1,721,137 research outputs found
Physical phenomena leading to melting of metals
No full textPrecursor phenomena of melting in pure metals and alloys have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). The examined materials were the pure metals In, Sn, Pb and Bi, and some alloys of the systems In-Sn and Pb-Bi with different compositions. MS tests have been carried out by means of a novel method developed by us that permits to operate in resonance conditions and employs hollow reeds of stainless steel containing the liquid metal. In all the metals a sharp drop of dynamic modulus and a Q-1 maximum were observed in a temperature range ΔT before melting that depends on the specific metal and its structure. Such anelastic behaviour is consistent with an increase of the interstitialcies concentration as predicted by the Granato’s theory. Moreover, HT-XRD evidenced that sudden grain re-orientation, shift and broadening of diffraction peaks occur just before the formation of the first liquid, therefore self-interstitials and vacancies seem to play a synergic role in melting. The increase of self-interstitials over ΔT has the effect of weakening interatomic bonds that favours the successive vacancy avalanche leading to the collapse of crystal lattice (melting)
A focus on dynamic modulus: Effects of external and internal morphological features
No full textThe present work examines the effects of external and internal morphological features on the dynamic elastic modulus and its measure. It consists of two parts. The first part considers the effect of geometrical features of probes and shows the key role of roughness as source of a systematic error leading to the underestimation of the Young’s modulus. The second one is focused on the effect of porosity. Several models which consider the porosity as an ideal regular microstructure and the relative equations describing the Young’s modulus vs. porosity have been reviewed and critically discussed. The values of the relative modulus Er predicted by different models are similar for materials with low porosity (p < 0.2) and isolated pores whereas they strongly diverge if p > 0.2 and interconnected pores are present. Moreover, such models fail to describe the elastic behavior of materials correctly also with low porosity (p ≈ 0.1) such as sintered steels in the case of pores with a preferred orientation and an irregular shape
A novel technique of mechanical spectroscopy for studies on metals in liquid and solid state
No full textMechanical Spectroscopy (MS) is a technique which allows to carry out internal friction and dynamic modulus tests on metals vs. temperature. MS has been extensively used for studying metals in solid state whereas few data on liquid metals are reported in literature and have been obtained by means of a modified inverted torsion pendulum. Present work describes a novel technique developed by us which permits to study metals from solid to liquid state without stopping experiments during solidification and melting. The metal is cast inside a container of stainless steel (AISI316) dosed at an end; after solidification the open extremity of the reed has been sealed. The container-metal system is mounted in free-damped mode and excited by flexural vibrations. Experimental data (damping Q' and dynamic modulus E) are corrected from the contribution of the container. This technique has been used for studying precursor phenomena of melting in pure metals and alloys and liquid-liquid transitions in several alloys
Anelastic phenomena preceding the melting of pure metals and alloys
No full textPrecursor phenomena of melting in pure metals (In, Pb, Bi and Sn) and alloys of the systems Pb-Bi and In-Sn with different compositions have been investigated by means of Mechanical Spectroscopy (MS), i.e. dynamic modulus and damping measurements. MS tests evidenced that a sharp drop of dynamic modulus E takes place in a temperature range ΔT before the formation of the first liquid: the modulus variation ΔE and the corresponding temperature range ΔT depend on the specific metal or alloy. The modulus drop is consistent with a relevant increase of interstitial concentration (self-interstitials assuming the dumbbell configuration), as predicted by the Granato’s theory of melting. The increase of damping in the same temperature range of modulus drop supports this explanation. Owing to their dumbbell configuration self-interstitials interact with the flexural vibration of samples and the periodic re-orientation under the external applied stress leads to energy loss and damping increase. The increase of self-interstitials has the effect to weaken interatomic bonds (modulus drop) and favours the collapse of crystal lattice (melting)
Flat-top cylinder indenter for mechanical characterization: a report of industrial applications
