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	Author	Delville, R.; Malard, B.; Pilch, J.; Sittner, P.; Schryvers, D.
	Title	Transmission electron microscopy study of microstructural evolution in nanograined Ni-Ti microwires heat treated by electric pulse			Type	A1 Journal article
	Year	2011	Publication	Diffusion and defect data : solid state data : part B : solid state phenomena	Abbreviated Journal
	Volume	172/174	Issue		Pages	682-687
	Keywords	A1 Journal article; Electron microscopy for materials research (EMAT)
	Abstract	Transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1mm diameter as-drawn Ni-Ti wires subjected to a non-conventional heat treatment by controlled electric pulse current. This method enables a finer control of the recovery and recrystallisation processes taking place during the heat treatment and accordingly a better control on the final microstructure. The best functional properties were obtained for heat-treated Ni-Ti wires having a nanograined microstructure (20-50 nm) partially recovered through polygonization and partially recrystallized. Such microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer time and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 100 nm. The density of dislocation defects increased significantly with increasing grain size of the microstructure. The activity of three <100>/{011} slip systems was identified in the largest grains of 500-1200 nm. An additional mode of plastic deformation, {114} compound austenite twinning, was observed in the largest grains of fully recrystallized microstructures. It is proposed that dislocation slip (and possibly deformation twinning) occurring in superelastic cycling is coupled with the stress-induced martensitic transformation.
	Address
	Corporate Author				Thesis
	Publisher		Place of Publication	Vaduz	Editor
	Language		Wos	000303359700105	Publication Date	2011-07-04
	Series Editor		Series Title		Abbreviated Series Title
	Series Volume		Series Issue		Edition
	ISSN	1662-9779;	ISBN		Additional Links	UA library record; WoS full record; WoS citing articles
	Impact Factor		Times cited	3	Open Access
	Notes				Approved	Most recent IF: NA
	Call Number	UA @ lucian @ c:irua:90154			Serial	3717
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	Author	Malard, B.; Pilch, J.; Sittner, P.; Delville, R.; Curfs, C.
	Title	In situ investigation of the fast microstructure evolution during electropulse treatment of cold drawn NiTi wires			Type	A1 Journal article
	Year	2011	Publication	Acta materialia	Abbreviated Journal	Acta Mater
	Volume	59	Issue	4	Pages	1542-1556
	Keywords	A1 Journal article; Electron microscopy for materials research (EMAT)
	Abstract	Microstructural changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in shape memory alloy technology, since they are responsible for setting the austenite shape and functional properties of the heat-treated alloy. In this work, microstructural evolution during non-conventional electropulse heat treatment of thin NiTi filaments was investigated in a unique high-speed in situ synchrotron X-ray diffraction experiment with simultaneous evaluation of the tensile force and electrical resistivity of the treated wire. The in situ results provide direct experimental evidence on the evolution of the internal stress and density of defects during fast heating from 20 °C to ∼700 °C. This evidence is used to characterize a sequence of dynamic recovery and recrystallization processes responsible for the microstructure and superelastic functional property changes during the electropulse treatments.
	Address
	Corporate Author				Thesis
	Publisher		Place of Publication	Oxford	Editor
	Language		Wos	000287265100023	Publication Date	2010-12-11
	Series Editor		Series Title		Abbreviated Series Title
	Series Volume		Series Issue		Edition
	ISSN	1359-6454;	ISBN		Additional Links	UA library record; WoS full record; WoS citing articles
	Impact Factor	5.301	Times cited	48	Open Access
	Notes				Approved	Most recent IF: 5.301; 2011 IF: 3.755
	Call Number	UA @ lucian @ c:irua:98372			Serial	1583
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	Author	Delville, R.; Malard, B.; Pilch, J.; Sittner, P.; Schryvers, D.
