Xiaobing Ren

Recent developments in the research of shape memory alloys

Most of shape memory alloys are functional intermetallics. They are now practically being used for couplings, actuators, medical guide wires etc., and are hopeful candidates for smart materials, which already exist. In the present paper, recent developments within nearly 10 years on shape memory alloys and martensitic: transformations, on which shape memory eAect and superelasticity are based, were concisely reviewed. Since Ti‐Ni alloys are the best practical shape memory alloys, we mostly discussed on the alloys, but we discussed more general problems as well. Furthermore, we discussed the ductility and density of point defects in intermetallics, since they are important problems in intermetallics in general. At the end we introduced some recent applications of shape memory alloys briefly. # 1999 Elsevier Science Ltd. All rights reserved.

The mechanism of multistage martensitic transformations in aged Ni-rich NiTi shape memory alloys

Abstract Usually aged Ni-rich NiTi alloys undergo martensitic transformations on cooling from high temperatures in two steps: B2 to R and then R to B19′ (normal behaviour). But under certain ageing conditions, the transformation can also occur in three or more steps (unusual multiple step behaviour). In the present study we use differential scanning calorimetry (DSC) for a systematic investigation of the evolution of transformation behaviour with ageing temperature and time. We demonstrate that during ageing of Ni-rich NiTi alloys, DSC curves exhibit two transformation peaks on cooling after short ageing times, three after intermediate ageing times and finally again two peaks after long ageing times (2–3–2 transformation behaviour). In the present study we propose a new explanation for the 2–3–2 transformation behaviour which consists of two basic elements: (1) The composition inhomogeneity which evolves during ageing as Ni 4 Ti 3 precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19′ (large). These two elements explain all features of the evolution of DSC charts during ageing including the number of distinct DSC peaks and their positions.

Elastic, piezoelectric, and dielectric properties of Ba(Zr0.2Ti0.8)O3- 50(Ba0.7Ca0.3)TiO3 Pb-free ceramic at the morphotropic phase boundary

There is an urgent demand for high performance Pb-free piezoelectrics to substitute for the current workhorse, the lead zirconate titanate (PZT) family. Recently, a triple point (also tricritical point) type morphotropic phase boundary (MPB) in Pb-free Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 system has been reported that shows equally as excellent piezoelectricity as soft PZT at room temperature (Liu and Ren6). In the present study, we measured a full set of elastic, piezoelectric, and dielectric properties for the MPB composition, Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 (BZT-50BCT), by using a resonance method. The resonant method gives piezoelectric properties d33 = 546 pC/N, g33 = 15.3 × 10−3 Vm/N, electromechanical coupling factor k33 = 65%, and the elastic constant s33E = 19.7 × 10−12 m2/N, c33E = 11.3 × 1010 N/m2, which are close to the properties of soft PZT (PZT-5A). Furthermore, the piezoelectric coefficients (k33, d33), the ferroelectric properties (coercive field, remnant polarization), and the elastic ...

Origin of rubber-like behaviour in metal alloys

Since 1932 it has been known that a number of ordered alloys show an unusual kind of deformation behaviour. These alloys (including Au–Cd, Au–Cu–Zn, Cu–Zn–Al, Cu–Al–Ni), after being aged for some time in a martensitic state (the low-symmetry phase of a diffusionless transformation), can be deformed like a soft and pseudo-elastic rubber (with a recoverable strain as large as a few per cent). Accompanying martensite ageing is the development of martensite stabilization (increase in the temperature of reverse transformation to the parent state), the avoidance of which is important in actuator applications of the shape-memory effect, (which these alloys also generally exhibit. The origin of this rubber-like behaviour and of the ageing effect has remained unclear. Here we show that this behaviour does not involve a change in the degree of long-range order, but is instead due to an atomic rearrangement within the same sublattice of the imperfectly ordered alloy during martensite ageing. This process is driven by a general tendency for the equilibrium symmetry of the short-range order configuration of lattice imperfections to conform to the symmetry of the lattice. This principle not only explains all the observed aspects of the rubber-like behaviour and the ageing effect in both ordered and disordered alloys, but may also further our understanding of some diffusion phenomena in other crystalline materials.

