Papers by Keyword: Martensitic Phase Transformation

Abstract: Industrial low alloy TRIP sheet steels contain blocky and lath-shaped retained austenite. In the present study, transformation behaviour of blocky and lath-shaped retained austenite during straining was investigated to clarify its effect on mechanical properties. Two types of TRIP steels containing almost the same amount but the different morphology of retained austenite were used. A steel containing large amount of lath-shaped retained austenite exhibits superior ductility, and sustains high work-hardenability in a high strain region. On the contrast, a steel containing large amount of blocky retained austenite exhibits low ductility. 　The work-hardenability increased steeply to the maximum at a low strain region, and then reduced in a high strain region. The stability of the blocky austenite has been found to be poor with respected to martensite transformation. The lath-shaped retained austenite remains until a high strain region whereas the blocky retained austenite transformed into martensite in a low strain region. Carbon content was higher in the lath-shaped retained austenite than in the blocky retained austenite. Stability of retained austenite is, however, inexplicable only by the carbon content, and would be affected by the different morphology and the resulting restraint conditions.

Abstract: Due to Plasticity induced Transformation in metastable β-Ti-10V-2Fe-3Al (wt.%) alloy (PiTTi) upon deformation, a noticeable improvement in mechanical properties is observed. Among the main factors controlling such effect are the β grain size and its composition. Such phase transforms into martensite upon quenching. Its martensite start temperature (Ms) varies in accordance with its composition. Following Ghosh and Olson’s theory, a thermodynamics based model to predict the compositional dependence of the Ms temperature is developed, and successfully validated for Ti-X (X = Fe, Cr, Mo, V, Nb, Zr and Al) binary alloys. The model has been used to design new alloys displaying a tailored PiTTi effect.

Abstract: In order to better understand the unique functional responses of shape memory alloys, improve the currently existing SMA modeling tools and used them beneficially in smart structure applications, it is desirable to investigate the deformation/transformation processes in these materials in action – i.e. under stress and temperature variation. In this work, an overview is presented on the applications of various recently developed or originally employed in-situ experimental methods and approaches to martensitic transformations in SMAs.

Abstract: NiTi shape memory alloys are a group of materials which have a lot of applications especially in aerospace industries and medical equipments because of their excellent properties. Shape memory effect (SME), pseudo-elasticity (PE), high corrosion resistance and biocompatibility is special properties of these alloys which lead to their extensive applications. The superior behavior of NiTi alloy is due to thermoelastic martensitic phase transformation. In the present paper, two NiTi shape memory alloys were prepared by non-consumable vacuum arc melting technique in copper water cooled crucible. One of them had commercial elements and the other had high purity elements. Metallographic investigation, chemical analysis, XRD and DSC were carried out on two alloys. Metallographic observation and XRD shows that structure at ambient temperature consists of austenite phase besides Ti2Ni, Ni3Ti intermetallic compounds and martensite phase. Transformation investigation determines that the impurity such as iron in commercial alloy causes two stage phase transformation B2→R→B19′.

Abstract: The present study investigates the relations between distributions of strain with those of martensitic volume fraction around fatigue cracks in SUS304 stainless steel. Specimens having random pattern created by spraying on their surfaces were fatigued at room temperature (297K) in air. The pictures around fatigue crack were taken during fatigue crack propagation experiments. Distributions of strain were calculated by the digital image correlation (DIC) method with these pictures. Distributions of martensitic volume fraction Vα’ were measured around fatigue cracks with ferrite scope. The distributions of strain resembled those of Vα’. The higher the strain becomes, the more the α’ martensitic phase was induced by the plasticity-induced transformation around fatigue cracks. It was revealed that the same value of Vα’max is induced by strain regardless of static or fatigue.

Abstract: Martensitic phase transformation can greatly affect the mechanical behaviors and the stress-strain response of shape memory alloys (SMAs). In this study, the effect of martensitic phase transformation on the deformation of a single-crystal TiNi SMA specimen with a triangle crack was investigated experimentally by means of moiré interferometry method. A typical displacement field and the corresponding strain field in areas both around and far from the tip of the crack were measured in a certain time during the loading process in which the tensile load is coupled with the stress-induced martensitic phase transformation. Some characteristics of the deformation and the martensitic phase transformation of the specimen are revealed. These results may provide a reliable support for revealing the fracture mechanism of single crystal TiNi SMAs, and may enable further development in putting forward the failure criterion of SMAs.

Abstract: A physically-based multi-scale model for martensitic transformation induced plasticity is presented. At the fine scale, a model for one transforming martensitic variant is established based on the concept of a lamellae composed of a martensitic plate and an austenitic layer. Next, the behaviour of 24 potentially transforming variants is homogenized towards the behaviour of an austenitic grain. As a simple example, the model is applied to deformation and transformation of a single austenitic grain under different deformation modes.

Abstract: The damping behavior of an Fe-28Mn-6Si-5Cr-0.5NbC (mass%) shape memory alloy was measured by low cycle fatigue tests during tension-compression loadings. A remarkable damping capacity was observed above the strain amplitude of 0.1%, and the specific damping capacity (SDC) parameter reached saturation at ~ 80% above 0.4%. The reversible motion of the γ/ε interfaces is considered to dominate the cyclic deformation behavior, while the work hardening during tension-compression loading is negligible. These characteristics are favorable for seismic damping devices that protect civil structures from earthquakes.

Abstract: 1.4 %C ultra high carbon steel (UHCS) was prepared in order to study the structure of martensite transformation and mechanical properties. Ultra-fine spherical carbide and ultra-fine austenite grain size were obtained. A great deal of lath martensite was observed after quenching. The phenomenon does not agree with the traditional knowledge that the lath martensite would disappear when carbon content is in excess of 0.8% in austenite. The strength, fatigue properties and fracture toughness have been measured. A good combination of strength, toughness and fatigue properties come from fine and uniform distributed carbide particles and ultra-fine austenite grain size. Fracture strength increases by 48%, yield strength increases by 15% and plasticity keep the same comparing with that of hardened and tempered 40CrNiMo. The carbon content of ultrahigh carbon steels (UHCS) is in the range of 1.0-2.1% [1, 2]. Traditional heat treatments for normal steels will cause the microstructure of UHCS to be coarse and do not produce optimal properties. With controlled rolling and special heat treatment, UHCS can be in ferrite, pearlite, bainnite or martensite structures, which all have different mechanical properties. The yield stress of a 1.8%C, 1.6%Al ferrite UHCS can reach 1500MPa, which is much higher than that of high strength and plain alloy steels [3]. The tensile strength of a 1.25%C-1.5%Cr pearlite UHCS can reach 1810Mpa and its elongation can be 18%. When it is treated into martensite, its compression strength reached to 4690Mpa and compression strain reached to 26% [1, 4], which is comparable to WC-12Co. Such good mechanical properties can be ascribed to the ultra fine grain sizes because of the undissolved carbide particles which resist growth of austenite grain during heating. Another reason could be the lath martensite structures. O.D.Sherby [4] had reported that there was a lot of lath martensite in quenched UHCS. The UHCS was considered not only as tool steels but also as good structure materials. Fracture and fatigue properties are important for structure materials. However, they have rarely been studied. The present paper is going to study the martensite structure and mechanical properties of a prepared 1.4% C UHCS.