Papers by Leandru-Gheorghe BUJOREANU
Mechanisms and machine science, 2024
Journal of Optoelectronics and Advanced Materials, 2008
The paper analyses the reproducible behaviour of three work generating applications of shape memo... more The paper analyses the reproducible behaviour of three work generating applications of shape memory electric actuators, trained for two way shape memory effect (TWSME) in bending. The first is a lifting device, the displacement-temperature variation of which is considered as a function of time while revealing the influence of applied load. The second is an electric circuit breaker (disjunctor) for electric overload protection, the functioning of which is compared to that of a bimetallic thermostat, while revealing the influence of applied voltage. The third is a composite with polymeric matrix, the geometrical evolutions of which are monitored as a function of temperature, while revealing the influence of matrix on the bending reproducible behaviour. A comparison is performed between the critical transformation temperatures of the three devices, by emphasizing the influence of extrinsic factors on their mechanical responses.
Batteries, Aug 18, 2022
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Revista De Metalurgia, Dec 30, 1999
The friction reversión mechanism during the ultrasonic vibration drawing (UVD) of wires has been ... more The friction reversión mechanism during the ultrasonic vibration drawing (UVD) of wires has been detailed for the case when the die is located at the oscillation máxima of the waves and actuated parallel to the friction forcé direction. The decrease of the drawing forcé for the UVD technology as compared to classical drawing has been explained by means of the intermittent contact in the metal-die forming área. A relationship has been derived for the UVD friction coefficient, u us that allowed the analytical determination of the drawing forcé. In the case of the Romanian RUL IV (AISI 52100) ball bearing steel wires, a good agreement has been found between the analytical and the experimental valúes of the drawing forces that have decreased, as compared to classical drawing, by more than 5 % for drawing rates lower than 0.66 m/s.
Scripta Materialia, Oct 1, 2008
The mechanism of reversible transformation-induced plasticity of an Fe-30Mn-6Si (mass %) shape me... more The mechanism of reversible transformation-induced plasticity of an Fe-30Mn-6Si (mass %) shape memory alloy, under tensile stress and subsequent compression, has been observed by optical and atomic force microscopy. The tensile stress-induced e (hexagonal close-packed) martensite reverts into c (face-centered cubic) austenite after compression to zero strain. Further compression to negative strains induces a different e martensite variant from that of the tensile-stress-induced e martensite.
Materials Today: Proceedings, 2019
Oligocrystalline Fe 43.5 Mn 34 Al 15 Ni 7.5 alloys, with an average grain size larger than their ... more Oligocrystalline Fe 43.5 Mn 34 Al 15 Ni 7.5 alloys, with an average grain size larger than their thickness/ diameter, experience superelasticity between 223 … 423K which recommends them for large scale potential applications in seismic-protection industries. Besides oligocrystallinity which is necessary in order to avoid the presence of triple junctions between grains, the body centered cubic (bcc) austenite matrix has to be reinforced by the coherent precipitation of ordered β-NiAl bcc, in such a way as to enable the reversible stress induced formation of γ' face centered cubic (fcc) martensite. The present paper describes the processing procedure of three alloys with nominal chemical composition Fe 43.5 Mn 34 Al 151.5 Ni 7.5 ∓ 1.5 After hot rolling, annealing, cold rolling, cyclic heat treatment around solvus temperature (enhancing γ-fcc phase decomposition and the occurrence of a discontinuous phenomenon called abnormal grain growth) and solution treatment with final water quenching (meant to avoid γ-fcc formation and to enable coherent precipitation), chemical composition effects have been analyzed by optical microscopy (OM). In each of the processing steps, the evolution of average grain size was monitored and correlated with chemical composition effects. A limited oligocrystalline structure was obtained at Fe 43.5 Mn 34 Al 16.5 Ni 6 after cyclic heat treatment.
The International Conference on Shape Memory and Superelastic Technologies (SMST), May 21, 2013
ABSTRACT Shape memory actuators are thermo-responsive elements able to develop work-generating sh... more ABSTRACT Shape memory actuators are thermo-responsive elements able to develop work-generating shape memory effect (SME). In a series of previous reports, the refining tendency of martensite plates with increasing the number of training cycles applied to Cu-Zn-Al shape memory alloy (SMA) actuators was discussed [1] and the effects of actual functioning conditions of trained actuators, within a hydraulic oil installation, were emphasized, while revealing some particular aspects of "temperature memory effect" [2]. In another series of studies, the effects of complex thermal cycles on the same Cu-Zn-Al SMAs revealed a progressive decay tendency of reverse martensitic transformation, associated with the reciprocal blocking of differentially oriented martensite needles [3] and eventually to an intensification of atomic migration [4]. Considering these results, the present study aims to further investigate the cumulated effects of applying large numbers of training cycles and of functioning conditions during oil heating-cooling cycles of Cu-Zn-Al SMA actuators, on the structure and morphology of martensite plates as well as on the evolution of atomic migration along martensitic profiles.
