Shape memory materials are a class of smart materials able to convert heat into mechanical strain (or strain into heat) by virtue of a martensitic phase transformation. Some brittle materials such as intermetallics and ceramics exhibit a martensitic transformation but fail by cracking at low strains and after only a few applied strain cycles. Here we show that such failure can be suppressed in normally brittle martensitic ceramics by providing a fine-scale structure with few crystal grains. Such oligocrystalline structures reduce internal mismatch stresses during the martensitic transformation and lead to robust shape memory ceramics that are capable of many superelastic cycles up to large strains; here we describe samples cycled as many as 50 times and samples that can withstand strains over 7%. Shape memory ceramics with these properties represent a new class of actuators or smart materials with a set of properties that include high energy output, high energy damping, and high-temperature usage.

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Shape Memory and Superelastic Ceramics at Small Scales

An introduction to tetragonal (t) → monoclinic (m) martensitic transformation in zirconia containing ceramics, especially tetragonal zirconia polycrystalline (TZP) was presented. Thermodynamics, crystallographics and kinetics of t → m martensitic transformation in TZP were emphasized. Transformation toughening and shape memory effect (SME) associated with t → m martensitic transformation in the TZP were reviewed. Perspective of future challenges was briefly mentioned at the end.

Martensitic transformation in zirconia containing ceramics and its applications

The effects of rapid quenching on the transformation behaviour of three melt-spun ribbons of Ni–Mn–Ga alloys has been studied by X-ray and electron diffraction, calorimetry, thermomechanical and magnetisation tests. One step martensitic transformation (MT) has been detected for each ribbon accompanied by the formation of modulated martensitic crystal structures with 7- or 10-layer periods, which differ from those observed for single crystals of the same compositions. The MT temperatures can be raised or lowered by ageing in the parent phase, depending on composition. The ageing also causes a significant increase of both Curie temperature and magnetization in all alloys. These effects indicate the occurrence of some atomic rearrangement processes leading to a more uniform element distribution and, probably, change of atomic order. The activation energies for these processes have been estimated to be between 0.2 and 0.7 eV.

Transformation and ageing behaviour of melt-spun Ni–Mn–Ga shape memory alloys

Instead of conventional quenching martensitic transformation from beta-Ti to alpha-Ti, unusual martensitic transformation from alpha-Ti to beta-Ti induced by rapid heating has been achieved simply by using an electric current pulse. A large amount of the high-temperature martensitic phase remained in a Ti-6Al-4V alloy. We verified that such transformation is structurally and kinetically achievable, which agrees well with the phenomenological crystallographic theory of martensitic transformation. (C) 2004 American Institute of Physics.

Martensitic transformation from α-Ti to β-Ti on rapid heating

The effect of grain boundaries and the crystal surface on the deformation and strength properties of nanocrystalline and nanosized materials, primarily metals, is reviewed Basic experimental facts about the mechanical behavior of nanocrystal materials are presented and the observed findings are theoretically analyzed using equations of dislocation kinetics that take the properties of grain boundaries into account as sources, sinks, and dislocation barriers Using a similar approach, size effects in the plastic deformation of micro- and nanocrystals are discussed by considering the crystal surface as a primary source of and sink for dislocations

http://iopscience.iop.org/article/10.3367/UFNe.0181.201111a.1129/pdf

Strength and plasticity of nanocrystalline materials and nanosized crystals

The relationship between the starting temperature of the martensitic transformation, M
s, and the grain size of the parent phase, d, in ZrO2-containing ceramics was investigated. The experimental results showed that in tetragonal zirconia polycrystals doped with CeO2 (8 mol%) and Y2O3 (0.25 mol%) (8Ce, 0.25Y-TZP), the M
s temperature displays a linear relationship with d
−1/2, its slope being negative. A new explanation for this phenomenon, the so-called the size effect, has been presented, in which the grain size of the parent phase affects the M
s temperature through the strength of the parent phase. Thermodynamic calculation of the relationship between M
s and d gives a result consistent with the experimental ones.

