Papers by Keyword: Laves Phase

Abstract: Effect of rare earth (RE) on creep rupture behavior of 316LN austenitic stainless steel (316LN steel) was investigated after crept at 700°C under the stress in the range from 125MPa to 200MPa, by the optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show RE addition in 316LN steel increased the creep rupture ductility at high stress, but reduced the creep rupture ductility at low stress. Under 200MPa, RE addition increased the creep rupture strain of 316LN steel from 0.558 to 0.787 but the creep rupture strain after crept under 150MPa was decreased from 0.875 to 0.566. The fracture mode of 316LN steel was also apparently impacted by the RE addition. The typical ductile fracture feature of homogeneous dense dimples was obviously observed in NRE steel after crept rupture under all stresses. While in 32RE steel, small amount of intergranular fracture fractographs under low stress appeared instead of partial dimples under high stress. Moreover, it is noted that RE addition in 316LN steel promotes to precipitate a great number of fine Laves particles within grains. These Laves particles strengthening the matrix resulted in the strain concentration on grain boundaries, which might sensitively induce crack initiation on grain boundaries during long-term creep under the low stress.

Abstract: The thermodynamic calculation software Thermo-Calc was used to study the influences of alloy elements Cr, Co, and Mo on the precipitated phases of a S280 ultrahigh strength stainless steel at the temperatures ranging from 400 °C to 1200 °C. The results showed that the precipitated phases in the steel were mainly composed of M₂₃C₆ carbide, M₆C carbide, intermetallic compound of Laves phase and σ phase. The temperature and the content change of alloying elements Cr, Co and Mo have little effect on the precipitation of M₂₃C₆ carbide, while have great effect on the precipitation of Laves phase and σ phase. By lowering the element content and adjusting the heat treatment temperature, the precipitation of σ phase in the alloy can be reduced．According to the results of thermodynamic simulation, after optimizing the chemical composition, the Ф300mm bar was trial-produced. The chemical composition, microstructure and mechanical properties of the alloy were analyzed and tested. The results show that, after tempering at 550 °C, the main precipitated phases in the S280 alloy were M₂₃C₆ carbide and intermetallic compound of Laves phase. The characterized nano-precipitates have good agreement with simulation results. The optimized design of S280 steel reached a tensile strength over 1930 MPa and a fracture toughness of 90 MPa•m^1/2.

Abstract: The multicomponent Tb_0.2Dy_0.8-xGd_xCo₂ and Tb_0.2Dy_0.8-xGd_xCo_0.9Al_0.1 alloys (x≤0.5) were studied in a large temperature range (80 – 350 K) and fields up to 1.8 T. Temperature dependencies of lattices parameters, surface topology features, Curie temperature and magnetocaloric effect near it, of these polycrystalline cubic Laves phase alloys have been obtained and analyzed. The effect of Gd and Al substitution within the rare earth and cobalt sublattices on the structural and magnetocaloric properties of Tb_0.2Dy_0.8Co₂ has been discussed.

Abstract: Magnesium-zinc based materials are characteristic with the creation of intermetallic phases, strongly influencing material mechanical properties. Mg-Zn powder mixture (10 % wt. Zn) was processed by the hot pressing method under 500 MPa at 300 °C. Microstructure of the prepared material was analyzed in terms of light optical microscopy and scanning electron microscopy. Chemical and phase composition of the processed material were analyzed by energy-dispersive X-ray spectroscopy and X-ray powder diffraction, respectively. Microhardness testing was adopted to characterize created structure mechanical properties on the microscopic level. Depending on the Mg-Zn powder mixture local chemical composition, the structural and chemical analysis of the processed material revealed that it consisted of magnesium and zinc rich areas, and MgZn₂ intermetallic phase. The MgZn₂ intermetallic phase belongs to the so-called Laves phases group with the general formula AB₂. Laves phases are characteristic with high hardness and the related high brittleness. Their presence in the material usually results in deterioration of mechanical properties such as strength and toughness. The microhardness of magnesium and zinc rich areas in the processed material was 58±1 HV 0.025 and 47 ±1 HV 0.025, respectively, while the value of the microhardness for MgZn₂ intermetallic phase was 323±12 HV 0.025. Different behavior and mechanical properties of the present phases was observed on the fracture surfaces of specimens broken during the 3-point bend test. While brittle fracture was a characteristic feature for MgZn₂ intermetallic phase, the rest of the material exhibited more ductile fracture behavior with characteristic transgranular failure.

