Ben Mala | Institut National Polytechnique de Toulouse (original) (raw)

Papers by Ben Mala

Materials Research Proceedings

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC.

X-Rays in Mechanical Engineering Applications, 2004

Large Central Scientific Facilities such as the ESRF (the European Synchrotron Radiation Facility... more Large Central Scientific Facilities such as the ESRF (the European Synchrotron Radiation Facility) and ILL (the European centre for neutron research), were set up to provide scientists with the advanced facilities they need to exploit neutron and synchrotron X-ray beams for scientific research. Engineers also conduct research at these Facilities, but this is less common as most practicing engineers generally have little or no knowledge of neutron or X-ray scattering, or of their considerable potential for engineering research, model validation, material development and for fatigue and failure analysis. FaME38 is the new joint support Facility for Materials Engineering, located at ILL-ESRF, set up to encourage and to facilitate engineering research by engineers at these facilities. It provides a technical and knowledge centre, a materials support laboratory, and the additional equipment and resources that academic and industrial engineers need for materials engineering research to be...

Materials Science Forum, 2005

FaME38 is a new facility at the ILL/ESRF in Grenoble with the aim of improving the accessibility ... more FaME38 is a new facility at the ILL/ESRF in Grenoble with the aim of improving the accessibility and effectiveness of neutron and synchrotron strain measurements. In addition to providing basic materials engineering facilities, it enables users from both commercial and academic backgrounds to prepare and to evaluate experiments on-site. The real success and impact of a strain scanning experiment depends on the quality of the collected data and its practical use. FaME38 provides a knowledge base and tools which can increase the efficiency of the measurement. These tools include a VAMAS standard sample base-plate, a materials support laboratory and enhanced visualisation software. The VAMAS base-plate allows pre-configuration of the sample position and set-up, as well as so-called “hot-swapping” of samples with minimum time required for re-configuration of the instrument. The visualisation tools feature web-based simulation and, in particular, 3D visualisation of both the experimental environment as well as the data. The use of the support facility can significantly reduce the set-up time, thus increasing the time available for measurement. The visualisation naturally enhances the understanding of the data and ties in with existing engineering code such as CAD/FEA software. We demonstrate how the use of additional technology can improve the effectiveness and impact of strain mapping experiments at neutron and synchrotron facilities by disseminating the workflow of a typical experiment undertaken in the FaME38 framework. This approach is aimed at paving the way toward technology-oriented application of synchrotron and neutron strain scanning.

Physical processes taking place during the final heat treatment of cold worked NiTi elements give... more Physical processes taking place during the final heat treatment of cold worked NiTi elements gives them parent austenite shape, microstructure and functional properties as shape memory or superelasticity. Since the heat treatment in large extent controls the functional properties of NiTi, these processes are of significant application interest but, surprisingly, very little is known about them. In this work, we will discuss the activity of physical processes taking place during the heat treatment performed on as drawn thin NiTi wires (d=0.1 mm) by passing electric current of constant power through them. The discussion is based on the results of: i) in-situ electrical resistance measurements during annealing using dedicated experimental equipment allowing to measure simultaneously the evolution of stress, strain, electric resistance, temperature on the NiTi wire exposed to high temperatures and stresses induced by electric power pulse, ii) in-situ synchrotron powder diffraction exper...

This paper reports selected results of the research motivated by SMA application development in f... more This paper reports selected results of the research motivated by SMA application development in frame of European research project PROSTONE aimed at innovation in stone processing equipment. In many devices used in the stone sector, stress inhomogeneity in machine parts and tools seems to cause problems. There is an idea to solve the problem of stress concentrations in metallic sheets occurring during operation through the insertion of small prestrained elliptical NiTi elements into elliptical holes in locations of maximum stress concentration which would counteract these stress concentrations. In order to evaluate the effect of the temperature, compression force and on the stress distribution in the steel plate, three different neutrons strain scanning experiments have been carried out. 1. the effect of a prestrain insert at room temperature, 2. the effect of the temperature with in-situ heating experiment at 3 different temperature (25°C, 50°C, 70°C), 3. the effect of stress and t...

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations, 2009

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testin... more High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni-Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni-Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni-Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni-Ti wire are comparable to those observed in straight annealed wires.

