Onset Melting Temperature Research Articles

The eutectic point in Pr-rich Pr–Cu–Al ternary alloys

A series of Pr-rich Pr–Cu–Al alloys was prepared by arc melting. The onset and offset melting temperatures ( T m and T l), corresponding to the solidus and liquidus temperature of the phase diagram, were determined by differential scanning calorimetry. A ternary eutectic alloy is found at the composition of Pr 68Cu 25Al 7 (at.%) with eutectic temperature at around 705 K. The ternary eutectic point was also confirmed by scanning electron microscopy observation and X-ray diffraction analysis.

Characterization of the melting and wetting of Sn-Ag-X solders

There is tremendous interest at present with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu) and tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-3.5Ag alloy, which melts at 221/spl deg/C. Differential scanning calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC.

Glass forming ability of La–Al–Ni–Cu and Pd–Si–Cu bulk metallic glasses

Onset ( T m) and offset ( T l) melting temperatures of glass forming La–Al–Cu–Ni–(Co) and Pd–Cu–Si alloys were measured by studying the melting of these alloys using differential thermal analysis. The compositional dependence of glass transition temperature T g, and the above two temperatures for these bulk amorphous alloys are presented. The effect of T l and T rg on glass forming ability for these bulk amorphous alloys will also be evaluated. The correlation between glass forming ability and T rg given by T g/ T l and T g/ T m will be discussed.

A Relationship between Glass-Forming Ability and Reduced Glass Transition Temperature near Eutectic Composition

Onset temperature (solidus), Tm and offset temperature (liquidus) Tl of melting of a series of bulk glass forming alloys based on Zr, La, Mg, Pd and rare-earth elements have been measured by studying systematically the melting behaviour of these alloys using differential thermal analysis (DTA) or differential scanning calorimetry (DSC). Bulk metallic glass formation has been found to be most effective at or near their eutectic points and less effective for off-eutectic alloys. Reduced glass transition temperature Trg given by Tg⁄Tl is found to show a stronger correlation with critical cooling rate or critical section thickness for glass formation than Trg given by Tg⁄Tm.

Influence of Ca2PbO4 on Phase Formation and Electrical Properties of (Bi,Pb)2Sr2Ca2Cu3O10/Ag Superconducting Composites

(Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) precursor powders with large and small amounts of Ca2PbO4 phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca2PbO4 on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature (Tm,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca2PbO4. Tapes with a small amount of Ca2PbO4 had a higher Tm,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca2PbO4. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current‐carrying capacity of the tapes were significantly improved by reducing the Ca2PbO4 content of the precursor powders. These results are of practical significance for the commercialization of Bi‐based high‐temperature superconductors.

Thermodynamics of La based La–Al–Cu–Ni–Co alloys studied by temperature modulated DSC

The onset and offset melting temperatures T m and T l, glass transition temperature T g and heat of fusion Δ H m of six La based La–Al–Cu–Ni–(Co) alloys were measured by DTA or DSC at a heating rate of 20 K/s. The absolute values of specific heat capacity for the undercooled liquid and the corresponding crystalline of these six alloys were obtained by means of temperature-modulated DSC (TMDSC). Entropy, enthalpy and Gibbs free energy differences between the undercooled liquid and crystalline of these alloys as a function of temperature have been calculated. Their glass forming ability was discussed from a thermodynamic point of view.

The effect of temperature cycling on the amylopectin retrogradation of starches with different amylopectin unit-chain length distribution

