Jo Dec 3, 2022
Ti2AlNb‒based alloys, so-called ordered orthorhombic (O (Cmcm)) phase alloys, have a chemical composition of Ti ‒ (18 ‒ 30) Al ‒ (12.5 ‒ 30) Nb (mole fraction, %). Since the discovery of orthorhombic (O) phase, Ti2AlNb‒based alloys have received special attention as a promising candidate for advanced aerospace and automotive application due to their high specific strength, excellent creep and oxidation resistance at elevated temperatures as well as good workability.
Up to now, many studies on the fabrication of Ti2AlNb-based alloys by casting, rolling and forging have been conducted, in which the major focus was set on the elimination of disadvantages such as microstructure segregation and inhomogeneity. It is because the thermodynamic properties of Ti, Al and Nb elements including a melting point, a density, a diffusion coefficient, etc. are very different.
Recently, powder metallurgy (PM) method such as spark plasma sintering (SPS) has been applied to the fabrication of Ti2AlNb‒based alloys, which made it possible to obtain fine and homogeneous microstructure. Several endeavors have been devoted to preparing PM Ti2AlNb‒based alloys from pre‒alloyed powder and elemental powders by vacuum hot pressing, and from pre‒alloyed powder by hot isostatic pressing (HIP). However, these as‒sintered compacts showed the microstructure with coarse grain size (>40μm), due to higher sintering temperature and longer sintering time. Therefore, the strength of these Ti2AlNb‒based alloys was not so high.
In recent years, several studies on the fabrication of high strength and ductility titanium alloys with nanostructured (NS) and ultrafine grained (UFG) microstructure have been reported. One process to achieve this microstructure consists of the preparation of NS or UFG powder by high energy ball milling (HEBM) and the consolidation of the powder into dense compacts by PM methods such as HIP and SPS. For instance, using the high energy ball-milled powder as a starting material, an ultrafine grained Ti‒6Al‒4V with high mechanical properties can be obtained.
In order to produce a high strength and ductility Ti‒22Al‒25Nb alloy from pre alloyed powder, Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has introduced HEBM with subsequent SPS. The process is as follows. Firstly, the Ti‒22Al‒25Nb pre‒alloyed powder with a weight of 75g was put in a stainless steel vial with bearing steel balls. HEBM was carried out in a high-energy planetary ball mill (QM-3SP4) under a high purity argon atmosphere for 20h at a rotation speed of 300 rpm. The HEBMed powder was directly loaded into a high-strength graphite mould, and subsequently consolidated by a LHPD250 SPS apparatus (FCT Co. Ldt., Germany) at SPS temperatures of 950℃ for SPS time of 10 min. After sintering, the sintered compact was cooled to the room temperature (RT) in the furnace. Finally, a sintered compact, with dimensions of φ50 × 15 mm, was obtained.
The Ti‒22Al‒25Nb alloy sintered at 950℃ for 10 min under the pressure of 50 MPa from the 20 h‒HEBMed powder showed the microstructure with a large amount of UFG/NS O-phase. The yield strength, tensile strength and elongation to failure were 1 092 MPa, 1 105 MPa and 9.4%, respectively. Compared with the referenced Ti‒22Al‒25Nb alloys, the Ti‒22Al‒25Nb alloy fabricated by HEBM and subsequent SPS exhibited higher strength and good ductility at the room temperature and high temperature. This indicates that HEBM with subsequent SPS is feasible for PM Ti‒22Al‒25Nb alloy with enhanced tensile properties.
More information about this is found in his paper “Fabrication of a high strength and ductility Ti‒22Al‒25Nb alloy from high energy ball-milled powder by spark plasma sintering” published in the SCI Journal “Journal of Alloys and Compounds”.
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Jo Dec 2, 2022
Ho Yong Ju, a researcher at the Faculty of Heat Engineering, introduced virtual reality technology to the practice at thermoelectric power plants, thus raising remarkably the teaching density and the cognition effect among his students.
The effectiveness is as follows.
First, as they can control any equipment in a virtual control room, they are able to notice the changes in the parameters of the plant and it becomes easier for them to be familiar with methods and characteristics of operation control.
Second, it saves time and practice procedures since all students can learn before necessary equipment in one room at the same time. In the past, it took a lot of time as the production site could hold only a few students in front of the machines at a time.
Third, it is possible for students to look closely at the structure and operation of all machines in a virtual situation. It leads to the raised efficiency of the practice.
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Jo Dec 1, 2022
Ryo Ok (5th century B.C ~ 4th century B.C.), a woman musician in the latter period of Ancient Korea, was a songwriter and an instrumentalist. She was one of the talented folk musicians. She was famous for writing and playing a lyric song “Konghuin” on a small konghu (a kind of traditional Korean stringed instrument) with 13 strings, while living with her husband near the ferry of the River Taedong in Pyongyang, the capital of Ancient Korea. Despite being poverty-stricken, she loved music so much that she always took pleasure in playing the konghu.