Abstract: Microstructural evolution in a 9Cr-3Co-3W-0.2V-0.06Nb-0.05N-0.005B steel crept at T=650°C under an applied stress of 140 MPa up to strains of 1, 3, 5.75 and 12%, which represent primary, secondary and tertiary creep stages and rupture, respectively, was studied. The steel was initially normalized from 1050°C, and finally tempered at 750°C for 3h. After tempering the boundaries of tempered martensite lath structure (TMLS) were decorated by M₂₃C₆ carbides, M₆C carbides and Laves phase particles. The 3% W additives provide the narrow size distribution of the boundary particles excepting M₆C carbides. The depletion of thermodynamically none-equilibrium content of W from the solid solution during creep leads to following events. (i) Continuous precipitation of small Laves phase particles occurs during all creep stages and results in the formation of bimodal size distribution. As a result, the average size of Laves phase particles remains unchanged during creep. (ii) Coarsening of M₂₃C₆ carbides starts to occur only at the transition to tertiary creep. (iii) Transformation of laths to subgrains followed by their growth is observed during all stages of creep. The density of particle located at lath/subgrain boundaries decreases from 5.6 to 2.6 μm^-1 during creep up to rupture. However, no full transformation of TMLS into subgrain structure has been revealed.

Abstract: Advanced tungsten modified 9%Cr ferritic steel (ASTM Grade P92) is a promising material for the next generation of fossil and nuclear power plants. Unfortunately, there are rather few published reports on damage processes in P92 steel during high temperature creep and the effect of damage evolution on the creep strength is not fully understood. In this work, the creep behaviour of P92 steel in as-received condition and after long-term isothermal ageing was investigated at 600 and 650°C using uniaxial tension creep tests. To quantify the effect of each damage process on the loss of creep strength, most of creep tests were followed by microstructural and fractographic investigations. It was found that the large Laves phase particles, which coarsened during creep exposure, served as preferential sites for creep cavity nucleation.

Abstract: The effects of Co from 0 to 11.60 % (in mass fraction) on the solidification and precipitation behaviors of IN 718 alloy had been investigated. The results showed that the volume fraction of the dendrite core increased with the addition of Co. In the alloys with 0-5.84 %Co, the addition of Co could restrain the precipitation of blocky Laves phase and promoted the formation of eutectic Laves phase. In the alloys with 9.00-11.60 % Co, the eutectic gray phase and small blocky Laves phase precipitated in the interdendritic region. The eutectic gray phase increased and small blocky Laves phase decreased with increasing Co. The parallel lath-like δ-Ni₃Nb phase was observed to precipitate in some interdendritic region without the formation of gray phase and Laves phase in the 9.00-11.60 % Co alloys. Further research found that Co slightly segregated in the dendrite core and markedly raised the solubility of element Mo in the dendrite core which resulted in reduced Mo in the residual liquid, and consequently, restrained Laves phase while promoted the precipitation of Mo-depleted gray phase and δ-Ni₃Nb phase. Furthermore, Co was seemed to elevate the solidification point of the γ matrix while decrease that of the Laves phase.

Abstract: High energy ball mill tests under the condition of the ball material mass ratio 13:1 and the rotate speed 400 r/min have been employed to investigate the process of mechanical alloying (MA) of Ta and Cr powder mixed in the mole ratio of 1:2.The microstructure evolution process and phase composition were explained useing scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that, the milled 20h powder existed in Ta (Cr) supersaturated solid solution and amorphous after 40h. Although the hours were spent on ball milling reached to 50h, Laves phase TaCr₂ had not been made during the process.

Abstract: The relationship between creep rupture strength and Laves phase precipitation and growth kinetics was investigated at 650 °C for two Fe-9Cr-3Co (wt.%) alloys differing mainly in the amounts of W and Mo added. In the alloy with 3.14 wt.% W added, Laves phase precipitated heterogeneously on grain boundaries and hence had little dispersion strengthening effect. Its stress exponent for rup-ture time became lower in the lower creep stress range tested. In the alloy with 1.31 wt.% W and 3.22 Mo added, Laves phase precipitated both heterogeneously on grain boundaries and homogenously within grains and there was no reduction in stress exponent for rupture time in the whole stress range tested. The Lave phase precipitation kinetics increased with increasing the total amount of W and Mo in the alloys. The differences in stress-rupture time relationship observed between the two alloys were discussed in relation to their differences in the Lave phase precipitation behaviour.

Abstract: The microstructure of as-cast 718 alloy after modified with Mo has been investigated by means of optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). It was found that the dendrite arm space was widened and the interdendritic area was reduced by increasing Mo addition. And no new phase was precipitated in the modified alloys by Mo addition in the range of the present test. The addition of Mo promoted the precipitated of Laves phase, restrained the formation of δ phase, and elevated melting point of the Laves phase.