The amylopectin retrogradation of six cereal starches (wheat, rye, barley (normal-amylose, high-amylose and waxy), and waxy maize), three potato starches (two normal-amylose, and a high-amylopectin potato), and a single pea starch was studied by differential scanning calorimetry (DSC). The recrystallization of amylopectin was measured after 2 and 4 days of storage. In order to affect the crystallinity of the amylopectin, samples were exposed to different temperature cycles during the storage period, favouring the nucleation or the propagation of the crystallites. The temperature cycles during the first 2 days were: 24 h at 6°C (facilitating nucleation), followed by 24 h at either 30 or 40°C (promoting propagation). For the 4 days storage test the temperature cycles were repeated. Moreover, the amylopectin unit-chain length distribution of the starches was determined by high-performance anion exchange chromatography (HPAEC) after debranching of isolated amylopectins. The recrystallization of amylopectin was greatly affected when gelatinized starch was treated with different time–temperature cycles. The melting enthalpy of recrystallized amylopectin (Δ H) decreased in most cases. The onset temperature of melting of recrystallized amylopectin ( T o) was controlled by the propagation temperature ( T p), and increasing the latter resulted in an increase in T o. As a result of the increase in T o, the melting range of the recrystallized amylopectin (Δ T r) decreased. A development of crystallites that melted at higher temperatures, noticed as an increase in the offset temperature ( T f) of melting, was observed for the cereal starches after all temperature treatments. The possibility of predicting how a specific T p would affect and increase the T o of recrystallized amylopectin when gelatinized starch was treated with the two day cycles was demonstrated. The DSC and HPAEC results showed that amylopectin unit-chains with DP 6 and the population of chains with DP 18–19 were positively correlated to Δ H, whereas the correlation to the population of chains of DP 8–11 was negative. Indications of a negative correlation between Δ H and chains of DP 22–34 as well as a positive correlation to chains with DP>40 were also found. The changes found in T f correlated to the same distinct amylopectin populations as the previous ones for the relation to Δ H. However, the signs of the correlation coefficients were changed.

The role of partial melting on superplasticity of Si 3N 4p/Al-Cu-Mg composite

For metal matrix composites containing ceramic reinforcements, stress concentration during tensile deformation occurs at interfaces between the matrix and ceramic, so that, cavitation is excessively developed at interfaces, resulting in premature fracture. In a previous paper, it was suggested that the dominant deformation process of high-strain-rate superplasticity for metal matrix composites if grain boundary sliding (GBS), and that liquid phases in high-strain-rate superplasticity serve both to relax the stress concentration and to limit the appearance of internal cavitation and subsequent failure. In their experimental observation, it has been observed that a large elongation is obtained at the temperature, that is close to the onset temperature for partial melting in the superplastic composites, but the elongation significantly decreases at slightly higher temperatures, which are close to the end temperature for partial melting. This indicates that there is an optimum amount of the liquid phase for obtaining high-strain-rate superplasticity in these materials. Thus, it is important to examine the nature of ceramic-matrix interfaces of superplastic composites in order to understand the origin of superplastic flow related to liquid grain boundaries during high-strain-rate superplastic deformation. The aim of this work is to study the nature of the interfaces and surfaces of the deformedmore » specimens in Si{sub 3}N{sub 4}p/A;-Cu-Mg composites to understand GBS related with liquid phases at interfaces during superplastic flow.« less

Partial melting effect on tensile elongation behavior and activation energy for plastic flow in high-strain-rate superplastic 2124 Al alloy and 2124 Al composites

