Determination of equilibrium solid-phase transition temperature using DTA

A theoretical model concerning the influence of heating/cooling rate on the solid-phase transition temperature is developed. The equilibrium solid-phase transition temperature is obtained y extrapolating the heating/cooling rate to 0 °C/min. It is supposed that the solid-phase transition is controlled by the Gibbs free energy and the diffusivity of elements involved in the transition. Comparison with differential thermal analysis (DTA) results of Zr and Ti alloys shows good agreement.     

Thermography of the phase transformations in a hypoeutectic Al-7% Si-0.5% Mg silumin at high pressures and temperatures

The phase equilibria in a hypoeutectic AK7 aluminum alloy (Al-7% Si-0.5% Mg silumin) are studied by differential barothermal analysis in the pressure range 5–131 MPa at temperatures up to 960 K. The baric dependences of the liquidus temperature during heating and the solidus and liquidus temperatures during cooling are found to have a minimum at 5 MPa. The solidus temperature during heating increases sharply as the pressure increases from 5 to 53 MPa and remains almost unchanged at a pressure p ≥ 53 MPa. The baric coefficients of the solidus and liquidus temperatures in the pressure range 5 ≤ p ≤ 53 MPa achieve ~0.6 K/MPa, which is much higher than the baric coefficient of the melting temperature of pure aluminum. A pT phase diagram is proposed for the AK7 alloy.     

Liquidus temperature determination in multicomponent alloys by thermal analysis

Thermal analysis is often used to determine equilibrium phase boundary temperatures such as the liquids. Accurate measurements require proper calibration procedures, which are standard for given instruments. In multicomponent alloys such as RENE N5 and PWA 1484 superalloys, a complex melting behavior associated with the solidification structure was exposed by examining the melting response at different heating rates. The observed variability in the melting signal is related to the sample processing history and is not addressed by the various standard calibration methods or supplemental procedures for different heating rates. The liquidus temperature can be determined under conditions approaching full compositional equilibrium by the application of an interrupted-heating thermal analysis protocol. The approach provides a new strategy for the reliable determination of phase boundary temperatures by thermal analysis.     

Experimental investigation and thermodynamic modeling of the Cr-Ni-Si system

The constitution of the Cr-Ni-Si ternary system is established via experiment and thermodynamic modeling. In the experimental section, 35 decisive alloys, the compositions of which are based on a preliminary calculation using available literature data, are prepared by arc melting of cold-pressed pellets and annealing at 900 °C for 25 days. Water-quenched samples are analyzed by using X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. In the modeling section, a consistent thermodynamic data set for the Cr-Ni-Si system is obtained by considering the present experimental results and reliable literature data. Comparisons between the calculated and measured phase diagrams show that the experimental information is satisfactorily accounted for by the thermodynamic modeling. The liquidus projection and reaction scheme for the entire ternary system are presented.     

Effect of composition on the solidification behavior of several Ni-Cr-Mo and Fe-Ni-Cr-Mo alloys

The microstructural development of several Ni-Cr-Mo and Fe-Ni-Cr-Mo alloys over a range of conditions has been examined. A commercial alloy, AL-6XN, was chosen for analysis along with three experimental compositions to isolate the contribution of individual alloying elements to the overall microstructural development. Detailed microstructural characterization on each alloy demonstrated that the observed solidification reaction sequences were primarily dependent on the segregation behavior of molybdenum (Mo), which was unaffected by the large difference in cooling rate between differential thermal analysis (DTA) samples and welded specimens. This explains the invariance of the amount of eutectic constituent observed in the microstructure in the welded and DTA conditions. Multicomponent liquidus projections developed using the CALPHAD approach were combined with solidification path calculations as a first step to understanding the observed solidification reaction sequences. Discrepancies between the calculations and observed reaction sequences were resolved by proposing slight modifications to the calculated multicomponent liquidus projections.     

钙元素对钢液相线温度的影响

用差热分析法(DTA)测定了含0.000 9%~0.003 0%Ca的0.14%~0.76%C-0.48%~1.48%Mn钢及不含Ca的0.15%~0.63%C-0.56%~1.32%Mn钢的液相线温度。通过比较分析测定值和计算值,得到钙元素对钢液相线温度的影响系数A值的平均数为-0.142 5,并对传统液相线温度计算公式进行了修正,提高钙处理钢水浇铸时过热度控制的精度。     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.     

On solidification of hypereutectic Al-Si alloys

Precipitation of primary silicon was studied in Al-Si hypereutectic alloys with 15, 18, and 25 wt. % silicon content. The alloys were solidified with different cooling rates from different super heat temperatures. The liquidus and eutectic temperature were evaluated from the cooling curves. The liquidus temperature was found to decrease with cooling rate. The evaluation of microstructure showed that the fraction of primary silicon decreased with increasing the cooling rate and super heat temperature. Furthermore, the morphology of the primary silicon changed as an effect of cooling rate and super heat temperature.     

