발간논문

Home > KJMM 논문 > 발간논문

The microstructure and properties of the rapidly solidified Al-8Fe-2Mo-1.5Si alloy for elevated temperature applications were investigated. The effects of Mo on thermal stability and Si on fracture toughness through the observation of microstructure, hardness test and tensile test were described. The mechanical properties of extrudates with the variation of extrusion ratio and size distribution of powder were investigated. Hardness of smaller powder(50㎛) was superior to that of larger powder(150㎛). The extrudate consolidated with finer powder size distribution (10∼100㎛) yields higher strength than that with a coarse size distribution(100∼200㎛). The tensile strength of 2nd extruded bar which was consolidated with finer powders was 520MPa at 25℃, 310MPa at 275℃. The deterioration of mechanical properties at elevated temperature was due to the decomposition of Zone A and metastable phase Al_6Fe into stable phase Al₃Fe and the growth of needle-like Al₃Fe precipitates. Quarternary Mo and Si containing alloys showed uniform dispersion of fine cuboidal-polygonal precipitates, bcc Al_(12)(Fe,Mo)₃Si phase which displayed a remarkable resistance to coarsening.

keyword :

Residual stresses of ZrO₂/AISI304 joint were determined by measuring the diffracted X-ray peak shift with an incident angle Ψ, sin²Ψ method and the results were compared with that of Finite Element Method (FEM). In X-ray diffraction method, radial stress in ceramic was mainly compressive decreasing with the distance from the center to edge. And the radial stress in stainless steel was also compressive unexpectedly. It`s because that tensile stress developed at A12024/AISI304 interface transformed rapidly into compressive component as distance from the interface increased, and it was confirmed in FEM results. In FEM calculation, high tensile axial stress was found to develop in ZrO₂edge close to interface and also in interlayer. In this study the fracture of joint occurred in the interlayer and it supported the FEM result that high axial stress was developed in the interlayer. There was qualitative agreement between analytical estimation and experimental result and the X-ray diffraction method with the FEM analysis seems to be effective to determine the residual stress in metal/ceramic joint.

keyword :

The effects of both a pre-ageing stretch and duplex ageing with pre-ageing stretch on strength and fracture toughness in the Al-Li-Cu-Zr alloy 8090 were observed. Both stretching prior to ageing and duplex ageing with pre-ageing stretch were found to effectively homogenize the distribution of S` and to increase the strength in this alloy. The decrease in toughness as the strength increases is little affected by the size of δ` and S` phases, but is primarily caused by the increase of yield stress arising from the prior stretch. The particle sizes of the δ` phase formed as a result of both the pre-ageing stretch and the duplex ageing treatment with pre-ageing stretch are only slightly dependent upon the change of deformation and the pre-ageing treatment, but is primarily dependent on the final ageing temperature.

keyword :

Amorphous ribbons of composition Fe_(73.5)Si_(13.5)B_9Cu₁Nb₃have been annealed above their crystallization temperature. When α-Fe is fully crystallized, a nanocrystalline material which has superior soft magnetic properties is obtained. To explain the variation of the magnetic properties of Fe_(73.5)Si_(13.5)B_9Cu₁Nb₃alloy during crystallization, a new method using DSC residual peak was contrived in this paper. Using this method, a relation between magnetic permeability(μ), coercivity(H_c), flux density(B) and the crystallized volume fraction of α-Fe was investigated. Also, the microstructural change of Fe_(73.5)Si_(13.5)B_9Cu₁Nb₃alloy with the variation of the crystallized volume fraction of α-Fe was investigated using Transmission Electron Microscopy(T.E.M.).

keyword :

High temperature strength of heat resistant Mo-base allay such as TZM and TZC, was considerably improved due to the effect of TiC, ZrC precipitate hardening. Since HfC is much more stable than TiC and ZrC, development of HfC strengthened Mo-base alloy has already been under way. In this study, the Mo-Hf-C alloys containing 0.31∼1.14 at% Hf and 0.08∼1.00 at% C were prepared by plasma arc melting. The recrystallization behavior was examined upon the amounts of added Hf and C. The precipitates which influence the recrystallization behavior were analyzed by TEM observation. HfC precipitated when annealed sheet was heat treated below 1600℃. When the sample was heat treated above 1600℃, HfO₂ precipitated instead of HfC. As the contents of Hf and C increased, 50%-1hr recrystallization temperature increased from 1400℃ to 1700℃.