Acta Crystallographica Section A Foundations of Crystallography, 2007

If necessary NiTi elements can be shape set at temperatures lower than 300oC. This is particularl... more If necessary NiTi elements can be shape set at temperatures lower than 300oC. This is particularly of interest for the development of hybrid NiTi-polyester medical devices which can not be shape set at higher temperatures since the polyester filaments would be damaged. A method how to do that is presented in this work. The results of dedicated thermomechanical tests combined with in-situ electrical resistivity, synchrotron X-ray and TEM measurements during shape setting performed when developing the method are discussed and examples of shape setting of textiles are given.

Materials Science Forum, 2006

This paper focuses on the study of the superelastic behavior associated to the stress induced mar... more This paper focuses on the study of the superelastic behavior associated to the stress induced martensite transformation in a Cu-12.5%Al-0.5%Be [wt. %] shape memory alloy. Neutron diffraction was used to track the evolution of stress in the (β1) austenitic phase during the onset of the stress-induced martensite phase change. A thin flat and a cylindrical specimen was analyzed, allowing us firstly to evaluate the stress evolution in the austenite phase during martensitic transformation with laboratory X-ray and neutron diffraction and secondly to compare differences between methods (sin2ψ, principal stress) for in-situ neutron diffraction experiments.

Surface and Coatings Technology, 2006

TiN/TiCN films were prepared on AZ31 by physical vapor deposition. Structural features and residu... more TiN/TiCN films were prepared on AZ31 by physical vapor deposition. Structural features and residual stress were determined by X-ray fluorescence (XRF), scanning electron microscopy (SEM) and based on the grazing incidence X-ray diffraction (GIXRD), respectively. Results show that films have (111) preferred orientation. With a increasing of the reaction gas ratio, the C þ N and Ti content increases first then decreases, and reach at the maximum value when the gas ratio is 15/5/3. Film have uniform thickness, and the hole shape defects appears in partial film and non-uniformly distribute on the surface of the film. The residual stress was tensile stress, and the residual tensile stress value decreased with the increase of the reaction gas ratio.

Review of Scientific Instruments, 2005

An ISO (International Organization for Standardization) TTA (Technology Trends Assessment) was pu... more An ISO (International Organization for Standardization) TTA (Technology Trends Assessment) was published in 2001 for the determination of residual stress using neutron diffraction which identifies sample alignment and positioning as a key source of strain measurement error. Although the measurement uncertainty by neutron and synchrotron x-ray diffraction for an individual measurement of lattice strain is typically of the order of 10–100×10−6, specimens commonly exhibit strain gradients of 1000×10−6mm−1 or more, making sample location a potentially considerable source of error. An integrated approach to sample alignment and positioning is described which incorporates standard base-plates and sample holders, instrument alignment procedures, accurate digitization using a coordinate measuring machine and automatic generation of instrument control scripts. The methodology that has been developed is illustrated by the measurement of the transverse residual strain field in a welded steel T...

Materials Science and Engineering: A, 2012

OATAO is an open access repository that collects the work of Toulouse researchers and makes it fr... more OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

Journal of Neutron Research, 2003

The Facility for Materials Engineering (FaME38) was inaugurated on the joini ILL-ESRF site in Nov... more The Facility for Materials Engineering (FaME38) was inaugurated on the joini ILL-ESRF site in November 2002 to encourage and lo support Ihe use of neutron.s atid synchrotron X-rays for materials engineering research. FaME3!^ actively eiicourage.s new engineering users and provides the extra support that they may require for all stages tif their research, its technical centre provides a location where engineers and scientists, research students and support slaff, can work and meet. Its laboralory i.s equipped with a coordinate measuring machine (CMM) and a scan simulator. A fatigue test machine and micro-stnictural characterisation facilities will he provided in 2(X)4, FaME38 also provides sample environments for use in situ on the beam-lines including a standard mounting system based on Ihe recommendation given in the ISO-VAMAS neutron diffraction standard [Technology Trends Asses.smenl (2001) "Polycrystalline materials-determination of residual stresses by neutron diffraciion". ISO/7TA 3). Through the use of standard hardware and engineer-friendly software interfaces and off-line sample shape delerminalion, mounting and alignment, user and beam-time efficiency is being dramatically improved.

Functional Materials Letters, 2009

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testin... more High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni – Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni – Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni – Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni – Ti wire are comparable to...

Advances in Science and Technology, 2008

In order to better understand the unique functional responses of shape memory alloys, improve the... more In order to better understand the unique functional responses of shape memory alloys, improve the currently existing SMA modeling tools and used them beneficially in smart structure applications, it is desirable to investigate the deformation/transformation processes in these materials in action – i.e. under stress and temperature variation. In this work, an overview is presented on the applications of various recently developed or originally employed in-situ experimental methods and approaches to martensitic transformations in SMAs.