Preparation of Si- Mn- N alloy by adding nitrogen to silicon- manganese alloy

Under the laboratory conditions, nine kinds of temperature regimes were designed to prepare Si- Mn- N alloy and the effect of heating rate, holding temperature, holding time and cooling rate on the increased nitrogen content in Si- Mn alloy was investigated by differential thermal analyzer. The results show that increased nitrogen content in Si- Mn alloy sample is 5. 45 mass%-37. 92 mass%, and the average value is 19. 33 mass%. The main factors influencing increased nitrogen content in Si- Mn alloy are heating rate, holding temperature and holding time. The optimum heating rate, optimal temperature range, and holding time for increasing nitrogen in Si- Mn alloys are 5??/min, 1100-1400?? and 4h, respectively. Under the optimum temperature range of 1100-1400??. and holding for 4h, the Si- Mn alloy has the largest nitrogen content. At this time, nitrogen content in the silicon- manganese alloy is 32. 8 mass%. and the main phase is Si3N4, MnSiN2 and Fe2Si by XRD analyzer. The nitrogen- enhancement process of Si- Mn alloy is a process in which solid- state and liquid- state Si- Mn alloys react with nitrogen. The nitrogen- enhancement rate of the liquid- state Si- Mn alloy reacting with nitrogen is faster.     

Estimation of liquidus temperatures for multicomponent silicates from activation energies for viscous flow

This article considers the viscous-flow phenomenon in molten silicate melts in the vicinity of their liquidus temperatures. An agglomeration of ionic species occurs in the liquid slag with decreasing temperatures and results in an increase in the viscosity and the activation energy for viscous flow. The latter was found to be nonlinear, increasing rapidly as the temperature is approached wherein a solid phase is likely to separate from the liquid. The second derivative of the activation energy for viscous flow with respect to temperature was found to show a discontinuity in the vicinity of the liquidus temperature. This has been verified in the case of viscous flow for both pure water and CaOSiO₂ melts. Experimental data for the viscosities of complex silicate melts and mold fluxes have also demonstrated the occurrence of a discontinuity in the second-derivative function, which is in accordance with the liquidus temperature as determined by differential thermal analysis (DTA). Thus, the second derivative offers a useful way of estimating the liquidus temperatures of multicomponent silicates, which are often difficult to determine due to supercooling effects.     

直接测定铝电解质初晶温度的研究

用冷却曲线法测定了实际铝电解质的初晶温度。为确证测温的准确性 ,与差热分析的结果进行了对比。自配电解质样品与文献所述的初晶温度值相当符合 ,实际样品的初晶温度要低于文献值。分析了产生误差的原因 ,对温度测定的重复性也作了研究。操作方式对电解质测温的重复性有很大的影响 ,处于同一操作情况下测得的结果吻合。冷却曲线法测定铝电解质初晶温度快速、准确 ,可为初晶温度的在线检测提供依据。     

Solidification thermal parameters affecting the columnar-to-equiaxed transition

Experiments were conducted to analyze the columnar-to-equiaxed transition (CET) during the upward unsteady-state directional solidification of Al-Cu and Sn-Pb alloys, under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat-transfer coefficients, tip growth rates, thermal gradients, and cooling rates. The observed results do not give support to CET criteria based individually either on tip growth rate or temperature gradients ahead of the liquidus isotherm. Rather, the analysis has indicated that a more convenient criterion should encompass both thermal parameters through the tip cooling rate. The columnar growth is expected to prevail throughout the casting for a tip cooling rate higher than a critical value, which depends only on the alloy system and was observed to be about 0.2 K/s for Al-Cu alloys and 0.01 K/s for Sn-Pb alloys in the present investigation.     

DTA‐Measurements to Determine the Thixoformability of Steels

Semi‐solid metallurgy (SSM), also known as “thixoforming” or “thixoprocessing”, is of special interest as a new potential manufacturing technology for components in the automobile, machine and electronic industries. The aim of this technology is to produce complex shapes which cannot be produced with conventional processing methods. An important process step of semi‐solid processing (SSP) is the reheating and isothermal holding of the billet within the solid‐liquid range in order to obtain the required fraction liquid content and the desired globular microstructure. Aside from the investigation of billet heating and the development of a suitable tool design, the development and evaluation of adequate microstructures over a wide temperature area is very important. The focus of this paper is to determine the semi‐solid area of different steels through Differential Thermal Analysis (DTA) measurements. To determine a process window for handling the alloys in the semi‐solid state, the DTA‐results can be combined with microstructure parameters. Subsequent quenching experiments show the development of the microstructure parameters (e.g. grain size, phase distribution, volume fraction, shape factor, matrix character, contiguity, and particle density of the primary solid and liquid phases). A comparison of the slopes of the determined solid‐liquid areas for different steels show the width of the melting or freezing intervals to evaluate the possible process windows. DTA‐experiments performed at different heating rates show the influence of faster heating and cooling rates on the solidus‐liquidus interval. To evaluate the suitability for the thixoforming processes, this paper describes, and then compares, the semi‐solid intervals of different steel grades, which have been investigated in the Department of Ferrous Metallurgy at the RWTH Aachen University. The tool steel HS 6‐5‐3 and the cold work tool steel X210CrW12 have a wide semi‐solid area, which can be explained due to the dissolution of different carbides. In contrast to this, the steels C45, 42CrMo4, 16MnCr5, 34CrNiMo4, 100Cr6, X220CrVMo13‐4 and the Alloy 33 show a much smaller semi‐solid area.     