It is now well recognized that high strain-rate superplasticity (HSRS) can be obtained from powderprocessed aluminum and aluminum matrix composites [1–6]. Though the origin of HSRS is still controversial, it has been proposed that the presence of a liquid phase is important for achievement of HSRS as it serves to help the accommodation process for grain boundary sliding by relaxing the high stress concentration around the matrix/particulate interfaces [7]. Mabuchi and Higashi [8] showed that the optimum superplastic temperature for the MMCs is often close to the onset temperature for partial melting. Mabuchi et al. [8–10] and Koike et al. [11] conducted differential scanning calorimeter (DSC) tests on the 20% Si3N4 2124 Al composites reinforced by particulates or whiskers and discovered that for each material two downward peaks appeared in the DSC trace that were small and sharp, and large and continuous, interpreted to represent the local melting at the reinforcement/matrix interfaces and the melting of the aluminum alloy matrix, respectively. The temperatures for partial melting were 816 K, 784 K and 849 K for the 20% Si3N4 (dp= 0.2μm, where dp is the size of particulate reinforcement), 20% Si3N4 (dp= 1μm) and 20% Si3N4w (whisker) 2124 Al composites. Recently, Zahid et al. [12] reported HSRS in a PM 18 vol% SiCp (dp= 3μm)/2124 Al composite where tensile elongations over 300%, with the maximum value of 450%, were obtained in the temperature range between 743 K and 783 K at 7× 10−2 s−1. Their DSC test, however, did not show the sharp peak for partial melting. The onset temperature for a continuous endothermic curve was 861 K, which is significantly higher than the optimum temperatures where superplasticity was obtained. Kim et al. [13] recently investigated the HSRS in a PM 20 vol% SiCp (dp= 3μm)/2124 Al composite at the temperatures between 773 K and 838 K. Tensile elongations of about 300% could be obtained at temperatures between 793 K and 823 K near 10−1 s−1, with the largest tensile elongation of 400% at 808 K. Kim and Hong [14] also examined the HSRS behavior of a PM 2124 Al alloy where tensile elongations over 300% were achieved at temperatures between 773 K and 823 K at the optimum strain rate of 10−2 s−1, with the tensile elongation as large as 700% at 823 K. The aim of the present study is to look for evidence for partial melting in the HSR superplastic 2124 Al alloy and 20 vol% SiCp/2124 Al composite using a DSC technique, and to compare the DSC results with those reported by other investigators. The tensile ductility and activation energy for superplastic flow are also discussed based on the DSC test results. The chemical composition of the matrix of the composite is 3.5%Cu-1.24%Mg-0.53%Mn-0.14%Fe0.01%Zn-bal Al by weight percent. The 2124 Al alloy and 20% SiC/2124 Al composite were prepared by hot pressing at 843 K and hot extrusion at 723 K. DSC tests were conducted on the as-extruded and the superplastically deformed samples in a ∼50 mg specimen during heating and cooling at a rate of 10 K min−1. Two different DSC schedules were arranged for each material: 1) the sample was heated to 993 K; 2) the sample was heated to 878 K, held there for 30 min, and cooled down to 673 K. Then it was heated again to 993 K. Fig. 1 shows the DSC results for the superplastically deformed 2124 Al alloy (803 K, 10−1 s−1, 280%) and 20% SiCp/2124 Al composite (808 K, 9× 10−2 s−1,

Sugar Accumulation as a Mechanism for Freezing Tolerance in Cold-Stored 'Danshaku' Potato Tuber (Solanum tuberosum L.)

Tissue samples from cold-stored (0°C) 'Danshaku' potato tuber which underwent low temperature sweetening (LTS) were analyzed by differential scanning calorimetry (DSC) using a heating rate of 5°C /min. Results showed that LTS significantly lowered the onset temperature of melting. Tubers stored at 10°C did not sweeten and showed insignificant reduction of onset temperature. Total sugar levels were significantly related to Tp, To and Te while off was significantly related to moisture content of the sample. The primary endotherm (average To=-3. 86°C) representing extracellular melting of water was observed for tubers stored at 10°C and 0°C ; a secondary endotherm representing freezing of intracellular water was observed in 41.18% of tissue samples from tubers stored at 10°C (average To=-3.22°C). Only 11.11% of tissue samples from tubers stored at 0°C showed a secondary endotherm (average To=-12.20°C), implying that high sugar levels reduced lethal intracellular freezing in cellular compartments. Hence, sugar accumulation induced by cold conditions may be a mechanism to delay freezing and reduce its harmful effects on cell compartmentation and function.

Effect of Polarity of Complexing Agents on Thermal and Rheological Properties of Rice Starch Gels

ABSTRACTThe ability of rice starch to complex with ligands of various polarities was studied to examine the mechanism of complex formation in an aqueous solution. Differential scanning calorimetry (DSC) showed that TNuS19 rice starch (27.9% amylose) formed inclusion complexes with all 12‐C complexing agents. The onset melting temperatures (To) of the complexes were ≈93–96°C. The saturation concentrations of added ligands with high polarity, lauric acid (LA), and lauryl alcohol (LOH), had a range of 2–4% (w/w) of the starch, and both of the corresponding melting enthalpies (ΔH) were ≈3.0 J/g. In contrast, the saturation concentrations of ligands with low polarity, methyl laurate (ML) and dodecane (DO), were ≈1–2% (w/w), and the ΔH were 1.87 and 1.80 J/g, respectively. This implied that solubility of ligands had a significant effect on the extent of complexation. The To and ΔH increased with an increase of annealing time at 85°C, and the optima for the partially reversible complex formation were 2 hr of annealing in all cases. When measured by a dynamic rheometer, the TNuS19 rice starch gel with added LA or LOH showed a higher storage modulus (G′) than that with no complexing agent added during heating. The G′ and tan δ of the complexed gel were further increased during 12 hr of storage. The increase of G′ indicated that the elastic structure of the concentrated rice starch gels could be improved by complex formation and annealing, whereas the increase of tan δ suggested incompatibility of starch components during storage.