keyword :

A new diffusion bonding method (named as Melting Induced Diffusion(MID) bonding) which aimed to make better Transient Liquid Phase(TLP) bonding was developed and applied to the bonding of 304/304 stainless steel. In this research boron powder was used as a insert metal. The width of reacted layer of the bonded joint had linear relationship with square root of bonding time. And the bonded ratio of the joint increased with bonding time. As a result of tension test, tensile strength far room temperature were about 200㎫.

keyword :

The Si₃N₄/STS304 joints vacuum-brazed with four types of Cu-Ag-Ti brazing alloy was shearing-tested. For the joints showing interfacial fracture the position, morphology and microchemistry of reaction products on the fracture surface of Si₃N₄- side was investigated by scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS). Types and fratures of interfacial reaction products observed on the fracture surface are following : first, titanium silicide that is a layer structure and contains the network-shaped microcrack ; second, iron titanium silicide that is a granular (grain size is about several hundred nanometer in dia.) and layer structure ; third, Fe-Ti compounds can be classified into Fe-Ti-Si-Cr and Fe-Ti-Cr systems, the former is the angular-type grains (of several micrometer in size) and the latter is a compound showing the brittle cleavage fracture ; fourth, Cu-Ti-Fe compound that is a granular (grain size is about several hundred nanometer in dia.) and layer structure. All the compound consisted of a granular structure showed a intergranular fracture. Interfacial fracture of Si₃N₄/STS 304 joints is classified into three types as following ; first, seperation (fracture) at the interface between Si₃N₄and titanium silicide ; second, complex fracture varying the path through each interface between reaction products ; third, a part of fracture occur in the second form, but a large part of fracture occur through the inner of reaction products. The shear strength of Si₃N₄/STS304 joints showing three types of interfacial fracture is below 42 ㎫, average 180 and 202 ㎫ respectively. Therefore, the more the strength of joints is high, the more fracture occur through the inner of reaction products than at the interface between reaction products.

keyword :

The effect of heating rate on the combustion temperature and microstructure of products formed by combustion synthesis was investigated. Combustion synthesis did not occur at the heating rate below 1℃ /min. Ignition temperature is nearly constant at the heating rate between 2℃/min and 60℃/min and then increased with heating rate. Both temperature rise caused by exothermic raction and the density of combustion synthesized product increased with increasing heating rate up to about 10℃/min. and then remained nearly constant. Below the heating rate of 10℃/min, Al₃Ni, Al₃Ni₂NiAl intermetallic compund is formed and Ni, Al remain, but over the heating rate, Ni₃Al intermetaltic compound is well obtained.

keyword :

The effect of green Compact porosity on the combustion temperature and microstructure of the product formed by combustion synthesis was investigated. The Combustion temperature of the reactant first increased up to about 33% porosity and then decreased. The dendritic morphology of Ni₃Al intermetallic Compound was observed in the specimen produced with initial porosity of 33%, whereas typical polygonal grain structure was observed in specimen produced with initial porosity of 16%. The porosity and grain size of Ni₃Al product obtained with heating rate of 600℃/min in Ar atmosphere could be decreased by an application of uniaxial pressure during combustion. (Received October 19, 1992)

keyword :

Cementation was investigated to recover gold and silver from the acidic thiourea solution with zinc, aluminum and iron powders. The results showed that the zinc powder was not of practical use in acidic solution because of the great consumption of zinc and thiourea decomposition, while aluminum and iron powders were substantially effective in practical aspects in comparison with zinc. Iron was found to be the most effective precipitant due to its little consumption and low cost. The dissolved oxygen becomes detrimental to cementation of precious metals in acidic thiourea solution. When the oxygen was expelled by introducing nitrogen gas, the fast cementation was carried out effectively with the decrease of metal consumption as cementing agents. In the cementation with iron powder, the activation energy of gold and silver were 4.4 ㎉/㏖ and 13.1 ㎉/㏖, respectively. Accordingly the reaction was controlled by mass transfer for the cementation of gold and by surface chemical reaction for the cementation of silver.

keyword :