Papers by Author: Jan Van Humbeeck

Abstract: This article is dedicated to the estimation of the relative stability for B2, B19`, B33 and Cm phase in ZrCu-ZrNi-ZrCo intermetallic compound row through the ab-initio electronic structure calculations and subsequent crystal structure Rietveld refinement. The information about electronic and crystal structure of phases in Zr-based intermetallics will allow selecting for this high temperature shape memory alloy such alloying elements that will significantly improve shape memory behavior through definite structural changes.

Abstract: Intermetallic compounds of ZrNi-ZrCo cross-section undergo martensitic phase transformation, which is accompanied by the imperfect shape memory effect at elevated temperatures. Martensitic transformation is also taking place in one of the constituents for the in-situ composites of ZrCo-Zr₂Co-Zr₂Ni-ZrNi region. High temperature shape memory behavior of Zr-Ni-Co composite material shows improvement in shape recovery comparing to quasibinary intermetallic compounds of ZrNi-ZrCo cross-section.

Abstract: A Ti-50.8at.%Ni wire produced using a co-drawing method and a commercial Ti-50.8at.%Ni wire were annealed between 450°C and 700°C. Grains with diameter less than 100nm were revealed by TEM for both wires before annealing treatment. However, the microstructural heterogeneity of the co-drawn wire is more obvious than that of the commercial wire. The transformation behavior and mechanical properties of both wires were found to be sensitive to the annealing temperature. Multi-stage martensitic transformation was observed in the co-drawn wire, compared with the one-stage A↔M transformation in the commercial wire after annealing at 600°C for 30min. Moreover, the ultimate tensile stress and plateau stress of the commercial wire were found to be higher than that of the co-drawn wire under the same annealing conditions. The differences of total elongation, plateau strain and pseudoelastic recoverable strain between the commercial wire and the co-drawn wire were also observed. The differences of the transformation behavior and mechanical properties between the commercial wire and the co-drawn wire are attributed to the microstructural difference between these two wires.

Abstract: The effects of long-range atomic order and pinning of interfaces on anelastic properties of Cu-based martensite have been distinguished by means of studying strain amplitude dependences of internal friction and Young’s modulus defect, assuming that low-and high-amplitude stages of the strain amplitude dependence correspond to the oscillations within and beyond pinning atmospheres.

Abstract: The internal friction of the hyperstabilized martensite demonstrates very low values, both above and below the nominal martensitic transformation temperature, due to a pronounced pinning effect. Over a wide temperature range it is comparable with the level of damping in the parent phase. A study of the temperature dependence of the non-linear ultrasonic internal friction and its strain amplitude hysteresis indicates that the diffusion, assisted by dislocations/interfaces, is quite pronounced and in Ni-Fe-Ga and Cu-Al-Be alloys it operates at temperatures around 20 K. The renucleation of the lamellar parent phase during the reverse martensitic transformation close to 600 K is accompanied by an internal friction peak which demonstrates a substantial transitory contribution. After renucleation of the parent phase the samples recover a conventional martensitic transformation with the internal friction level in the martensite comparable to the one in non-stabilized samples. Observations of a relaxation peak in the parent phase of different alloys for temperatures just below the renucleation stage of the reverse transformation point to the essential role of diffusion in the nucleation of the parent phase in hyperstabilized martensites.

Abstract: This chapter analyzes applicability of different models of anelasticity to damping capacity of shape memory alloys both in the martensitic state and during the martensitic transformation. The chapter focuses mainly on recent observations made in Cu-based and NiTi alloys. From the latest works it is evident that the high damping capacity can not only be related to the hysteretic mobility of interfaces between martensitic variants but may be associated as well with internal defects of variants.

Abstract: The ageing phenomena and its effect on the thermoelastic martensitic transformation was investigated in three Cu-base SMAs. The transformation temperatures shifted to higher temperatures due to aging in the beta-phase. To increase the alloy’s ductility, a definite amount of Mn (4 wt%) and Fe (2 wt%) were added to the ternary alloy. The thermoelastic martensitic transformation was found in the not-aged samples of the CuAlNiMn and CuAlNiMnFe alloys. This transformation was destroyed due to ageing heat treatments by a fairly unknown exothermic process. The thermoelastic martensitic transformation appeared again in the aged CuAlNiMn and CuAlNiMnFe samples after keeping them on room temperature for a few months. This phenomena was investigated by DSC, SEM, TEM, and XRD.

Abstract: IF steel processed by equal channel angular pressing to an equivalent strain of 9.2 via route BA was annealed for different times at 500°C and 600°C. At both temperatures the microstructural evolution shows continuous recrystallization followed by grain growth, in absence of primary recrystallization. At 600°C a slightly bimodal grain size distribution develops.