Effect of phase change from nonequilibrium to equilibrium phase on densification process and formation of nanocrystallin material

In this investigation，densification behavior of alloyed powder having non-equilibrium phase was discussed。Especially，effect of phase change from nonequilibrium phase to equilibrium phase on densification process of pressure sintering.The alloy composition of powder used here was Ti-37.5mol%Si，that was obtained by using mechanical alloying technique.Long time milling more than 540 ks made the mixture of the powder amorphous state.3600ks milled amorphous powder was used.In addition to the amorphous powder，lamellar structure(Ti+Si)powder milled for 360 ks was also used as to compare the densification behavior of amorphous powder.At first，phase formation characteristics of amorphous and lamellar powder were discussed。Based on the evidence of phase change to stable intermetallic compounds，densification behavior of MAed powder was investigated using vacuum hot pressing(VHP)method。Before heating of VHP，hot pressing pressure(10-200 MPa)was applied.With keeping the pres…More sure constant，pressed powder was heated at the rate of 5-20K/min up to 1273K，then kept for 10.8ks。MA powders consisting amorphous phase and(Ti+Si)lamellar structure showed extraordinary densification during phase formation period that was measured by DTA analysis.The following effects were examined to make clear the densification behavior.Those are(1)Exothermic heating during phase formation，(2)Powder re-arrangement，(3)Thermal expansion of compacts having nonequilibrium phases，(4)Pressure and heating rate dependence on densification。It is concluded that the enhanced densification during phase formation period was due to the enhanced deformation of powder particle.After the phase change from nonequilibrium to stable phase，in the case of amorphous powder，densification ceased foray little while due to the formation of Ti_5Si_3intermetallic compound up to 1050K.On the other hand，lamellar powder having reduced Ti phase kept to increase density.When the temperature reached 1050K，densification restarted again。It is thought that the densification progressed at these higher temperature over 1050K by the mechanism of grain boundary sliding(Fine Microstructure Superplasticity)due to the nano size microstructure about 100nm.The VHP compact made from amorphous powder showed nano size microstructure expressed hardness of HV1336 and fracture toughness K_&lt；Ic&gt；=1.84 MPa·m&lt；0.5&gt；.Less：Less

In this investigation, densification behavior of alloyed powder having non-equilibrium phase was discussed. Especially, effect of phase change from nonequilibrium phase to equilibrium phase on densification process of pressure sintering. The alloy composition of powder used here was Ti-37.5 mol %Si, that was obtained by using mechanical alloying technique. Long time milling more than 540 ks made the mixture of the powder amorphous state. 3600 ks milled amorphous powder was used. In addition to the amorphous powder, lamellar structure (Ti + Si) powder milled for 360 ks was also used as to compare the densification behavior of amorphous powder.At first, phase formation characteristics of amorphous and lamellar powder were discussed. Based on the evidence of phase change to stable intermetallic compounds, densification behavior of MAed powder was investigated using vacuum hot pressing (VHP) method. Before heating of VHP, hot pressing pressure (10-200MPa) was applied. With keeping the pres … More sure constant, pressed powder was heated at the rate of 5-20 K/min up to 1273 K, then kept for 10.8 ks. MA powders consisting amorphous phase and (Ti + Si) lamellar structure showed extraordinary densification during phase formation period that was measured by DTA analysis. The following effects were examined to make clear the densification behavior. Those are (1) Exothermic heating during phase formation, (2) Powder re-arrangement, (3) Thermal expansion of compacts having nonequilibrium phases, (4) Pressure and heating rate dependence on densification. It is concluded that the enhanced densification during phase formation period was due to the enhanced deformation of powder particle.After the phase change from nonequilibrium to stable phase, in the case of amorphous powder, densification ceased foray little while due to the formation of Ti_5Si_3 intermetallic compound up to 1050 K. On the other hand, lamellar powder having reduced Ti phase kept to increase density. When the temperature reached 1050K, densification restarted again. It is thought that the densification progressed at these higher temperature over 1050 K by the mechanism of grain boundary sliding (fine microstructure superplasticity) due to the nano size microstructure about 100 nm.The VHP compact made from amorphous powder showed nano size microstructure expressed hardness of HV1336 and fracture toughness K_<Ic> = 1.84MPa・m^<0.5>. Less