L. Delaey

Asymmetry of stress–strain curves under tension and compression for NiTi shape memory alloys

Abstract The stress–strain curves of polycrystalline martensitic NiTi shape memory alloys are often different for loading under tension and compression. Under tension, a flat stress-plateau occurs, while under compression, the material is quickly strain hardened and no flat stress-plateau is observed. Cyclic deformation under tension–compression also shows that it is more difficult to deform the material during compression than during tension, where an asymmetric stress–strain loop is obtained. TEM observations show that, under tension to 4% strain, martensite variants are partially reoriented via migration of variant interfaces with formation of dislocation networks mainly along the junction plane areas, and no significantly plastic deformation has been observed inside the martensite twin bands. While under compression to 4% strain, a high density of dislocations has been generated in both the martensite twin bands and the variant accommodation area, and no significant martensite reorientation via variant interfacial migration has been observed. This shows that the deformation mechanism of martensitic polycrystalline NiTi SMAs under tension is different from that under compression.

Some aspects of the properties of NiTi shape memory alloy

Abstract The damping capacity, tensile behaviour and the transformation characteristics of a NiTi shape memory alloy rolled sheet as a function of annealing conditions have been studied. Results show that both the martensite phase damping capacity and the stress for martensite phase reorientation are a function of annealing temperature. Attention has also been paid to the correlation between the damping capacity of the shape memory alloy and its other properties. Experimental results show that a higher damping of the martensite phase generally corresponds to a lower reorientation stress in the stress-strain curve, suggesting that the martensite phase damping capacity and the stress for martensite phase reorientation are interrelated. This result may provide the possibility of predicting one shape memory property from others in the same alloy.

Thermoelasticity, pseudoelasticity and the memory effects associated with martensitic transformations: Part 3 Thermodynamics and kinetics

The pertinent thermodynamic relations, nucleation and growth and the influence of stress are considered and applied to thermoelasticity and the memory effects.

Sol-gel preparation of high-Tc Bi-Ca-Sr-Cu-O and Y-Ba-Ca-O superconductors

High‐Tc superconducting oxides have been prepared by a liquid‐mix technique using etylene‐diamine‐tetra‐acetic acid (EDTA) as a complexing agent. Bi‐Sr‐Ca‐Cu oxides and Y‐Ba‐Cu oxides were made using this technique giving a better compositional homogeneity, a more precise control of the cation stoichiometry and a decreased firing temperature as compared to conventional material produced by a solid‐state reaction. This technique extends the amorphous citrate process to those systems where no citrate complex exists. EDTA binds with most metallic elements of the periodic table, making this technique a versatile tool in the production and study of these new ceramic materials. Therefore, the method is easily adapted to the preparation of new superconducting oxides.

Thermomechanical cycling, two way memory and concomitant effects in Cu-Zn-Al alloys

Abstract The interrelationships between thermomechanical cycling (training), the two way memory effect (TWME) and other, concomitant, effects of training are still points of controversion. A theoretical analysis shows that the results obtained in previous studies are influenced by artifacts resulting from the testing modes. In the present study, those artifacts are avoided by using a uniform deformation mode on a specially designed testing apparatus. The relationships between the training parameters, the TWME and concomitant effects are quantitatively determined. It is shown that the TWME is directly determined by the transformation strain induced during training. Local martensite stabilization, residual deformations of the high temperature shape and shifts of the transformation temperatures show no causal relationship with the TWME and are considered as negative side-effects of training. Those undesirable side-effects can be minimized, without affecting the TWME, by the proper choice of the training parameters. From this, criteria are deduced for the optimum training procedure.

Review: on the structure and microstructure of quenched beta-brass type alloys

Quenched beta-phase alloys in systems based on the noble metals are often thermodynamically metastable. A review of the information derived from transmission electron microscopy indicates the presence of various anomalous contrast and diffraction effects in such alloys. Contrast effects may be referred to as mottling, striation, cross-hatching, etc., and they may change in intensity with respect to different extinction contours, foil thickness and foil tilting. Diffraction effects refer to diffuse streaking in certain directions, the appearance of additional maxima or their splitting, and slight deviations from cubic symmetry. The possible causes for the presence of such features are discussed in the present paper. Some effects are considered to be at least in part the result of surface rippling, caused by electropolishing, but other effects are associated with the presence of precipitates, or with atomic displacements from ideal bcc packing. A detailed analysis of the diffraction effects suggests that structural features must be present analogous to the hexagonal ω-phase (known in transition alloys), and the nearly hexagonal α′ phase (known in bainitic structures). The stability of the beta-phase alloys is considered in terms of the above features, concluding that both vibrational and electronic entropy terms may be contributing, and that instability at low temperatures may be manifested in a number of ways not all of which fit into conventional phase-transformation categories.

The self-accommodating character of the β1 copper-aluminum martensite

The phenomenological martensite theory is applied to the β1 to β1 martensitic transformation in Cu-Al. Crystallographic and morphological aspects of the resulting martensitic microstructure are discussed and verified with X-ray pole figures, which are combined with a single surface trace analysis based on the use of polarized light. From the analysis of the orientation of any single martensite plate related to that of one or more neighboring plates, it can be proved that the martensite microstructure in each former β-grain is composed of at most six self-accommodating martensite plate groups, each of which consists of four different martensite plate variants.

Hysteresis in shape-memory alloys

We present an overview of hysteresis phenomena in the martensitic transformation, and their relevance in the thermomechanical behaviour of shape-memory alloys. The first part of the paper introduces the concept of hysteresis, and the related phenomena of branching, dissipation and memory. The second part deals with revising some aspects of the thermomechanical behaviour of shape-memory alloys, emphasizing the hysteretic behaviour of single crystals and polycrystals under different driving conditions. The last part of the work is dedicated to the problem of modelling hysteresis phenomena in shape-memory alloys. Our focus is on phenomenological approaches which, as shown in the paper, account for the memory properties observed in hysteretic trajectories and open the possibility of deriving a generic energy balance for systems with hysteresis.

Thermodynamic calculation of Nb-Ti-V phase diagram

Abstract Phase equilibrium in Nb-Ti-V system is calculated using thermodynamic descriptions of the lower-order systems. The thermodynamic descriptions of Nb-Ti and Nb-V are obtained by optimising available experimental information. The parameters for Ti-V are taken from a recent assessment. Phase diagram of the limiting binaries, liquidus projection, solidus projection, and two isothermal sections are computed.

The modelling of the mechanical alloying process in a planetary ball mill: comparison between theory and in-situ observations

In order to understand the mechanical alloying (MA) process in a planetary ball mill (PBM), the kinetics of a ball during milling have been analysed and the calculated ball trajectory is compared with in-situ observations of ball movement. Three different milling modes are deduced from the modelling. Although an experimentally efficient milling mode can be explained from the modelling, there is no good agreement between theory and in-situ observations in the mode of operation of most commercially available PBMs. The present study shows that, in these mills, the MA process should be described in terms of attrition and wear and not in terms of impact as is usually done.