The size effect of the martensitic transformation in ZrO2-containing ceramics

Vacancy behavior in Cu-26 Zn-4 Al and Cu-14 Al-4 Ni alloys during ageing is investigated and compared by means of positron annihilation (PA) and electrical resistivity measurements. For ageing in the martensitic state after direct quenching, the experimental results show that the S parameter of CuZnAl specimens, measured at liquid nitrogen temperature increases at first and then decreases, while that of CuAlNi specimens remains unchanged. The different behavior of quenched-in vacancies in these two alloys reveals that the clustering of quenched-in vacancies is responsible for the stabilization of martensite at the early stage in CuZnAl alloys. The reason of the insensibility to stabilization for the CuAlNi alloy is discussed.



Das Verhalten von Leerstellen in Cu–26 Zn-4 Al- und Cu-14 Al-4 Ni-Legierungen wahrend des Alterns wird mittels Positronenannihilation und elektrischer Widerstandsmessungen untersucht und verglichen. Bei Altern im Martensit-Zustand nach direktem Abschrecken nimmt der bei der Temperatur des flussigen Stickstoffs gemessene S-Parameter von CuZnAl zunachst zu und dann ab, wahrend in CuAlNi-Proben S unverandert bleibt. Das unterschiedliche Verhalten der Leerstellen in diesen beiden Legierungen zeigt, das Clusterbildung im Anfangszustand fur die Martensit-Stabilisierung in CuZnAl-Legierungen verantwortlich ist. Der Grund fur die Unempfindlichkeit von CuAlNi-Legierungen gegenuber einer Stabilisierung wird diskutiert.

The Behavior of Quenched‐in Vacancies and Stabilization of Martensite in Copper‐Based Shape Memory Alloys

The Recovery Behavior of Quenched‐in Vacancies in CuZnAl Alloy

As known to all,shape memory behaviour has been observed in a variety of metallic alloys. This phenomenon, however, is not only observed in that field but also in other materials recently, such as in polymers,intermetallic compounds and ceramics, especially in zirconia ceramics. Swain [1] observed the shape recovery of a bent magnesia-partiallystabilized zirconia (Mg-PSZ) bar upon heating above a certain temperature. The maximum deflection which was nearly completely recovered on heating is about 300 μm. Chen et al [2] investigated the pseudoelasticity and shape memory effect (SME) in ceria-stabilized tetragonal zirconia polycrystals (Ce- TZP) containing 12 mol% CeO 2 .Wang et al [3] also observed the SME in Ce-TZP containing 10 mol% CeO 2 ,.Here we would like to introduce some of results conducted recently in our group on SHE in Ce-TZP ceramics.

The Effect of Some Factors on Ms and SME in Ce-TZP Ceramics

Vacancy behaviors during ageing of Cu-26Zn-4A1 and Cu-14Al-4Ni alloys have been investigated and compared by means of positron annihilation (PA) and electrical resistivity measurement. For ageing in martensitic state after direct quenching, it is observed that the S parameter values of Cu-Zn- Al specimens, measured in liquid nitrogen, increase at first and then decrease, while those of Cu-Al-Ni remain unchanged. The activation energies calculated from the S parameter for increasing and decreasing stages are o.4lev and o.63ev respectively, and the former can be corresponding to the formation energy of vacancy clustering, while the latter may be regarded as the migration energy of effective vacancies. A mechanism is put forward that the clustering of quenched-in vacancies results in a decreasing of the ordering degree and a reduction of the stored energy in martensite, which is responsible for the early stage of the stabilization of martensite in Cu-Zn-Al alloys. However, the fact that Cu-Al-Ni alloy is not subject to the stabilization is assumed to be owing to the immobility of supersaturated vacancies in its martensitic state which may be associated with the strong binding force between Ni and Al atoms.

Bohong Jiang

Papers

The size effect of the martensitic transformation in ZrO2-containing ceramics

The Behavior of Quenched‐in Vacancies and Stabilization of Martensite in Copper‐Based Shape Memory Alloys

The Recovery Behavior of Quenched‐in Vacancies in CuZnAl Alloy

The Effect of Some Factors on Ms and SME in Ce-TZP Ceramics

Effect of Vacancy Clustering on Stabilization of Martensite in Copper Based Shape Memory Alloys