06Cr19Ni10奥氏体不锈钢的DTA试验与组织分析

 根据铬当量、镍当量的计算结果推断了06Cr19Ni10奥氏体不锈钢的凝固模式,并采用差热分析技术（DTA）对06Cr19Ni10奥氏体不锈钢在10和30℃/min的加热冷却速度下的凝固过程进行了研究,对DTA曲线中的吸热峰和放热峰进行了分析,并采用激光共聚焦显微镜（LSCM）和扫描电子显微镜（SEM）对样品进行了测定。分析结果表明：06Cr19Ni10奥氏体不锈钢的实际凝固模式为FA型,即铁素体奥氏体型。随着冷却速率的增加,奥氏体形核率增大,残余的δ铁素体在形态上更加细小分散。该研究结果对实际生产中改善铸坯组织,提高铸坯质量具有着一定的意义。     

Thermal stress and strain effects on phase transition temperatures in differential thermal analysis testing

The effects of thermal stress and strain on the solid-phase transition temperature in differential thermal analysis (DTA) were investigated both theoretically and experimentally. The thermal stress is caused by the nonuniform distribution of temperature in the sample, and the strain is caused by the phase transformation. The experimental materials included high-purity Zr, commercially pure Ti, and high-purity Fe. It was found that the effect of thermal stress on the solid-phase transformation temperature was negligible due to the small size of the sample. An equation relating the phase transition temperature and the heating rate was developed which contains both strain energy and grain boundary energy considerations. Equilibrium phase transition temperatures calculated using this equation for Zr, Ti, and Fe are in good agreement with the phase diagrams and previous work.     

Solidification of undercooled Sn-Sb peritectic alloys: Part II. Heterogeneous nucleation

The undercooling behavior of fine droplet samples of Sn-rich, Sn-Sb alloys was investigated using differential thermal analysis (DTA). Undercooling levels measured during cooling from the liquid state follow the trend of the intermetallic phase liquidus, suggesting that solidification of all droplet samples (even those which solidify to yield a supersaturatedβ-tin product) was probably initiated with formation of primary intermetallic phase. Heterogeneous nucleation thermal cycling treatments were then used to measure the relative catalytic potency of primary intermetallic phases in this system for nucleation ofβ-tin during cooling. Crystallization reactions below the equilibrium peritectic temperature of 250 °C, at 187 °C and 230 °C, have been interpreted as corresponding to nucleation ofβ on Sn₃Sb₂ and SnSb substrates, respectively. The behavior observed in the Sn-Sb system can be generalized to guide the interpretation of heterogeneous catalysis and the analysis of solidification pathways in other peritectic alloy systems. Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

Liquid-liquid phase separation and remixing in the Cu-Co system

This article deals with the metastable liquid-liquid separation in the Cu-Co system. Several samples with different compositions were investigated by differential scanning calorimetry. High undercooling with respect to the liquidus was reached by means of the glass fluxing technique. The alloys were cycled with several heating and cooling runs in order to determine the temperature of the liquid-liquid separation and of the remixing. For each composition, demixing and remixing temperatures were found to be equal. Nucleation rate calculations of the liquid phase separation were carried out to explain the experimental results. The liquid-liquid separation in the Cu-Co system was found to be a nucleation process occurring with no detectable undercooling below the binodal line.     

Determination of the melting and solidification characteristics of solders using differential scanning calorimetry

Differential scanning calorimetry (DSC) is used in the present study to determine the onset temperature of phase transformation and the enthalpy of fusion of various solder alloys. The solders studied are Sn-Pb, Sn-Bi, Ag-Sn, In-Ag, and Sn-Pb-Bi alloys. Very notable undercooling, such as 35 °C, is observed in the solidification process; however, a superheating effect is not as significant in the heating process. Besides the direct measurements of reaction temperature and heat of fusion, the fraction solid vs temperature has also been determined using a DSC coupled with a mathematical-model method. The heating and cooling curves of the solders are first determined using DSC. By mathematically modeling the heat transfer of the DSC cells, the heat evolution and absorption can be calculated, and then the melting and solidification curves of the solder alloys are determined. The three ternary alloys, Sn-35 wt pct Pb-10 wt pct Bi, Sn-45 wt pct Pb-10 wt pct Bi, and Sn-55 wt pct Pb-10 wt pct Bi, displayed similar DSC cooling curves, which had three reaction peaks. However, the solid fractions of the three alloys at the same temperature in the semisolid state, which had been determined quantitatively using the DSC coupled with a mathematical method, were different, and their primary solidification phases were also different.