Yong-Ho Park

Effect of MA on microstructure and synthesis path of in-situ TiC reinforced Fe–28at.% Al intermetallic composites

Abstract An in-situ composite consisting of titanium carbide and iron aluminide was synthesized by mechanical alloying of elemental powders and pulse discharge sintering. The microstructure and synthesis path of in-situ TiC reinforced Fe–28at.% Al composite were examined as a function of milling time and heat treatment temperature. Mechanical alloying of the elemental powders promoted bcc solid solution formation, and the solid solution completed at milling of 400 h. During sintering Fe 3 Al and TiC precipitated from the supersaturated Fe solid solution containing Al, Ti and C. Microstructure of the sintered body made of powder milled for 200 h consists of three regions, that is, large TiC particles of about 5 μm in size, Fe 3 Al matrix with TiC particles of submicron size and particle-free Fe 3 Al region. For the sintered body after milling of 400 h, however, the large TiC particles disappeared due to the complete solid solution during mechanical alloying. Sintering of the powders milled for the period shorter than 50 h formed intermediate phases, such as Al 5 Fe 2 , Al 3 Ti and Fe 3 AlC x , by heat treatment up to 1273 K, while that of the powder milled for 400 h showed direct precipitation of Fe 3 Al and TiC from the Fe solid solution.

High temperature oxidation behavior of Ti3Al–Nb alloys prepared by pulse discharge sintering

Abstract Titanium aluminides have received considerable attentions as the basis for new high temperature structural materials. Several compositions based on Ti3Al–Nb have been developed to achieve specific strength and stress rupture properties which exceed those of Ni base alloys such as INCO 718 over temperature range 823–973 K. Recently, pulse discharge sintering (PDS) process technique was successfully developed to manufacture net shape TiAl valves. This research was aimed at investigating the high temperature oxidation of Ti3Al–Nb alloys prepared by PDS. The oxidation rates were measured from 1073–1273 K in air. The kinetics and activation energy for the isothermal oxidation of Ti3Al–Nb alloys revealed that Nb added Ti3Al tended to form finer oxide grain and denser oxide layer and the oxidation rate could be clearly reduced by increasing Nb content (up to 13 at.% in the study). The parabolic rate law was adequate to describe the high temperature oxidation behavior for Ti3Al–Nb alloys in air.

Phase evolution and formation process of compound during ball milling of TiSi powder mixtures

Abstract Titanium and silicon powder mixtures (Ti−3.67−80)at.%Si) were vibratory ball milled to investigate the effect of milling conditions and initial powder compositions on the phase formation during milling. During milling, the temperature of a mill container was measured with a thermocouple attached to the surface of the container. An exothermic temperature spike was observed only when the powder with Ti−40at.%Si composition was milled for 60 h, aged for 48 h and milled again for 32.5 h. During this temperature spike, an amorphous phase which had been formed after the secondary milling time of 32.5 h rapidly changed into a crystalline phase and fine particles were formed. The temperature spike suggests the occurrence of the combustion synthesis reaction during milling. On the contrary, no temperature spike was observed in continuous milling condition.

Application of Ultrasonic Image to the Evaluation of Temperature Distribution in Metal Powder Compacts During Spark Plasma Activated Sintering

Spark Plasma Activated Sintering, SPAS, is a newly developed sintering method for both metal and ceramic powders. An advantage of the SPAS is that a powder can be consolidated in a very short time within several hundreds seconds, because the powder is pressurized in a mold and is heated by direct electric resistance heating of the powder, using pulsative electric current. Another advantage of the SPAS is that the sintering temperature is lower than conventional sintering methods such as hot press and HIP. For instance, a titanium-aluminide powder has to be sintered at 1473K by HIP, while well sintered at 1223K by the SPAS. However, so called “sintering temperature” of the SPAS is usually measured at the mold in place of the powder itself, because direct measurement of the powder’s temperature is much difficult from a technical point of view. It is considered that the powder is heated more rapidly than the mold and therefore the heat flows from the powder to the mold during sintering. Consequently, a temperature distribution is generated in the powder and therefore “true sintering temperature” cannot be clear.