Chun-Hao Koo

Bio: Chun-Hao Koo is an academic researcher from National Taiwan University. The author has contributed to research in topics: Superalloy & Alloy. The author has an hindex of 15, co-authored 27 publications receiving 689 citations.

Antibacterial properties, corrosion resistance and mechanical properties of Cu-modified SUS 304 stainless steel

Abstract: This investigation studies the effects of Cu content and ageing treatment on the microstructural, mechanical, corrosion and antibacterial properties of SUS 304 austenitic stainless steel. Cu was added respectively to SUS 304 stainless steels in proportions of 1.5, 2.5, 3.5, 4.5 and 5.5 wt.%. A vacuum arc remelting furnace was used to remelt SUS 304 stainless steel with various added Cu contents. These ingot alloys underwent hot rolling and various heat treatments, and were then cut into test specimens. A series of microstructural investigation, tensile tests, corrosion tests and antibacterial tests were conducted to study the properties of Cu-containing SUS 304 austenitic stainless steel. Microstructural observations reveal that the amount of retained δ-ferrite in the as-cast SUS 304 steel decreases as the Cu content increases. After hot rolling, the retained δ-ferrite disappears and α′-martensite forms in the austenitic matrix. The results of the tensile tests reveal that the ultimate tensile strength (UTS) declines as the Cu content increases below 2.5 wt.%. However, the ultimate tensile strength increases with the Cu content above 2.5 wt.%. X-ray diffraction analysis reveals that adding Cu suppresses the formation of strain-induced martensite (α′-martensite). The corrosion test indicates that the pitting potential declines as the Cu content in SUS 304 steels increases. The results of the antibacterial test reveal that adding a proper amount of Cu (such as 2 wt.%) gives SUS 304 stainless steel an excellent antibacterial property.

Pack cementation coatings on Ti3Al–Nb alloys to modify the high-temperature oxidation properties

Abstract: Ti 3 Al-based alloys have excellent high-temperature strength, but their poor oxidation resistance restricts the applications of these alloys at high temperature. In this study the microstructure, oxidation resistance, and mechanical properties of pack cementation Al, Si, and Cr coatings on Ti 3 Al-based alloys, i.e. Ti–25Al–10Nb and Ti–25Al–13Nb, were investigated and discussed. The morphology of aluminized coating is a single layer of the TiAl 3 phase. The siliconized coating shows a multilayer structure with the composition of TiSi 2 , TiSi, TiAl 2 and TiAl from the outer surface to the substrate. The chromized coating is a Cr-rich beta phase. The high-temperature oxidation resistance of Al and Si coatings is quite superior, and their temperature limit of oxidation resistance is up to 1100°C. In contrast to Al and Si coatings, the oxidation resistance of the chromized coating is poor, since it is unable to improve the oxidation resistance of the Ti–25Al–10Nb alloy. The high-temperature tensile test of Ti–25Al–13Nb with Al or Si coatings shows that the hard coating layer may reduce the ductility of the base alloy. The alloy with the Al or Si coating is suitable for applications which operate at high temperatures but low stress levels.

Improvement of high temperature oxidation and corrosion resistance of superalloy IN-738LC by pack cementation

Abstract: This study systematically analyzes the experimental results of the resistance of various coatings on superalloy IN-738LC to high temperature oxidation and corrosion. Three different pack cementation processes are carried out: pack-aluminization process, two-step aluminization–chromization process and siliconization process (with or without TiB2 in the packing mixture). Rhodium or palladium is deposited on the superalloy IN-738LC specimen by electroplating prior to the pack-aluminization processes. NiAl and Ni2Al3 phases are the main constituents of the aluminized coatings. Single Cr-rich phase forms on the surface of aluminized–chromized coatings. The Ni2Si, Ni3Si and TiSi phases exist after simple siliconization while Ni2Si, Ni3Si, TiSi, TiSi2 and TiB phases form after siliconization with TiB2 in the packing mixture. Among the three different categories of pack cementation coatings, aluminized coatings have the best isothermal and cyclic oxidation resistance at high temperature. When the temperature is over 850 °C, aluminized–chromized coating layers lose their protection against oxidation due to the spalling or cracking of the oxide scales during the test for cyclic oxidation. Siliconized coatings are brittle and crack severely due to thermal shock. Therefore, siliconized coatings are not suitable for superalloy IN-738-LC. Hot corrosion tests were carried out at 900 °C, a temperature above the melting point of Na2SO4 (884 °C). All three categories of pack cementation coatings show degradation of the corrosion scales and internal attack. Nevertheless, the aluminized coatings were still protective under hot corrosion conditions. The ranking of high temperature oxidation and corrosion resistance of all pack cementation coatings, from best to worst, is as follows: Rh–Al, simple Al, Pd–Al, Si+TiB2, simple Si, Pd–Al–Cr, Rh–Al–Cr and simple Al–Cr samples.

