Papers by Keyword: Critical Cooling Rate

Abstract: In the actual industrial production process, it is usually appropriate to reduce the cooling rate and control the residual stress. In this study, the Time-Temperature-Property curve of Al-Mg-Si alloy sheets was measured by interruption quenching and subsequent artificial aging method. The microstructure evolution of Al-Mg-Si alloy was carefully characterized using optical microscopy (OM) and transmission electron microscopy (TEM). It was found that the nose temperature of the TTP curve drawn by experiment was ~360°C, closing to the nose temperature of ~365°C obtained from the simulated TTT curves. The number of equilibrium phase rapidly increased with the increasing of holding time, while no obvious equilibrium phase formation at the low temperature region and high temperature region. The critical cooling rate is 14.3°C/s, the determination of the critical cooling rate has important reference value for the control of alloy sheet during quenching process in the actual industrial production. The quenching sensitive region of the Al-Mg-Si alloy sheet is between 290°C and 440°C.

Abstract: A new bulk amorphous alloy, Y₃₆Nd₂₀Al₂₄Co₂₀, with a diameter of 5 mm was successfully fabricated by the method of equiatomic substitution for the Y element in Y₅₆Al₂₄Co₂₀ amorphous alloy. The values of the supercooled liquid region ∆T_x(=T_x-T_g ), the reduced glass transition temperature T_rg (=T_g/T_l) and the parameter γ (=T_x/(T_g+T_l)) for Y₃₆Nd₂₀Al₂₄Co₂₀ bulk amorphous alloy are 60K, 0.605 and 0.415, respectively. The critical cooling rate of the Y₃₆Nd₂₀Al₂₄Co₂₀ bulk amorphous alloy was determined to be 40 K/s, providing an indication that this alloy has a high glass-forming ability.

Abstract: In this work, the critical cooling rate R_c for glass formation of a series of Fe_80-xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) alloys was determined by means of constructing CCT curves using Uhlmanns method. The calculated critical cooling rates for x = 0, 5, 10, 15, 20 at.% are 621, 441, 548, 894, 922 K/s, respectively. These results well coincide with the maximum diameters of Fe_80-xCo_xP₁₃C₇ amorphous alloys determined by experiments varying with the content of Co. The calculated R_c was also on the reasonable order of magnitudes. In addition, the values of three common GFA criterions of T_rg, ΔT_x and γ were calculated according to the thermodynamic data determined from DSC and DTA curves of Fe_80-xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) bulk amorphous alloy. The validity of these GFA criterions in the series of Fe_80-xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) alloys were investigated and it was pointed out that these three GFA criterions were not able to explain the experimental results of the maximum diameters of Fe_80-xCo_xP₁₃C₇ amorphous alloys varying with the content x of Co.

Abstract: The law of phase change of bearing-B steel during continual cooling was studied by adopting dilatometer. The CCT curves of bearing-B steel were drawn, and the effects of RE on critical cooling rates were studied. The experimental results show that the start temperatures of martensite TM was decreased from 438 to 404°C. The critical cooling rate was simultaneously decreased from 33 to 15°C/s.

Abstract: A new method to evaluate the critical cooling rate, Rc of Fe-based metallic glass alloy was proposed and discussed. [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 alloy particles were prepared with narrow size distribution and high sphericity by Pulsated Orifice Ejection Method in Ar, He and 50%Ar+50%He mixed atmosphere, respectively. Phase transition of a particle from amorphous to amorphous-crystalline and fully crystalline occurred with the increase of particle diameter. Rc of the formation of fully amorphous phase was estimated to be in the range of 700-1100 K/s, lower than that measured by time-temperature transformation diagram of bulk metallic alloy. No change of Rc occurred in Ar, He or 50%Ar+50%He mixed atmosphere, which proved it an effective method to evaluate the critical cooling rate of Fe-based metallic glass alloy.

Abstract: Evaluation of glass-forming ability (GFA) is important in the development of amorphous alloys. Based on phase field theory, the kinetic model of liquid-to-solid phase transition is build, and the time-temperature-transformation (TTT) diagram is plotted according to the phase field simulations of isothermal phase transformation kinetics for a model system. Furthermore, the critical cooling rate for glass formation is calculated on the basis of the TTT curve and is taken as the intrinsic criteria of reflecting the GFA for metallic melts.