Acta Materialia, 2010

Transmission electron microscopy, electrical resistivity measurements and mechanical testing were... more Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1 mm diameter as-drawn Ni-Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20-50 nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200 nm.

Acta Materialia, 2011

Microstructure changes taking place during the heat treatment of cold-worked NiTi alloy are of ke... more Microstructure changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in SMA technology since they are responsible for setting the austenite shape and functional properties of the heat treated alloy. Although these microstructure changes probably occur with extremely fast kinetics, conventional heat treatments of NiTi in a furnace commonly last for several minutes or hours. Short heat treatments can be performed e.g. via Joule heating by electric current or by laser treatments. In this work, microstructure changes taking place during non-conventional fast heat treatment of thin NiTi filaments by short pulse of controlled electric power were investigated by high speed in-situ synchrotron X-ray diffraction with simultaneous evaluation of tensile force and electrical resistance of the treated wire. The results provide direct experimental evidence on the evolution of the internal strain (stress) and density of defects during the fast heating of the wire from 20 o C to ~700 o C. This evidence is used, together with the earlier reported results of TEM investigations of microstructures in treated wires, to identify a sequence of dynamic processes responsible for the microstructure changes during the fast heating. Based on this knowledge an interpretation of the experimentally recorded variation of the tensile stress and electric resistance during the treatment is proposed.

Materials Science and Engineering: A, 2010

The present work deals with a non-conventional application of multifunctional materials such as s... more The present work deals with a non-conventional application of multifunctional materials such as shape memory alloy in engineering components. The concept of active inserts has been adopted in order to redistribute compressive stresses emerged in cutting disc during its operation. According to the present design, the small pre-strained elliptical NiTi elements were placed into openings of steel cutting disc in places with expected maximum stress concentration. To study the stress interaction of the NiTi inserts with steel matrix in detail, the in-situ method of neutron diffraction was employed for residual stress mapping. The diffraction experiments were focused substantially on scan of internal stresses around inserts and their evolution with increased temperature. The performed studies confirm the potential ability of NiTi insert to induce the compressive stress within steel matrix with applied temperature.

Materials Research Proceedings

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC.

X-Rays in Mechanical Engineering Applications, 2004

Large Central Scientific Facilities such as the ESRF (the European Synchrotron Radiation Facility... more Large Central Scientific Facilities such as the ESRF (the European Synchrotron Radiation Facility) and ILL (the European centre for neutron research), were set up to provide scientists with the advanced facilities they need to exploit neutron and synchrotron X-ray beams for scientific research. Engineers also conduct research at these Facilities, but this is less common as most practicing engineers generally have little or no knowledge of neutron or X-ray scattering, or of their considerable potential for engineering research, model validation, material development and for fatigue and failure analysis. FaME38 is the new joint support Facility for Materials Engineering, located at ILL-ESRF, set up to encourage and to facilitate engineering research by engineers at these facilities. It provides a technical and knowledge centre, a materials support laboratory, and the additional equipment and resources that academic and industrial engineers need for materials engineering research to be...

Materials Science Forum, 2005

FaME38 is a new facility at the ILL/ESRF in Grenoble with the aim of improving the accessibility ... more FaME38 is a new facility at the ILL/ESRF in Grenoble with the aim of improving the accessibility and effectiveness of neutron and synchrotron strain measurements. In addition to providing basic materials engineering facilities, it enables users from both commercial and academic backgrounds to prepare and to evaluate experiments on-site. The real success and impact of a strain scanning experiment depends on the quality of the collected data and its practical use. FaME38 provides a knowledge base and tools which can increase the efficiency of the measurement. These tools include a VAMAS standard sample base-plate, a materials support laboratory and enhanced visualisation software. The VAMAS base-plate allows pre-configuration of the sample position and set-up, as well as so-called “hot-swapping” of samples with minimum time required for re-configuration of the instrument. The visualisation tools feature web-based simulation and, in particular, 3D visualisation of both the experimental environment as well as the data. The use of the support facility can significantly reduce the set-up time, thus increasing the time available for measurement. The visualisation naturally enhances the understanding of the data and ties in with existing engineering code such as CAD/FEA software. We demonstrate how the use of additional technology can improve the effectiveness and impact of strain mapping experiments at neutron and synchrotron facilities by disseminating the workflow of a typical experiment undertaken in the FaME38 framework. This approach is aimed at paving the way toward technology-oriented application of synchrotron and neutron strain scanning.