Effects of kerosene or distilled water as dielectric on electrical discharge alloying of superalloy Haynes 230 with Al–Mo composite electrode

Abstract: The effects of kerosene and distilled water as dielectrics on the electrical discharge surface alloying of superalloy Haynes 230 are investigated. The 85 at.% Al and 15 at.% Mo composite electrode provided the surface alloying materials. The alloying results obtained using both positive and negative electrode polarities were compared. With negative electrode polarity and alloying in kerosene, many discontinuous piled-layers comprised mostly of Al3Mo8 and AlMo3 phases accumulate on the surface of the N-AlMo-Kero specimen, but alloying in distilled water was unsuccessful because of the difficulty of discharging under such an EDA condition. With positive electrode polarity, the alloyed layers constituted mainly of NiAl phase are formed on the EDA specimens, in either kerosene or distilled water. The alloyed layer of P-AlMo-Kero contains a mixture of NiAl, Al8Mo3, Cr23C6, and Al4C3, while the alloyed layer of P-AlMo-Water contains NiAl, AlCr2, Al5Cr, and Al2O3 phases. The P-AlMo-Water exhibits the highest hardness, whereas the P-AlMo-Kero has the smallest surface roughness of all the EDA specimens. The superalloy Haynes 230 and the EDA specimens are subjected to isothermal oxidation at 1000 °C in static air. Analyses of oxidation kinetics indicate that the P-AlMo-Water and the P-AlMo-Kero specimens are more resistant to oxidation than the unalloyed superalloy; and the P-AlMo-Kero specimen has the best oxidation resistance among all tested specimens.

Characteristics and Mechanical Properties of Polycrystalline CM 247 LC Superalloy Casting

Abstract: The CM 247 LC superalloy was remelted and cast to obtain the desired polycrystalline test bars by controlling casting parameters, followed by the investigation of precipitation morphology and mechanical properties. The experimental results show that by well-controlled casting parameters the CM 247 LC owns excellent castability to form a superalloy with fine grain structure, high strength as well as superior creep resistance.

Recent developments in stainless steels

Abstract: This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Magnesium and its alloys applications in automotive industry

Abstract: The objective of this study is to review and evaluate the applications of magnesium in the automotive industry that can significantly contribute to greater fuel economy and environmental conservation. In the study, the current advantages, limitations, technological barriers and future prospects of Mg alloys in the automotive industry are given. The usage of magnesium in automotive applications is also assessed for the impact on environmental conservation. Recent developments in coating and alloying of Mg improved the creep and corrosion resistance properties of magnesium alloys for elevated temperature and corrosive environments. The results of the study conclude that reasonable prices and improved properties of Mg and its alloys will lead to massive use of magnesium. Compared to using alternative materials, using Mg alloys results in a 22% to 70% weight reduction. Lastly, the use of magnesium in automotive components is increasing as knowledge of forming processes of Mg alloys increases.

An introduction to mathematical statistical and its applications / Richard J. Larsen, Morris L. Marx

