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One of the most serious problems in heat resistant alloy steel would be the control of accelerated corrosion caused by using it at high temperature. In this paper were studied chromium diffusion coating (chromizing) by powder method for austenitic heat resistant alloy used for boiler or gas turbine. When Nickel free austenitic heat resistant steel containing cobalt 2% was chromized by a mixture of chromium powder, alumina-powder and ammonium chloride. The thickness of the layer was limited to a small value mainly because of low diffusion constant of chromium in this alloy. The maximum depth of the layer on alloy steel obtained by powder method was 50 microns and surface chromium concentration was 69 wt. % and Activation energy at 1,200℃ was 37.4 Kcal/mole.
Submicron iron powders were produced by reducing ferric chloride(FeCl₃) with hydrogen gas. The dependence of reduction rate on the various parameters were inves-tigated by measuring the conductivity of water through which the reaction gas was passed out. The reduction rate increased with increasing the temperature up to 900℃ and increas-ing both the FeCl₃ partial pressure and H₂: Ar gas ratio up to 3 : 1 The particle size and distribution were affected by all of the above three parameters.
Molybdenum was deposited onto nickel substrate by chemical vapor deposition from MoCl_5 and H₂ gas mixture. Effects of temperature, flow rate of the gas, and MoCl_5 partial pressure on the deposition rate, microhardness, and x-ray pattern were thoroughly investigated. The deposition rate of Mo was found to be zero order reaction and was increased with increasing temperature, flow rate of the gas, and MoCl_5 partial pressure It was apparent that the reaction mechanism changes at 900℃, at temperatures lower than 900℃ the activation energy is 35.8Kcal/mole, whilst at temperatures higher than that is 3.6Kcal/mole. In the region of diffusion control, the deposition rate of Mo was proportional to the square of total flow rate. The microhardness of deposited layer was increased and the preferred orientation(220) distinctively appeared with increaing above three factors, (reaction temperature, flow rate of the gas, and Mocl_5 partial pressure) however, the former was highest and the latter was lowest at 800℃.
Pseudoelasticity behaviour and passivetion treatment on the shape memory alloys are known to have a great deal of influence on the stability of transformation temperature, shape recovery and improvement of corrosion resistance characteristics in their practical applications. The present work was aimed to examine the pseudoelasticity characteristics by tensile test, and corrosion resistance characteristics by corrosion test in Ti-Ni-X(X-Al, Cu) alloys with/ without passivation treatment. The results of this study are as follows, 1. The results of tensile test for Ti-Ni-X(X-Al, Cu) alloys show that the shape recovery is nearly perfect in martensite state and critical stress for slip is increased with increasing strain. Also, the pseudoelasticity behaviour appears in Ti_(50.0)Ni_(40.0)Cu_(10.0) alloy by unloading at temperature above Af and Ti_(50.0)Ni_(49.0)Al_(10.0) alloy by cold working over 25%. 2. The results of the anodic polarization corrosion test show that the corrosion resistance characteristics of Ti-Ni based alloys can be increased by application of passivation treatment and Ti-Ni-X(X=Al, Cu) alloys has better corrosion resistance characteristics than Ti-Ni equiatomic composition alloy.
The effects of indium on the phase transformation of 2090 Al-Li alloys has been studied with a differential scanning calorimeter (DSC), a transmission electron microscope(TEM), and a micro-hardness tester at the temperature ranges of 130℃∼220℃. The results show that indium addition to the 2090 Al-Li alloy retards the formation of prior precipitate(precursor) and δ´phase, whereas it accelerates the formation of T₁ and θ´(especially θ´phase). Above 190℃, the formation of δ´phases is retarted than those at 130℃ and at 160℃. The addition of 0.15wt% indium increases the hardness at the early stage of aging. The cause of this effect is attributed to the formation of GP zone. The main phases, in peak aged condition of indium free 2090 alloys are δ´and T₁; indium bearing 2090 alloys are θ´and T₁ (especially θ´Phases).