Revista De Metalurgia, Dec 30, 2002
PROCEEDINGS OF THE INTERNATIONAL CONFERENCE OF GLOBAL NETWORK FOR INNOVATIVE TECHNOLOGY AND AWAM INTERNATIONAL CONFERENCE IN CIVIL ENGINEERING (IGNITE-AICCE’17): Sustainable Technology And Practice For Infrastructure and Community Resilience, 2017
Equal channel angular pressing (ECAP) is a well-established method for grain refinement in metall... more Equal channel angular pressing (ECAP) is a well-established method for grain refinement in metallic materials by large shear plastic deformation, being the most promising and effective severe plastic deformation (SPD) technique. ECAP is a discontinuous process, so the billet removal implies a new development of the procedure: the new sample pushes out the previous sample. In resuming the process the head and the tail ends of the work piece which becomes strongly distorted and receiving different amount of strain have to be removed. Due to the path difference in material flow between upper and lower region of the outlet channel, a non-uniform strain and stress distribution across the width of the workpiece leaving the plastic deformation zone (PDZ) is achieved. A successful ECAP requires surpassing two obstacles: the necessary load level which directly affects tools and a favorable stress distribution so the material withstanding the accumulated strain of repeated deformation. Under back pressure (BP), mat...
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, May 1, 2008
The structure of a Cu–13.67Al–4.28Ni–0.54Mn–0.14Fe (wt.%) shape memory alloy was analyzed in the ... more The structure of a Cu–13.67Al–4.28Ni–0.54Mn–0.14Fe (wt.%) shape memory alloy was analyzed in the initial condition, during a heating–cooling cycle, and during a room temperature loading–unloading mechanical cycle. A fully reversible martensitic transformation was revealed which was associated with a shape recovery degree reaching a saturation value at 97.7% for a relative permanent deflection of 90 %. By X-ray diffraction, the
Isij International, 2008
Fig. 7. Correspondence between thermal hysteresis (DT) and relative volume of reversible martensi... more Fig. 7. Correspondence between thermal hysteresis (DT) and relative volume of reversible martensite (VRM).
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, May 1, 2008
Abstract The paper reports some austenitization effects on the α-phase morphology in a tempered m... more Abstract The paper reports some austenitization effects on the α-phase morphology in a tempered martensitic Cu–21.57 Zn–7 Al (wt.%) shape memory alloy (SMA). By means of differential scanning calorimetry, X-ray diffraction and optical and electron microscopy, it has been shown that the effects of the stress-state induced in austenite persist even after martensitic transformation and tempering at 773 K. The two analyzed austenitization states, hot-pressed and homogenized, respectively, aimed to reproduce two steps of an industrial technologic flow for SMA manufacturing. Basically, austenitic hot-working diminished the size of disoriented α-phase precipitates while austenitic homogenization enabled the formation of a Widmanstatten structure with oriented long α-phase needles, among which those corresponding to α(1 1 1) intersect at about 70° and those corresponding to α(2 0 0) at 90°.
Journal of Materials Engineering and Performance, Sep 27, 2018
The paper analyzes work hardening behavior of T105Mn120 Hadfield steel under dynamic conditions. ... more The paper analyzes work hardening behavior of T105Mn120 Hadfield steel under dynamic conditions. The specimens were investigated under two states: (a) untreated (as cast) and (b) solution treated to 1100°C. Dynamic flexural behavior was examined by means of three-point-bending tests performed with a dynamic mechanical analyser (DMA), and structural analysis was done by x-ray diffraction, optical and scanning electron microscopy; DMA tests were performed under two variants: (a) temperature scans, between 2 150 and 400°C and (b) isothermal strain sweeps, up to 0.15% strain amplitudes. The former emphasized the critical temperatures of thermally induced reversible martensitic transformation and antiferromagneticparamagnetic phase transition, while the latter enabled to monitor the storage modulus increase due to the work hardening caused by dynamic bending. Strain sweeps tests revealed the effects of both dynamic bending frequency and number of cycles. The largest work hardening effect, obtained after five strain sweep cycles applied at the frequency of 5 Hz, was associated with the finest distribution of precipitated carbides, observed by differential scanning calorimetry and the formation of slip micro-bands, illustrated on OM micrographs and SEM energy dispersion spectroscopy maps.