Abstract: 1. Introduction 1.1 An Overview 1.2 Some Examples 1.3 A Brief History 1.4 A Chapter Summary 2. Probability 2.1 Introduction 2.2 Sample Spaces and the Algebra of Sets 2.3 The Probability Function 2.4 Conditional Probability 2.5 Independence 2.6 Combinatorics 2.7 Combinatorial Probability 2.8 Taking a Second Look at Statistics (Monte Carlo Techniques) 3. Random Variables 3.1 Introduction 3.2 Binomial and Hypergeometric Probabilities 3.3 Discrete Random Variables 3.4 Continuous Random Variables 3.5 Expected Values 3.6 The Variance 3.7 Joint Densities 3.8 Transforming and Combining Random Variables 3.9 Further Properties of the Mean and Variance 3.10 Order Statistics 3.11 Conditional Densities 3.12 Moment-Generating Functions 3.13 Taking a Second Look at Statistics (Interpreting Means) Appendix 3.A.1 MINITAB Applications 4. Special Distributions 4.1 Introduction 4.2 The Poisson Distribution 4.3 The Normal Distribution 4.4 The Geometric Distribution 4.5 The Negative Binomial Distribution 4.6 The Gamma Distribution 4.7 Taking a Second Look at Statistics (Monte Carlo Simulations) Appendix 4.A.1 MINITAB Applications Appendix 4.A.2 A Proof of the Central Limit Theorem 5. Estimation 5.1 Introduction 5.2 Estimating Parameters: The Method of Maximum Likelihood and the Method of Moments 5.3 Interval Estimation 5.4 Properties of Estimators 5.5 Minimum-Variance Estimators: The Crami?½r-Rao Lower Bound 5.6 Sufficient Estimators 5.7 Consistency 5.8 Bayesian Estimation 5.9 Taking A Second Look at Statistics (Beyond Classical Estimation) Appendix 5.A.1 MINITAB Applications 6. Hypothesis Testing 6.1 Introduction 6.2 The Decision Rule 6.3 Testing Binomial Dataâ H0: p = po 6.4 Type I and Type II Errors 6.5 A Notion of Optimality: The Generalized Likelihood Ratio 6.6 Taking a Second Look at Statistics (Statistical Significance versus â Practicalâ Significance) 7. Inferences Based on the Normal Distribution 7.1 Introduction 7.2 Comparing Y-i?½ s/ vn and Y-i?½ S/ vn 7.3 Deriving the Distribution of Y-i?½ S/ vn 7.4 Drawing Inferences About i?½ 7.5 Drawing Inferences About s2 7.6 Taking a Second Look at Statistics (Type II Error) Appendix 7.A.1 MINITAB Applications Appendix 7.A.2 Some Distribution Results for Y and S2 Appendix 7.A.3 A Proof that the One-Sample t Test is a GLRT Appendix 7.A.4 A Proof of Theorem 7.5.2 8. Types of Data: A Brief Overview 8.1 Introduction 8.2 Classifying Data 8.3 Taking a Second Look at Statistics (Samples Are Not â Validâ !) 9. Two-Sample Inferences 9.1 Introduction 9.2 Testing H0: i?½X =i?½Y 9.3 Testing H0: s2X=s2Yâ The F Test 9.4 Binomial Data: Testing H0: pX = pY 9.5 Confidence Intervals for the Two-Sample Problem 9.6 Taking a Second Look at Statistics (Choosing Samples) Appendix 9.A.1 A Derivation of the Two-Sample t Test (A Proof of Theorem 9.2.2) Appendix 9.A.2 MINITAB Applications 10. Goodness-of-Fit Tests 10.1 Introduction 10.2 The Multinomial Distribution 10.3 Goodness-of-Fit Tests: All Parameters Known 10.4 Goodness-of-Fit Tests: Parameters Unknown 10.5 Contingency Tables 10.6 Taking a Second Look at Statistics (Outliers) Appendix 10.A.1 MINITAB Applications 11. Regression 11.1 Introduction 11.2 The Method of Least Squares 11.3 The Linear Model 11.4 Covariance and Correlation 11.5 The Bivariate Normal Distribution 11.6 Taking a Second Look at Statistics (How Not to Interpret the Sample Correlation Coefficient) Appendix 11.A.1 MINITAB Applications Appendix 11.A.2 A Proof of Theorem 11.3.3 12. The Analysis of Variance 12.1 Introduction 12.2 The F Test 12.3 Multiple Comparisons: Tukeyâ s Method 12.4 Testing Subhypotheses with Contrasts 12.5 Data Transformations 12.6 Taking a Second Look at Statistics (Putting the Subject of Statistics togetherâ the Contributions of Ronald A. Fisher) Appendix 12.A.1 MINITAB Applications Appendix 12.A.2 A Proof of Theorem 12.2.2 Appendix 12.A.3 The Distribution of SSTR/(kâ 1) SSE/(nâ k)When H1 is True 13. Randomized Block Designs 13.1 Introduction 13.2 The F Test for a Randomized Block Design 13.3 The Paired t Test 13.4 Taking a Second Look at Statistics (Choosing between a Two-Sample t Test and a Paired t Test) Appendix 13.A.1 MINITAB Applications 14. Nonparametric Statistics 14.1 Introduction 14.2 The Sign Test 14.3 Wilcoxon Tests 14.4 The Kruskal-Wallis Test 14.5 The Friedman Test 14.6 Testing for Randomness 14.7 Taking a Second Look at Statistics (Comparing Parametric and Nonparametric Procedures) Appendix 14.A.1 MINITAB Applications Appendix: Statistical Tables Answers to Selected Odd-Numbered Questions Bibliography Index