Abstract: High-strength sheet including 2MnB5 Boron and magnesium alloy sheet is the material commonly used in modern machinery, which is easy to induce problems such as excessive rebound, cracking, forming force increase, easy mould wear and the like. The heat analysis of 2MnB5 Boron and magnesium alloy sheet hot stamping forming process and experiments indicate that the transition process from Austenite to Martensite by controlling the sheet heating and cooling temperature is the foundation of heat forming. Only when the cooling rate reaches or surpasses the critical cooling rate, Austenite can be transformed to Martensite directly. Critical cooling rate of sheet is related to the elements of critical water flow rate, mould cooling system design, cooling medium, dented mould medium and the like. Under the condition that the elements of mould structure, cooling system, cooling medium and the like are defined, critical cooling rate is a constant value. As a result, through controlling critical water flow rate, hot forming transition process and hot forming requirements can be guaranteed to overcome the excessive rebound, cracking, forming force increase, easy mould wear and the like in hot forming process.

Abstract: Along nucleation → crystalline growth →crystalline fraction → critical cooling rate, the relationship between the nucleation, elements, cooling rate and the Glass Forming Ability of (Cu-Zr) based glass alloys is quantitatively studied with thermodynamics method, and a better method to evaluate the critical cooling rate of glass alloys is also proposed in this paper. The computed results show that: (1) with the increase of element number, the steady state nucleation rate drops gradually. From Cu-Zr, Cu-Zr-Al, Cu-Zr-Al-Ni, to Cu-Zr-Al-Ni-Ti, the peak value of nucleation rate decreases from 10²¹ mol^-1s^-1 to 10¹³ mol^-1s^-1. It is also found the nucleation rate both drops with the substitution of Ni with Cu or Al with Zr; (2) with the increase of cooling rate, the nucleation rate drops sharply. When the cooling rate reaches 10³K/s, the nucleation rates of Cu₆₄Zr₃₆, Cu₅₄Zr_42.5Al_3.5, Cu₅₅Zr₄₀Al₅ and Cu₃₀Zr₅₅Al₁₀Ni₅ drop to 10⁹mol^-1s^-1, 10⁶mol^-1s^-1, 10⁷mol^-1s^-1 and 10³mol^-1s^-1 accordingly.

Abstract: In this study, the multi-component white cast irons with 5wt% of Mo, W, 2 wt% of C, Co each and varying Cr content from 1 to 9 wt% and V content from 3 to 9 wt% independently were employed and effect of Cr and V contents on the behavior of continuous cooling transformation (CCT) was investigated. When the Cr content increases to 5wt%, the critical cooling rate of pearlite transformation (VC-P) does not change much, but the VC-P decreases gradually as the Cr content is increased over 5 wt%. The critical cooling rate of bainite transformation (VC-B) increases with an increase in the Cr content. In the case of V effect, on the other hand, the VC-P decreases as the V content increases. However, the VC-B decreases with an increase in the V content up to 4mass%, but over 4wt%V, it increases, and at 9wt%V, the precipitation of ferrite phase occurred. Ms temperature decreases continuously as the Cr content increases to 5wt%, and then increases. As the V content increases, Ms temperature rises gradually. In the case that Mf point appears, the Mf temperature goes up with an increase in the Cr and V contents.

Abstract: A modified criterion γ’ (=Tx/(1.5Tg+Tl) of glass-forming ability (GFA) for metallic glasses is suggested on the basis of present criterion γ (=Tx/(Tg+Tl) that correlates well with some systems including metallic glasses, some glassy oxides and some cryo-protective aqueous solutions. Results show that the modified criterion γ’ shows stronger correlation with the critical cooling rate Rc for metallic glasses than the criterion γ, which is demonstrated by a value increase in the statistical correlation parameter R2 from 0.9022 to 0.9037. Furthermore, a modified equation is given to reflect the relationship between γ’ and Rc.