Materialwissenschaft Und Werkstofftechnik, Nov 1, 2012
The surface micro‐reliefs of primary martensite plates, representative for two shape memory alloy... more The surface micro‐reliefs of primary martensite plates, representative for two shape memory alloys (SMAs) with different crystalline structures were compared from qualitative and quantitative point of view by scanning electron microscopy and atomic force microscopy, respectively. Qualitative evaluations revealed larger widths and heights of the primary plates of ε hexagonal close packed (hcp) martensite, in an Fe‐Mn‐Si‐Cr‐Ni SMA than those of β2′ orthorhombic (9R) martensite, in a Cu‐Zn‐Al SMA. Quantitative evaluations were based on systematic dimensional measurements of the width and height of primary plate profiles. The measurements were performed on one hundred and twenty five profiles, five on each martensite plate belonging to five typical groups of primary plates, with length above 50 micrometers, of both ε hcp and β2′ 9R martensites. In order to compare the topographies of the two types of plates a statistical evaluation of the dimensional intervals of width and height of measured plates was performed.
International Journal of Materials Research, May 1, 2010
The influence of microstructure on the stress–strain behavior of an Ni-rich NiTi shape memory all... more The influence of microstructure on the stress–strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni50.7Ti49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni4Ti3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress–strain–temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading–unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K.
Materials Science and Technology, Jun 1, 2000
ABSTRACT In order to explain the stabilised shape memory behaviour observed during the thermal an... more ABSTRACT In order to explain the stabilised shape memory behaviour observed during the thermal and mechanical cycling of a Cu–14·86Zn–5·81Al–0·5Fe (wt-%) alloy, X-ray diffraction and optical microscopy studies have been performed. The former have shown that the previously reported reorientation of the α brass phase crystallites from α(111) to α(200), observed during the heating up to 500°C, also occurs during a complete heating–cooling cycle up to 325°C. Optical microscopy studies confirm the increasing tendency for α phase preferred formation when the temperature is increased from 325 to 500°C. This texturing effect that occurs during thermal cycling is accompanied by an increase in the amount of α phase with a reduction in the amount of β 2 austenite. Finally, a tensile completely recoverable pseudoelastic strain of almost 0·6%, determined during a loading–unloading cycle applied to a heat treated specimen that contained no austenite, has been ascribed to the reversible formation of a specific martensite variant.
Materials Today: Proceedings
IOP Conference Series: Materials Science and Engineering, 2018
As a novel severe plastic deformation (SPD) method, multiple direct extrusion (MDE) has been prop... more As a novel severe plastic deformation (SPD) method, multiple direct extrusion (MDE) has been proposed as a new technique based on conventional deformation, consisting in repetitive extrusion processes of initial-squared cross-sectional workpieces, followed by transversal cutting, overlapping the resulting halves and resuming the extrusion by reinserting samples in the same die. Four cycles of MDE (route B) were applied to commercial copper and the potential for grain refinement and improvement in mechanical properties were evaluated. Because after the 3 rd pass MDE(route A) a buckling instability was noticed, a series of comparative experiments by simple and combined rolling (R) and MDE, respectively, were undertaken. Tensile testing conducted at room temperature using a computer-controlled testing machine was carried out to evaluate the mechanical properties after each MDE and/or R cycle. Tensile strength and elongation at fracture after MDE, R and MDE + R, respectively, were analysed. It was found that after MDE cycle (route B), tensile strength is more than two times higher than that of the initial state and the elongation at fracture decreases not less than 25%. Combining MDE and R does not give a new increasing in strength that remains almost the same but decreases elongation at fracture.
Journal of Alloys and Compounds, 1998
During the heating up to 5008C of a Cu-Zn-Al based Shape Memory Alloy, a marked texturing effect,... more During the heating up to 5008C of a Cu-Zn-Al based Shape Memory Alloy, a marked texturing effect, caused by the preferred formation of the a-phase has been revealed by means of X-ray diffraction and DTA. Kinetic analyses were also performed.
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Papers by Leandru-Gheorghe BUJOREANU