Physical processes taking place during the final heat treatment of cold worked NiTi elements give... more Physical processes taking place during the final heat treatment of cold worked NiTi elements gives them parent austenite shape, microstructure and functional properties as shape memory or superelasticity. Since the heat treatment in large extent controls the functional properties of NiTi, these processes are of significant application interest but, surprisingly, very little is known about them. In this work, we will discuss the activity of physical processes taking place during the heat treatment performed on as drawn thin NiTi wires (d=0.1 mm) by passing electric current of constant power through them. The discussion is based on the results of: i) in-situ electrical resistance measurements during annealing using dedicated experimental equipment allowing to measure simultaneously the evolution of stress, strain, electric resistance, temperature on the NiTi wire exposed to high temperatures and stresses induced by electric power pulse, ii) in-situ synchrotron powder diffraction exper...

This paper reports selected results of the research motivated by SMA application development in f... more This paper reports selected results of the research motivated by SMA application development in frame of European research project PROSTONE aimed at innovation in stone processing equipment. In many devices used in the stone sector, stress inhomogeneity in machine parts and tools seems to cause problems. There is an idea to solve the problem of stress concentrations in metallic sheets occurring during operation through the insertion of small prestrained elliptical NiTi elements into elliptical holes in locations of maximum stress concentration which would counteract these stress concentrations. In order to evaluate the effect of the temperature, compression force and on the stress distribution in the steel plate, three different neutrons strain scanning experiments have been carried out. 1. the effect of a prestrain insert at room temperature, 2. the effect of the temperature with in-situ heating experiment at 3 different temperature (25°C, 50°C, 70°C), 3. the effect of stress and t...

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations, 2009

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testin... more High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni-Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni-Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni-Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni-Ti wire are comparable to those observed in straight annealed wires.

Acta Crystallographica Section A Foundations of Crystallography, 2007

If necessary NiTi elements can be shape set at temperatures lower than 300oC. This is particularl... more If necessary NiTi elements can be shape set at temperatures lower than 300oC. This is particularly of interest for the development of hybrid NiTi-polyester medical devices which can not be shape set at higher temperatures since the polyester filaments would be damaged. A method how to do that is presented in this work. The results of dedicated thermomechanical tests combined with in-situ electrical resistivity, synchrotron X-ray and TEM measurements during shape setting performed when developing the method are discussed and examples of shape setting of textiles are given.

Materials Science Forum, 2006

This paper focuses on the study of the superelastic behavior associated to the stress induced mar... more This paper focuses on the study of the superelastic behavior associated to the stress induced martensite transformation in a Cu-12.5%Al-0.5%Be [wt. %] shape memory alloy. Neutron diffraction was used to track the evolution of stress in the (β1) austenitic phase during the onset of the stress-induced martensite phase change. A thin flat and a cylindrical specimen was analyzed, allowing us firstly to evaluate the stress evolution in the austenite phase during martensitic transformation with laboratory X-ray and neutron diffraction and secondly to compare differences between methods (sin2ψ, principal stress) for in-situ neutron diffraction experiments.

Surface and Coatings Technology, 2006

TiN/TiCN films were prepared on AZ31 by physical vapor deposition. Structural features and residu... more TiN/TiCN films were prepared on AZ31 by physical vapor deposition. Structural features and residual stress were determined by X-ray fluorescence (XRF), scanning electron microscopy (SEM) and based on the grazing incidence X-ray diffraction (GIXRD), respectively. Results show that films have (111) preferred orientation. With a increasing of the reaction gas ratio, the C þ N and Ti content increases first then decreases, and reach at the maximum value when the gas ratio is 15/5/3. Film have uniform thickness, and the hole shape defects appears in partial film and non-uniformly distribute on the surface of the film. The residual stress was tensile stress, and the residual tensile stress value decreased with the increase of the reaction gas ratio.