Time-resolved structural study of the Ge(111) high-temperature phase transition

The dynamic behavior of the Ge(111) high-temperature phase transition has been investigated using 100 ps time-resolved reflection high-energy electron diffraction (RHEED). A laser pulse heats the surface while a synchronized electron pulse is used to obtain the surface diffraction pattern at some time during and after the heating laser pulse. For slow heating, the RHEED results show that the high-temperature phase transition spreads from ∼1020 K to ∼1070 K. For 100 ps laser heating, the time-resolved RHEED measurements show that the phase transition occurs in the transient temperature range from 1083±23 K to 1138±32 K. The time-resolved RHEED results are in agreement with incomplete melting in which only the topmost layer melts during the phase transition. Thus, overheating of the topmost bilayer beyond the onset temperature of incomplete melting by 63±23 K was observed for 100 ps laser heating.

Melting temperature depression for low ether content polyether-polyester block copolymers with amide linkages

Melting behavior of a series of polyether-polyester block copolymers with low ether contents was studied. Significant melting-reorganization-remelting was suggested by means of differential scanning calorimetry. The equilibrium melting temperature of the copolymers was obtained by using the Hoffman-Weeks extrapolation. It has been found that the use of the on-set melting temperature for the Hoffman-Weeks plots is less reliable than the use of the corrected peak melting temperature. Copolymerization of polyetheramides with polyesters leads to a significant melting temperature suppression for the polyester crystal. We hypothesize that the locations of the junction points between blocks can be treated as chain ends for the crystallizable polyester blocks: the polyether-polyesters grow to form extended chain-like crystals with lamellar thickness limited by the polyester block length. For comparison, the nonlinear Hoffman-Weeks treatment, recently proposed by Marand, has also been applied to examine the behavior of melting temperature depression. Although the Tmo obtained from nonlinear extrapolation is much higher than the Tmo obtained from the Hoffman-Weeks extrapolation, similar trend regarding the composition dependence of Tmo have been found.

The Role of Liquid Phase in Cavitation and Fracture in High-Strain-Rate Superplastic Si<sub>3</sub>N<sub>4p</sub>/Al Alloy Composite

A 30 pct Si 3 N 4p /Al-Mg-Si composite has been investigated for high-strain-rate superplastic behavior and cavitation. A maximum elongation of 723 pct was attained at a temperature of 823 K close to the onset temperature for melting. Extension of cavities transverse to the tensile direction was small and perhaps this fact was responsible for the large elongation. It is likely that the stress concentrations are sufficiently relaxed by a liquid phase and that the development of cavities transverse to the tensile direction is limited.

The Effect of Deformation Temperature on Microstructure and Superplastic Flow in Al–Cu–Mg/Si<SUB>3</SUB>N<SUB>4</SUB>p Composites

The effect of deformation temperature on microstructure and superplastic flow in Al-Cu-Mg alloy composite reinforced with Si 3 N 4 particles (Si 3 N 4 p), was investigated. The composites tensile-tested at 758, 773 and 788 K, exhibiting high-strain-rate superplasticity, show high resistance to grain growth during the tensile deformation. However, the composite tensile-tested at 798 K, exhibiting much lower elongation than the composites tensile-tested at lower temperatures, shows considerable grain growth during the tensile deformation. The EDS analysis shows that concentration of the additive elements occurs at the interfaces and the analytical values depend on the sampling points. The largest elongation is obtained at 788 K, which is close to the onset temperature for partial melting determined by DTA. On the other hand, the elongation significantly decreases at 818 K, which is close to the end temperature for partial melting.

Preparation and Properties of Benzyl Corn Starches

Benzyl corn starches (DS 0.02–0.16) were prepared by reacting starch with benzyl chloride in an alkaline aqueous medium at 65°C, and the pasting properties of the starches were investigated. Reaction yield with 20% benzyl chloride (based on starch weight) for 12 h was increased from 26.7% to 60.5% as the NaOH addition increased from 3% to 8% (based on starch weight), and became relatively constant thereafter. With 20% benzyl chloride and 8% NaOH, degree of substitution (DS) reached 0.16. Light transmittance and viscosity of starch paste were decreased as DS increased. Low viscosity and opaqueness of the paste indicated that the benzyl groups increased hydrophobicity, and reduced solubility of the starch. Onset temperature (To) for starch melting measured by a Differential Scanning Calorimeter thermogram was decreased as DS increased up to 0.1. But above DS 0.1, To increased. Melting enthalpy was also decreased as DS increase because of the incomplete gelatinization. Solubility in aqueous methanol (80%) increased near 2% as DS reached 0.16. Maximum wavelength and absorbance of the iodine complexes of benzyl starch were decreased with benzylation.

