Papers by Keyword: Martensitic Phase Transformation

Abstract: The present paper is dedicated to the first observation of the martensitic transformation (MT) and associated shape memory effect (SME) in HfIr equiatomic intermetallic compound. Differential scanning calorimetry allowed determining MT temperatures, which are above 700 K. It was also shown that MT taking place at such high temperatures is associated with SME measured in 3 point bending. MT temperatures and crystal structure changes in the homogeneity range of HfIr compound were also studied. SME behavior changes in the homogeneity range of HfIr compound are discussed

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: This paper reports on the tension-compression asymmetry of [001]-oriented Co49Ni21Ga30 single crystals at elevated temperatures. Maximum strains of -4.8 % and 8.6 % in compression and tension, respectively, were found. A linear Clausius-Clapeyron relationship was observed for both stress-states where the smaller slope in tension resulted in a significant increase of the phase transformation temperatures with stress, which reached 180 °C under a constant stress level of 150 MPa. In addition, the material demonstrated a large pseudoelastic temperature range of about 300 °C under both stress state conditions. The results in this study unequivocally indicate the potential of these alloys for applications where elevated temperatures and stress levels prevail.

Abstract: The main deformation mechanism of shape memory alloys (SMA) is martensitic phase transformation (PT). To model martensitic phase transformations in such materials, several models have been developed. In the micromechanical model developed by Levitas and Ozsoy (2009), complete system of equations that describes evolution of stresses in phases and crystallographic parameters, as well as macroscopic stress–strain response for martensitic phase transformations under complex multiaxial loadings have been formulated. However, the experimental verification is incomplete. The objective of this study is to compare the experimental results in the literature for CuAlNi SMA with the results obtained from this micromechanical model. In the experiment used for comparison (Shield, 1995), a single crystal CuAlNi specimen is subjected to uniaxial load in several directions at different temperatures. According to the results obtained by using the constitutive equations of the model, the model describes the stress induced phase transformations very well when compared with these experiments. The uniaxial loading-unloading hysteresis loop shows good agreement with the experiments.

Abstract: Superelastic NiTi polycrystalline tubes, when subjected to quasi-static stretching, transform from an initial austenite phase to a high-strain martensite phase by the formation and growth of a macroscopic self-organized helical domain as deformation progresses. This paper performed an experimental study on the effects of the externally applied stretching and tube geometry (length L, wall-thickness h and tube radius R) on the martensitic helical domains in the tubes under very slow (isothermal) stretching. The evolution of the helical domains with the applied strain in different tube geometries are quantified by in-situ optical measurement. We demonstrate that the shape of the self-organized helical domain and its evolution are governed by the competition between bending strain energy and domain front energy in minimizing the total energy of the tube system. The former favors a long slim helical domain, while the latter favors a short fat helical domain. The experimental results provide a strong support to the recently developed theoretical relationship.

Abstract: At different temperatures ranging from ‑60°C to 200°C a cast CrMnNi-TRIP steel was deformed by uniaxial tension. The resulting microstructure was investigated using XRD, EBSD and LOM. The correlation of the phase transformation with the deformation temperature was examined. Depending on temperature, a transition in the deformation mechanisms was observed. Starting with the generation of deformation bands, accompanied by martensitic phase transformation, followed by twinning, the deformation mechanism turned to conventional dislocation glide with raising temperature. Between -60°C and 20°C the TRIP (TRansformation Induced Plasticity)-effect is the dominating deformation mechanism, whereas between 20°C and 200°C the TWIP (Twinning induced plasticity) effect is observed. The geometrical arrangement of martensite within the microstructure is considered within this study. The amount of α'-martensite is mainly responsible for the hardening rate and the resulting mechanical properties.

Abstract: TiNi shape memory alloys exhibiting high damping capacity are currently expected to be used as structural materials for energy dissipation or vibration control applications. In this paper, the characterization of damping behaviour of a binary TiNi SMA was performed by dynamic mechanical analyzer (DMA) instrument and differential scanning calorimetry (DSC) equipment. Damping tests measuring Tanδ, storage modulus E' and loss modulus E" of Ti49.2Ni50.8 binary shape memory alloy were investigated at different temperature, using different frequency and strain amplitude. It shows that quenching rate has a significant effect on the damping capacity of TiNi SMA by exhibiting different phase transformation behavior. Internal friction values (Q^-1) corresponding to cubic B2 parent phase to rhombohedral R phase transformation, B2-R, and R-B19' monoclinic martensite transformation are as high as 0.177 and 0.078, respectively. The occurrence of R-phase significantly softens the storage modulus and thus promotes the damping capacity of TiNi SMAs.

Abstract: Properly annealed Ti₅₀Ni₂₅Cu₂₅ amorphous ribbons have excellent shape memory founction, superelastic and internal friction. In this article, recent progress in research on the amorphous alloy ribbons of Ti₅₀Ni₂₅Cu₂₅ preparation method, phase composition, microstructure of the research status were reviewed. The review mainly includes the phase composition, microstructure evolution, supporting the process of structure change and phase change after heat treatment on Ti₅₀Ni₂₅Cu₂₅ crystallized amorphous ribbons.Shortcomings and future researching fields were also pointed out.

Abstract: The effect of thermal cycling of the Ti - 49.8 at. % Ni alloy deformed by rolling at 500 оС on dilatation of the material was investigated. It was shown that R → B2 and B19' → B2 phase transformations take place during heating due to retaining of R-phase at the lowest temperature of cycling. The evolution and subsequent stabilization of two-way shape memory effect (TWSME) upon thermal cycling are caused by decreasing of the residual B19'- martensitic phase formed during rolling. It was revealed that recoverable strain of the alloy doesn’t exceed 0.8 % even after forty cycles.

Abstract: Grind-hardening is a new integrated machining technology which utilizes grinding heat to quench the non-quenched steel directly. In this paper, the technology is applied in the process of rack form grinding. A comprehensive numerical model is developed to simulate the temperature distributions of the rack under the dry grind-hardening conditions with finite element method(FEM). The temperature dependency of the thermal properties, the triangular heat distribution of the heat flux, latent heat and the air convection are taken into account. The simulated hardness penetration depth(HPD) is deduced from the local temperature distribution, time history of workpiece according to martensitic phase transformation theory. This provides a reliable method for the proper selection of process parameters in order to produce enough heat at the contact zone, enabling the treatment of the rack.