Martensitic transformation and effects of thermal fatigue cycles on the transfer mation temperature and shape recovery of Ti-Ni-Cu alloys were investigated by means of electrical resistance measurement, thermal fatigue tests and transmission electron micros copy. Thermo-mechanical treatment at 600℃ for one hour after cold rolling showed the best shape recovery effect irrespective of Cu concentration. Substitution of a small amount of Cu for Ni in the equiatomic composition of Ti-Ni showed folling results ; Ms-temperature of the Ti-Ni-Cu alloy increased slightly with the increase of Cu content, The range of transformation temperature for Ti_(50)Ni_(40)Cu_(10) alloy(i.e., T_(Mf-Af)) was reduced to 7℃. After stress relief treatment at 750 ℃ for one hour, the Ti_(50)Ni_(45)Cu_5 alloy showed good two-way memory effect. Change of the M_s temperature was observed to a certain number of thermal cycles, and it became nearly constant after a critical number of thermal fatigue cycles.
The effects of low temperature aging on the impact toughness and mechanical properties of Al-1.9%Li-3.0%Cu-0.15%Zr alloy have been investigated. It shows that the impact absorption energy and elongation are increased in the order of over-aged, peak-aged and under-aged specimen at the various aging temperature, and these values in the specimen peak-aged at lower aging temperature are increased. In the specimens aged at 120℃ and 140℃, dimples are observed in the matrix and around the grain boundary. Intergranular fracture has been observed noticeably in the 160℃ peak aged specimen. These results are attributed to the microstructural changes by the low temperature aging treatment that forms more dense and homogeneous distribution of T₁ phases on the dislocations within the matrix and more narrow precipitate free zone in the grain boundary at 120℃ and 140℃ than at 160℃.
The effect of cold working on the aging behaviors and mechanical Properties of the Al-2Li-3Cu-0.12Zr alloy have been studied. This results show that the formation of T₁ and θ′ phases are accelerated by introduction of 5% cold rolling after as-quenched state two step aging treatment is most effective in aging response and impact energy increase among tested conditions. This results are attributed to the two step aging treatment which has the more dense and homogeneous distribution of strengthening phases such as T₁ and θ′. It is thought to be due to the participation of lithium to the formation of T₁ phase in early stage of aging treatment.
The shape memory characteristics in TiNi alloys are greatly effected by the alloy composition and heat treatment condition. The present work was aimed to investigate the effect of Pd_x(X=5, 10, 15, 20) addition on the shape memory characteristics of TiNi alloys by means of eletrical resistance measurement, X-ray diffraction, differential scanning calorimetry and electron dispersive analysis X-ray measurement. The results obtained from this study are as follows ; 1. The martensitic transformation start temperature, Ms of Ti-Ni_(50-x)-Pd_x alloys decreased considerably with the increase of Pd content up to 10at%, whereas increased largely with the increase of Pd content in the alloys with Pd content more than 15at%. 2. The Ms temperature of Ti-Ni_(50-x)-Pd_x alloys with cold working was significantly lower than that, of the fully annealed alloys because high density dislocation has been introduced by the cold working which suppressed the martensitic transformation.
The p-type thermoelectric powders of 25%Bi₂Te₃-75%Sb₂Te₃ doped with 4wt%Te were prepared by high pressure gas atomizer. The new process with rapid solidification followed by pressing and sintering was investigated to produce homogeneous material. Characteristics of the material were examined with XRD, DSC, SEM, EPMA-line scan and bending test. Property variations of the materials were investigated as a function of variables, such as powder size, sintering temperature and annealing effect. Quenched powders and sintered materials consist of Bi₂Te₃ type of single phase. When the process parameters were optimized, the maximum figure of merit was 2.78×10^(-3)K^(-1). The bending strength of the material produced in this work was 4.68 ㎏f/㎟.