No full textFIMEC (flat-top cylinder indenter for mechanical characterisation) is an instrumented indentation test employing a cylindrical punch. It has been used to determine the mechanical properties of metallic materials in several applications of industrial interest. This work briefly describes the technique and the theory of indentation with a flat-ended punch. The flat indentation of metals has been investigated through experimental tests, and an equation has been derived to calculate the yield stress from the experimental data in deep indentation. The approach is supported by many data on various metals and alloys. Some selected case studies are presented in the paper: (i) crank manufacturing through pin squeeze casting; (ii) the evaluation of the local mechanical properties in a carter of complex geometry; (iii) the qualification of Al billets for extrusion; (iv) stress- relaxation tests on CuCrZr heat sinks; (v) the characterization of thick W coatings on CuCrZr alloy; (vi) the measure of the local mechanical properties of the molten-zone (MZ) and the heat-affected zone (HAZ) in welded joints. The case studies demonstrate the great versatility of the FIMEC test which provides information not available by employing conventional experimental techniques such as tensile, bending, and hardness tests. On the basis of theoretical knowledge and large amount of experimental data, FIMEC has become a mature technique for application on a large scale in industrial practice
Liquid Pb-Bi eutectic alloy: Study of short-range order
No full textLiquid Pb-Bi eutectic (LBE) alloy has been selected as a coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system (ADS). This new reactor type might be one of the possible solutions for the nuclear waste problem because it is able to transmute high level radioactive waste and long lived actinides. The ADS technology however requires special operating conditions. The materials need to resist temperatures ranging between 200-550°C under a high energy neutron flux and in contact with the LBE. This condition increases liquid metal corrosion and embrittlement, in fact the limitation of ADS life is due to the relatively low corrosion resistance of structural materials in the LBE environment. The compatibility of structural materials with liquid LBE at high temperature is one of the key issues for the commercialization of such fast reactors. Possible structural changes of the liquid may affect the interaction of LBE with structural materials. Therefore, in previous works LBE was investigated by internal friction (IF) and dynamic modulus measurements far above the eutectic temperature (125 °C) by using a new method developed by us. After melting the alloy exhibits a steady modulus decrease up to 350 °C, here a remarkable drop is observed covering a temperature range of about 170 °C. Finally, above 520°C, modulus continues to decrease with slope very close to the initial one. In correspondence of the modulus drop two IF maxima were detected: The first centered at -350 °C, the second at -460 °C. Anomalies of the liquid metal have been evidenced by other investigators as well. It is believed that the structure of the alloys is heterogeneous after melting, with residual minor crystals still existing and, when the critical temperature is reached, the residual conglomerates are broken and the uniform structure appears. To better understand the nature of LBE structural evolution vs. temperature High Temperature X-Ray Diffraction (HT-XRD) measurements have been carried out up to 720 °C; from diffraction patterns the radial distribution function (RDF) has been calculated (some G(r) curves are reported in Fig. 1). RDF provides information about possible changes of liquid metal. The average distance rt between the 1st nearest neighbour atoms is of particular relevance: The position (Fig. 2) and shape of 1st RDF peak progressively change as temperature increases with strict correspondence with the dynamic modulus drop previously observed by us. The trend of the ratio r2 / r1 vs. temperature (Fig. 3) showed that just after melting r2 /r1 is ~ 1.41, increases till to reach the value of -1.61 at 720 °C. The result indicates that the short-order of liquid LBE gradually changes from a cuboctahedral configuration (the ratio is ) just after melting to an icosahedral one (the ratio corresponds to the golden ratio φ = 1.618 ) at 720 °C. The two structures of the liquid are shown in Fig. 4. To describe structural re-arrangement of atoms in the liquid from cuboctahedral to icosehedral configuration a model has been realized (Fig. 5). The fitting of the first RDF peak by pair functions Pij(r) permitted to identify and determine the single contributions at increasing temperature. From melting up to 350 °C, the mixed Pb-Bi pairs are not present. They appear at 350 °C and their contribution to RDF 1st peak becomes more important as temperature increases. In fact, the increasing number of Pb-Bi pairs corresponds to a more homogeneous elemental distribution in the alloy
Influence of probe test orientation, sintering degree and morphological anisotropy of porosity on the Young’s modulus of homogeneous sintered steel
No full textSynergic role of self-interstitials and vacancies in Indium melting
Full text linkPrecursor effects of indium melting have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). MS tests evidenced a sharp drop of dynamic modulus in the temperature range between 418 K and 429 K (melting point). At 429 K, HT-XRD showed partial grain re-orientation, peak profile broadening, in particular in the lower part, and peak shift towards lower angles. Experimental results are consistent with density increase of self-interstitials and vacancies in the crystal lattice before melting. Self-interstitials and vacancies play a synergic role in the solid–liquid (S-L) transformation. The increase of self-interstitials over a temperature range of about 10 K before melting has the effect of weakening interatomic bonds (modulus drop) that favors the successive vacancy formation. Finally, the huge increase of vacancy concentration above 428 K leads to the collapse of crystal lattice (melting)
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