Alessandra Varone

Electron Beam Welding of IN792 DS: Effects of Pass Speed and PWHT on Microstructure and Hardness

Electron Beam (EB) welding has been used to realize seams on 2 mm-thick plates of directionally solidified (DS) IN792 superalloy. The first part of this work evidenced the importance of pre-heating the workpiece to avoid the formation of long cracks in the seam. The comparison of different pre-heating temperatures (PHT) and pass speeds (v) allowed the identification of optimal process parameters, namely PHT = 300 °C and v = 2.5 m/min. The microstructural features of the melted zone (MZ); the heat affected zone (HAZ), and base material (BM) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and micro-hardness tests. In the as-welded condition; the structure of directionally oriented grains was completely lost in MZ. The γ’ phase in MZ consisted of small (20–40 nm) round shaped particles and its total amount depended on both PHT and welding pass speed, whereas in HAZ, it was the same BM. Even if the amount of γ’ phase in MZ was lower than that of the as-received material, the nanometric size of the particles induced an increase in hardness. EDS examinations did not show relevant composition changes in the γ’ and γ phases. Post-welding heat treatments (PWHT) at 700 and 750 °C for two hours were performed on the best samples. After PWHTs, the amount of the ordered phase increased, and the effect was more pronounced at 750 °C, while the size of γ’ particles in MZ remained almost the same. The hardness profiles measured across the joints showed an upward shift, but peak-valley height was a little lower, indicating more homogeneous features in the different zones.

Young's Modulus Profile in Kolsterized AISI 316L Steel

AISI 316L steel, subjected to a low temperature carburizing treatment (kolstering), has been examined by Mechanical Spectroscopy (MS) and nanoindentation to determine the Youngs modulus of the surface hardened layer (S phase). MS results showed that the average value of elastic modulus of S phase is 202 GPa, a little higher than that of the untreated material.Nanoindentation tests, carried out with loads of 5, 15 and 30 mN, evidence a modulus profile vs depth: E is ~ 400 GPa at a distance from the surface of ~ 110 nm, then decreases to reach the value of the steel substrate (190 GPa) at 33 μm.These results, together with X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) measurements of carbon concentration profile, can be explained by considering the presence of a very thin surface layer, different from S phase and consisting of a mixed structure of Diamond-like carbon (DLC) and tetrahedral carbon (taC).Furthermore, the same experiments have been carried out also after heat treatments at 450 °C to correlate the modulus change to the decomposition of the metastable S phase leading to the formation of (Cr,Mo)C and Cr23C6 carbides in a Cr-depleted austenitic matrix.

Mechanical Spectroscopy Investigation of Liquid Pb-Bi Alloys

Liquid Pb–Bi eutectic alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system. The alloy has been characterized in liquid state from melting (125 °C) to 650 °C by mechanical spectroscopy. Experiments have been carried out using hollow reeds of austenitic stainless steel filled with the Pb-Bi alloy and sealed at the extremities. From 350 °C to 520 °C modulus shows a remarkable drop accompanied by a broad internal friction maximum. In the same temperature range radial distribution functions, determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The anelastic phenomena have been attributed to a structural re-arrangement of liquid metal. For comparison, other alloys of the Pb-Bi system with hypo-eutectic composition have been investigated.

Structural Changes of Liquid Pb-Bi Eutectic Alloy

Liquid Pb–Bi eutectic (LBE) alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system (ADS). The alloy has been characterized in liquid state from melting (125 °C) to 750 °C by mechanical spectroscopy, i.e. internal friction (IF) and dynamic modulus measurements. The experiments have been carried out using hollow reeds of austenitic stainless steel filled with Pb-Bi alloy and sealed at the extremities. Dynamic modulus showed a remarkable drop in the range 350-520 °C. In the same temperature range radial distribution functions (RDFs), determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The phenomenon has been explained as a structural re-arrangement of atoms in the liquid metal.