Review of Scientific Instruments, 2005

An ISO (International Organization for Standardization) TTA (Technology Trends Assessment) was pu... more An ISO (International Organization for Standardization) TTA (Technology Trends Assessment) was published in 2001 for the determination of residual stress using neutron diffraction which identifies sample alignment and positioning as a key source of strain measurement error. Although the measurement uncertainty by neutron and synchrotron x-ray diffraction for an individual measurement of lattice strain is typically of the order of 10–100×10−6, specimens commonly exhibit strain gradients of 1000×10−6mm−1 or more, making sample location a potentially considerable source of error. An integrated approach to sample alignment and positioning is described which incorporates standard base-plates and sample holders, instrument alignment procedures, accurate digitization using a coordinate measuring machine and automatic generation of instrument control scripts. The methodology that has been developed is illustrated by the measurement of the transverse residual strain field in a welded steel T...

Materials Science and Engineering: A, 2012

OATAO is an open access repository that collects the work of Toulouse researchers and makes it fr... more OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

Journal of Neutron Research, 2003

The Facility for Materials Engineering (FaME38) was inaugurated on the joini ILL-ESRF site in Nov... more The Facility for Materials Engineering (FaME38) was inaugurated on the joini ILL-ESRF site in November 2002 to encourage and lo support Ihe use of neutron.s atid synchrotron X-rays for materials engineering research. FaME3!^ actively eiicourage.s new engineering users and provides the extra support that they may require for all stages tif their research, its technical centre provides a location where engineers and scientists, research students and support slaff, can work and meet. Its laboralory i.s equipped with a coordinate measuring machine (CMM) and a scan simulator. A fatigue test machine and micro-stnictural characterisation facilities will he provided in 2(X)4, FaME38 also provides sample environments for use in situ on the beam-lines including a standard mounting system based on Ihe recommendation given in the ISO-VAMAS neutron diffraction standard [Technology Trends Asses.smenl (2001) "Polycrystalline materials-determination of residual stresses by neutron diffraciion". ISO/7TA 3). Through the use of standard hardware and engineer-friendly software interfaces and off-line sample shape delerminalion, mounting and alignment, user and beam-time efficiency is being dramatically improved.

Functional Materials Letters, 2009

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testin... more High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni – Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni – Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni – Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni – Ti wire are comparable to...

Advances in Science and Technology, 2008

In order to better understand the unique functional responses of shape memory alloys, improve the... more In order to better understand the unique functional responses of shape memory alloys, improve the currently existing SMA modeling tools and used them beneficially in smart structure applications, it is desirable to investigate the deformation/transformation processes in these materials in action – i.e. under stress and temperature variation. In this work, an overview is presented on the applications of various recently developed or originally employed in-situ experimental methods and approaches to martensitic transformations in SMAs.

Acta Materialia, 2010

Transmission electron microscopy, electrical resistivity measurements and mechanical testing were... more Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1 mm diameter as-drawn Ni-Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20-50 nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200 nm.

Acta Materialia, 2011

Microstructure changes taking place during the heat treatment of cold-worked NiTi alloy are of ke... more Microstructure changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in SMA technology since they are responsible for setting the austenite shape and functional properties of the heat treated alloy. Although these microstructure changes probably occur with extremely fast kinetics, conventional heat treatments of NiTi in a furnace commonly last for several minutes or hours. Short heat treatments can be performed e.g. via Joule heating by electric current or by laser treatments. In this work, microstructure changes taking place during non-conventional fast heat treatment of thin NiTi filaments by short pulse of controlled electric power were investigated by high speed in-situ synchrotron X-ray diffraction with simultaneous evaluation of tensile force and electrical resistance of the treated wire. The results provide direct experimental evidence on the evolution of the internal strain (stress) and density of defects during the fast heating of the wire from 20 o C to ~700 o C. This evidence is used, together with the earlier reported results of TEM investigations of microstructures in treated wires, to identify a sequence of dynamic processes responsible for the microstructure changes during the fast heating. Based on this knowledge an interpretation of the experimentally recorded variation of the tensile stress and electric resistance during the treatment is proposed.

Materials Science and Engineering: A, 2010

The present work deals with a non-conventional application of multifunctional materials such as s... more The present work deals with a non-conventional application of multifunctional materials such as shape memory alloy in engineering components. The concept of active inserts has been adopted in order to redistribute compressive stresses emerged in cutting disc during its operation. According to the present design, the small pre-strained elliptical NiTi elements were placed into openings of steel cutting disc in places with expected maximum stress concentration. To study the stress interaction of the NiTi inserts with steel matrix in detail, the in-situ method of neutron diffraction was employed for residual stress mapping. The diffraction experiments were focused substantially on scan of internal stresses around inserts and their evolution with increased temperature. The performed studies confirm the potential ability of NiTi insert to induce the compressive stress within steel matrix with applied temperature.