Preparation and Properties of Benzyl Corn Starches

Benzyl corn starches (DS 0.02–0.16) were prepared by reacting starch with benzyl chloride in an alkaline aqueous medium at 65°C, and the pasting properties of the starches were investigated. Reaction yield with 20% benzyl chloride (based on starch weight) for 12 h was increased from 26.7% to 60.5% as the NaOH addition increased from 3% to 8% (based on starch weight), and became relatively constant thereafter. With 20% benzyl chloride and 8% NaOH, degree of substitution (DS) reached 0.16. Light transmittance and viscosity of starch paste were decreased as DS increased. Low viscosity and opaqueness of the paste indicated that the benzyl groups increased hydrophobicity, and reduced solubility of the starch. Onset temperature (To) for starch melting measured by a Differential Scanning Calorimeter thermogram was decreased as DS increased up to 0.1. But above DS 0.1, To increased. Melting enthalpy was also decreased as DS increase because of the incomplete gelatinization. Solubility in aqueous methanol (80%) increased near 2% as DS reached 0.16. Maximum wavelength and absorbance of the iodine complexes of benzyl starch were decreased with benzylation.

Superplastic behavior and cavitation in high-strain-rate superplastic Si3N4p /Al-Mg-Si composites

High-strain-rate superplastic behavior has been investigated for Si3N4p /Al-Mg-Si (6061) composites with a V f =20 and 30 pct, respectively, where V f is the volume fraction of reinforcements. A maximum elongation was attained at a temperature close to the onset temperature for melting for both composites. The maximum elongation for the 30 vol pct composite was larger than that for the 20 vol pct composite. Development of cavities transverse to the tensile direction is responsible for the lower maximum elongation of the 20 vol pct composite. However, development of the transverse cavities was limited to the optimum superplastic temperature for the 30 vol pct composite. The differential scanning calorimetry (DSC) investigation showed that a sharp endothermic peak appeared for the 30 vol pct composite, indicating that sufficient partial melting occurs. It is, therefore, likely that the stress concentrations are sufficiently relaxed by a liquid phase and that the development of transverse cavities is limited for the 30 vol pct composite.

Effects of freezing and thawing methods and storage time on thermal properties of freshwater prawns (Macrobrachium rosenbergii)

Thermal stability of proteins in frozen and thawed freshwater prawns was measured by differential scanning calorimetry. The onset and peak melting temperatures corresponding to myosin denaturation, as well as total enthalpy of denaturation of prawn muscle, decreased after freezing–thawing treatments. There were no significant differences in the thermal properties of prawns with changes in the rate of freezing, ie blast (fast) vs still (slow) freezing. However, the thermal properties were influenced by the rate of thawing. Rapid thawing using a combination of microwave and tap water resulted in a lower (P⩽0·05) thermal stability of prawn proteins compared to slow (refrigeration) or moderately fast (tap water) thawing methods. Keeping prawn shells intact or not intact during freezing–thawing did not alter the thermal properties of the prawn proteins. © 1997 SCI.

THE ROLE OF LIQUID PHASE IN CAVITATION IN A Si3N4p/Al-Mg-Si COMPOSITE EXHIBITING HIGH-STRAIN-RATE SUPERPLASTICITY

Cavitation was quantitatively investigated at 8 MPa and 833 K for a high-strain-rate superplastic Si3N4p/Al-Mg-Si composite, where the testing temperature of 833 K was slightly above the onset temperature for partial melting. Most of the cavities were very small (⩽ 1 μm), and cavity development was not significant. Based on a theoretical analysis, when stress concentrations around reinforcements are relaxed by the presence of a liquid phase, excessive cavity development is suggested to be limited. This view is in agreement with the experimental results. © 1997 Acta Metallurgica Inc.