Anelastic Phenomena Preceding the Melting of Pure Metals and Alloys

Precursor 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).

Physical Phenomena Leading to Melting of Metals

Precursor 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).

Layered Double Hydroxides: Tailoring Interlamellar Nanospace for a VastField of Applications

Fifty-eight years ago Fenman, during an American Physical Society meeting at the California Institute of Technology, anticipated the problem of modifying and governing the world of the infinitely small. He said: “What I want to talk about, is the problem of manipulating and controlling things on a small scale… What I have demonstrated is that there is room—that you can decrease the size of things in a practical way. I now want to show that there is plenty of room. I will not now discuss how we are going to do it, but only what is possible in principle… We are not doing it simply because we haven’t yet gotten around to it.” Useless to say how profound his sensibility for science was. We’ve just begun to walk in this enormous field, toward the assembly of devices atom by atom. What we did till now is still rudimentary. Anyhow we believe that Layered Double Hydroxides could play a role in manufacturing these nanometric equipments. Layered Double Hydroxides (LDHs) are 2D ionic lamellar nano-materials belonging to the group of anionic clays. Their structure consists of positively charged brucite-like layers and intercalated anions. The layered structure, together with the flexibility to intercept different anionic species in variable compositions, both inorganic and organic, has attracted increasing interest. In order to meet specific requirements in very distant fields, considerable efforts were made to tailor the physical/chemical properties of LDHs and to design engineered LDH for several applications, ranging from anticorrosion coatings, flame-retardants, catalysis, to water treatment/purification, and biomedical applications. Furthermore they have been applied in energy harvesting and conversion, thanks to the possibility of substituting the composing metals with transition metals. Within the framework of this contribution, we first briefly review the development of synthesis processes (§1). In Paragraph 2, examples of the LDHs applications are reported. We will than focus on our laboratory experimental activities, showing the growth of the structures either on printed circuit tracks for applications of LDHs as gas sensors and biosensors. One more application is in nanostructured-modified textiles.

Effect of Heat Treatments on TiH2: Surface Composition and Hydrogen Release

The present work investigates the effect of heat treatments in air on the surface and structure of titanium hydride (TiH2) and hydrogen desorption. TiH2 has been heated in air at 440 and 540 °C for increasing time up to 180 min. to obtain the samples representative of 12 different oxidation conditions. The samples have been then examined by Temperature Programmed Desorption (TPD), X-Ray Diffraction (XRD) and Photoelectron Spectroscopy (XPS). Experimental results are presented and discussed.

Early Instability Phenomena of IN792 DS Superalloy

Microstructure stability of the directionally solidified Ni base IN792 superalloy has been investigated by Mechanical Spectroscopy (MS), i.e. internal friction (IF) and dynamic modulus measurements. Repeated IF test runs from room temperature to 1173 K have been carried out on the same samples and a Q-1 maximum has been always observed above 700 K. Its position does not depend on the resonance frequency. After each run the values of modulus and Q-1 at room temperature change indicating that a progressive irreversible transformation occurs. Damping phenomena have been attributed to the rearrangement of dislocation structures in disordered matrix which modifies dislocation density and average distance of pinning points. The results are supported by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations.

IN792 DS superalloy: Optimization of EB welding and post-welding heat treatments

Electron beam (EB) welding has been used to realize the seams on 2 mm thick plates of directionally solidified (DS) IN792 superalloy. A grid of the samples has been prepared by varying the pass speed v from 1 to 2.5 m/min, while the other process parameters (power P = 1 kW, acceleration voltage T = 50 kV, beam current I = 20 mA) were kept constant. Experiments were carried out both at room temperature and with pre-heating at 200 °C or 300 °C.Once found the best process conditions (pre-heating at 300 °C; v = 2.5 m/min) the effect of post-welding heat treatments at 700 and 750 °C for increasing time up to 2 hours has been investigated.