A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

Abstract Single crystal elastic constants of Ti–Ni alloys without (quenched) and with (aged) Ti 3 Ni 4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti–Ni–Cu and Ti–Ni–Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19′ martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c 44 , in additional to c ′, is a common feature for all Ti–Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19′, suggesting that softening in c 44 is responsible for the unique B19′ martensite, which found no analogy in other β phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2–B19′ transformation anisotropy shows a decrease with lowering temperature; prior to B2–B19 an anisotropy increase, while prior to B2–R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19′) are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti–Ni-based alloys: B2–R–B19–B19′.

Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics

In this letter, we use transmission electron microscopy to study the microstructure feature of recently reported Pb-free piezoceramic Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 across its piezoelectricity-optimal morphotropic phase boundary (MPB) by varying composition and temperature, respectively. The domain structure evolutions during such processes show that in MPB regime, the domains become miniaturized down to nanometer size with a domain hierarchy, which coincides with the d33-maximum region. Further convergent beam electron diffraction measurement shows that rhombohedral and tetragonal crystal symmetries coexist among the miniaturized domains. Strong piezoelectricity reported in such a system is due to easy polarization rotation between the coexisting nano-scale tetragonal and rhombohedral domains.

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

We designed a Pb-free pseudo-binary system, Ba(Sn0.12Ti0.88)O3-x(Ba0.7Ca0.3)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases. The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 � 530 pC/N at room temperature. In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics. V C 2011 American Institute of Physics.

Large recoverable electrostrain in Mn-doped (Ba,Sr) TiO3 ceramics

In this letter we demonstrate that with a different principle, BaTiO3 ceramics, so far considered as inferior piezoelectrics compared with Pb(Zr,Ti)O3 (PZT), can show a large recoverable electrostrain. This principle utilizes a point-defect-mediated reversible domain switching mechanism, which can in theory generate 0.368% strain for BaTiO3 ceramics at the best condition. Experimental results showed that, after aging at room temperature, 1.0mol% Mn-doped (Ba0.95Sr0.05)TiO3 ceramics generate a large recoverable nonlinear strain of about 0.12%–0.15% at a field of 3kV∕mm. This value exceeds that of conventional hard PZT piezoelectric ceramics. A microscopic model for the domain-related electrostrain effect in ceramics is proposed. It is also found that the large electrostrain effect is quite stable with respect to both changing frequency and fatigue cycles. Large electrostrain remains recoverable down to 0.05Hz and after 10000cycles. These results demonstrate the potential of our approach in achieving large ...

A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength

S-T-R-E-T-C-H Me Most metals show elastic strain limits well below 1%, beyond which permanent plastic deformation occurs. Metal nanowires can be elastically stretched to much higher strains, on the order of 4 to 7%. However, when placed inside a metal matrix to form a composite, these nanowires can no longer be stretched to the same extent, even when the nanowires are well distributed and show good bonding with the matrix. Hao et al. (p. 1191; see the Perspective by Zhou) used a shape memory alloy as the matrix material to produce a much better (more elastic) composite. The use of a shape-memory metal alloy as a matrix better exploits the inherent elastic properties of niobium nanowires. [Also see Perspective by Zhou] Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Youngs modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

Triple-point-type morphotropic phase boundary based large piezoelectric Pb-free material—Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3

We report a large piezoelectric Pb-free system—Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3, designed upon a triple-point morphotropic phase boundary (TMPB) idea. The system shows anomalies of both ac (dielectric and piezoelectric) and dc (P-E hysteresis) properties around TMPB, especially the d33 achieving � 550 pC/N at room temperature. Moreover, the detected non-zero thermal hysteresis around triple point shows a first order transition nature and non-isotropic polarization state, as well as verified by our theoretical Landau-type deduction, thus being considered as an important factor to influence the piezoelectricity along TMPB. Our work may stimulate the study on triple-pointrelated critical phenomena in other ferroic systems. V C 2012 American Institute of Physics.