	Title	Transmission electron microscopy investigation of dislocation slip during superelastic cycling of NiTi wires			Type	A1 Journal article
	Year	2011	Publication	International journal of plasticity	Abbreviated Journal	Int J Plasticity
	Volume	27	Issue	2	Pages	282-297
	Keywords	A1 Journal article; Electron microscopy for materials research (EMAT)
	Abstract	Superelastic deformation of thin NiTi wires containing various nanograined microstructures was investigated by tensile cyclic loading with in situ evaluation of electric resistivity. Defects created by the superelastic cycling in these wires were analyzed by transmission electron microscopy. The role of dislocation slip in superelastic deformation is discussed. NiTi wires having finest microstructures (grain diameter <100 nm) are highly resistant against dislocation slip, while those with fully recrystallized microstructure and grain size exceeding 200 nm are prone to dislocation slip. The density of the observed dislocation defects increases significantly with increasing grain size. The upper plateau stress of the superelastic stressstrain curves is largely grain size independent from 10 up to 1000 nm. It is hence claimed that the HallPetch relationship fails for the stress-induced martensitic transformation in this grain size range. It is proposed that dislocation slip taking place during superelastic cycling is responsible for the accumulated irreversible strains, cyclic instability and degradation of functional properties. No residual martensite phase was found in the microstructures of superelastically cycled wires by TEM and results of the in situ electric resistance measurements during straining also indirectly suggest that none or very little martensite phase remains in the studied cycled superelastic wires after unloading. The accumulation of dislocation defects, however, does not prevent the superelasticity. It only affects the shape of the stressstrain response, makes it unstable upon cycling and changes the deformation mode from localized to homogeneous. The activity of dislocation slip during superelastic deformation of NiTi increases with increasing test temperature and ultimately destroys the superelasticity as the plateau stress approaches the yield stress for slip. Deformation twins in the austenite phase ({1 1 4} compound twins) were frequently found in cycled wires having largest grain size. It is proposed that they formed in the highly deformed B19′ martensite phase during forward loading and are retained in austenite after unloading. Such twinning would represent an additional deformation mechanism of NiTi yielding residual irrecoverable strains.
	Address
	Corporate Author				Thesis
	Publisher		Place of Publication	Oxford	Editor
	Language		Wos	000284921800007	Publication Date	2010-05-17
	Series Editor		Series Title		Abbreviated Series Title
	Series Volume		Series Issue		Edition
	ISSN	0749-6419;	ISBN		Additional Links	UA library record; WoS full record; WoS citing articles
	Impact Factor	5.702	Times cited	157	Open Access
	Notes	Fwo; Iap			Approved	Most recent IF: 5.702; 2011 IF: 4.603
	Call Number	UA @ lucian @ c:irua:84651			Serial	3709
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	Author	Delville, R.; Malard, B.; Pilch, J.; Schryvers, D.
	Title	Microstructure changes during non-conventional heat treatment of thin NiTi wires by pulsed electric current studied by transmission electron microscopy			Type	A1 Journal article
	Year	2010	Publication	Acta materialia	Abbreviated Journal	Acta Mater
	Volume	58	Issue	13	Pages	4503-4515
	Keywords	A1 Journal article; Electron microscopy for materials research (EMAT)
	Abstract	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 NiTi 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 (2050 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.
	Address
	Corporate Author				Thesis
	Publisher		Place of Publication	Oxford	Editor
	Language		Wos	000279787100020	Publication Date	2010-06-02
	Series Editor		Series Title		Abbreviated Series Title
	Series Volume		Series Issue		Edition
	ISSN	1359-6454;	ISBN		Additional Links	UA library record; WoS full record; WoS citing articles
	Impact Factor	5.301	Times cited	110	Open Access
	Notes	Multimat; FWO IAA			Approved	Most recent IF: 5.301; 2010 IF: 3.791
	Call Number	UA @ lucian @ c:irua:83279			Serial	2062
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	Author	Malard, B.; Pilch, J.; Sittner, P.; Gartnerova, V.; Delville, R.; Schryvers, D.; Curfs, C.
	Title	Microstructure and functional property changes in thin Ni-Ti wires heat teated by electric current: high energy X-ray and TEM investigations			Type	A1 Journal article
	Year	2009	Publication	Functional materials letters	Abbreviated Journal	Funct Mater Lett
	Volume	2	Issue	2	Pages	45-54
	Keywords	A1 Journal article; Electron microscopy for materials research (EMAT)
	Abstract	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 NiTi wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn NiTi 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 NiTi 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 NiTi wire are comparable to those observed in straight annealed wires.
	Address
	Corporate Author				Thesis
	Publisher		Place of Publication		Editor
	Language		Wos	000271077000001	Publication Date	2009-07-22
	Series Editor		Series Title		Abbreviated Series Title
	Series Volume		Series Issue		Edition
	ISSN	1793-6047;1793-7213;	ISBN		Additional Links	UA library record; WoS full record; WoS citing articles
	Impact Factor	1.234	Times cited	21	Open Access
	Notes	Multimat			Approved	Most recent IF: 1.234; 2009 IF: 2.561
	Call Number	UA @ lucian @ c:irua:77656			Serial	2052
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