How “Konghuin” was written and what it is about are found in a number of books at home and abroad including “Kogumju”, “Haedongyoksa”, “Osansollim”, “Yolhailgi”, “Akso”, etc. One day Ryo Ok was told about a pathetic old man from her husband, who had returned from all day’s work on the ferry dock of the River Taedong. When the old man threw himself away into the River Taedong, his wife improvised a song playing the konghu with grief over his death. Feeling pity for the old couple that was drowned in the river, Ryo Ok, with the konghu in her bosom, composed “Konghuin” picturing in her mind the old grief-stricken lady playing the konghu. Ryo Ok enjoyed singing it along to the konghu in the moonlight or at sunrise and she taught it to her neighboring pupil Ryo Yong. From then, the song was spread across the country and even to the neighboring countries under the title of “Konghuin”.
The fact that many poets and writers in many generations have taken great interest in the song “Konghuin” and that different opinions of it are given in several documents gives a clear proof of the contributions of the ancient art of our country made to the development of the eastern art.
Ri Yong Ho, section head at the Academy of Social Sciences
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Jo Nov 28, 2022
The research team led by Kim Hyok Chol, an institute head at the Faculty of Electrical Engineering, and Kim Nam Chol, a researcher at the Faculty of Mechanical Science and Technology, who designed and manufactured our style two-way high-speed circuit breaker for underground electric vehicles, is working for further achievements.
A two-way high-speed circuit breaker was designed and fabricated by combining a digital cut-off controller.
The device can achieve reliable high-speed cutoff even under severe operating conditions of underground electric vehicles. Accurate cutoff setting current and low mass and volume are other advantages of this device. In particular, successful bidirectional blocking is guaranteed.
The high-speed circuit breaker consists of a maintenance and release device for high-speed interruption, a blanking room, a locking control unit, and a contact system, which can sense both the current entering the electric vehicle and the current entering the grid during regenerative braking, so that the circuit can be shut down within 18 ms once an accident occurs.
The shutoff coil of the hold and trip device has no vibration-induced malfunction because there is no current flow during normal operation, and it takes less than 1ms to output a shutoff signal immediately after the current-cut signal from the current sensor enters the shutoff control device.
The turn-off controller has a double current sensing and a circuit to turn off analog and digital double, and the current setting has been simplified on a button.
The contactor is a two-point open type with an inclination angle of 45° and there is an arc-extinguishing component and a main contractor at each contactor. There is little electrical wear at the main contactor because there is no structural repulsion during turn-on.
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Jo Nov 27, 2022
Pyongyang, a place of exquisite beauty, has a long history of 5 000 years.
Pyongyang was the capital of Ancient Korea founded by Tangun, the founding father of the Korean nation. Born in Pyongyang, Tangun founded a country and set up the capital in Pyongyang in the early 30th century B.C. He named it Joson, which meant a country with the sunrise in the east.
According to several old books including “Samguksagi” (Chronicles of the Three Kingdoms), “History of Koryo” and “Sejong Sillok” (Chronicles of King Sejong), Tangun held the court in Pyongyang and named the country Joson, and Pyongyang is a city with a long history which the founder king of our countrymen chose as a capital. Furthermore, a lot of names of places associated with the birth and growth of Tangun are used and the Mausoleum of King Tangun was discovered in Kangdong, Pyongyang. These prove that Pyongyang was King Tangun’s birthplace and the capital of the country.
The name “Pyongyang” is the translation in Chinese characters of the Korean native word “Puruna”, where puru referred to a large field or a walled city and na meant a place. In a word, “Puruna” means “a plain or land of fields”. The area around Pyongyang had a lot of fertile fields and a river, so it was easy of access. At that time, “Puruna” was understood as a large walled city or the capital. Pyongyang was also called other names like “Phyongchon” or “Phyongna” as a result of different translations of the same Korean word “puruna” in Chinese characters.
It is recorded in historical books “Samguksagi” and “History of Koryo” that in the period of Ancient Korea Pyongyang was also called “Wanggom Walled City”, which meant a capital city chosen by the founder king.
In the period of the Three Kingdoms, too, Pyongyang was in the position of a capital for both its natural and geographical conditions and economic and cultural conditions. The reason that Koguryo chose Pyongyang as a provisional capital in 247 and as a real capital in 427 lies in the fact that Pyongyang had enough conditions as well as a long history to be a capital.
During the Koryo dynasty, Pyongyang had other names “Sogyong” or “Sodo”, both of which meant a capital in the west. In fact, Pyongyang was not the formal capital of Koryo, but it was given as much importance as the capital. Wanggon, who founded the first unified state in 918, was planning to move the capital to Pyongyang when he recognized that Pyongyang was more important than Kaegyong in the aspects of geographical position and economical and cultural conditions.
During the feudal Joson dynasty, Pyongyang was called yet other names “Sogyong” or “Ryugyong”.
Pyongyang was famous for its scenic beauty with willows and flowers. In particular, the view of drooping branches of willows and all sorts of flowers in bloom in early spring was spectacular. Therefore, people called Pyongyang “Ryugyong”, which is translated as a capital with a lot of willows.
Pyongyang was still given importance during this period, when it was the centre of feudal rule, commodity exchange, distribution of agricultural products and feudal handicrafts.
As mentioned above, Pyongyang has been the capital or a city in the same position as a capital for as long as 5 000 years. Today, the Korean people are proud of and ardently love Pyongyang, the capital city with a long history of fifty centuries.
Kim Kwang Jo, researcher at the Academy of Social Sciences
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