired a violet and then green color, which is due to the predominant release of titanium ions (+4) and nickel (+2).

Study on Formation Kinetics and Mechanism of Barium-Calcium Phosphoaluminate Mineral

Barium-calcium phosphoaluminate ((C,B)8A6P) mineral was synthesized by introducing Ba2+ into the lattice of calcium phosphoaluminate (C8A6P) mineral and it owned the better hydration activity than C8A6P. In this study, the formation kinetics and mechanism of (C,B)8A6P mineral was firstly systematically explored. The experimental results indicated that calcination temperature had a significant effect on the formation of (C,B)8A6P mineral. The conversion rate of (C,B)8A6P mineral was less than 0.5 and reaction rate was less than 9.4 × 10-7 with the holding time of 8 h at 1500-1530 °C. Nevertheless, the conversion rate and reaction rate reached 0.8 and 9.4 × 10-7, respectively, with the holding time of only 2 h when the calcination temperature ranged from 1540 °C to 1560 °C. In addition, the Jander equation was feasible for the formation of (C,B)8A6P with activation energy of 1310 kJ ∙ mol-1 at 1500-1530 °C, while the Ginstling equation was feasible for use with activation energy of 324 kJ ∙ mol-1 when the calcination temperature ranged from 1540 to 1560 °C.

Influence of Dwell Time and Pressure on SPS Process with Titanium Aluminides

Spark plasma sintering (SPS) or the field-assisted sintering technique (FAST) is commonly used to process powders that are difficult to consolidate, more efficiently than in the conventional powder metallurgy process route. During the process, holding time and applied holding pressure influence the product’s microstructure and subsequently its properties. In this study, in addition to the temperature impact, the influence of pressure and dwell time on the consolidation behaviour of titanium aluminide (TiAl) powders during the SPS process is investigated. Commercially available pre-alloyed TiAl48-2Cr-2Nb (GE48) and TiAl44-4Nb-0.7Mo-0.1B (TNM) powders were used, which have a high application potential in, for example, the aerospace industry. The results were evaluated based on microstructural analyses, hardness measurements and relative density calculations. It was shown that the investigated parameters significantly influence the sintering results, especially in the low temperature range. Depending on the temperature field in the sample, complete sintering is not achieved if the dwell time is too short in combination with too low a pressure. Above a certain temperature, the impact of holding pressure and holding time is significantly lower.