Phase diagram ofTi50−xNi50+x: Crossover from martensite to strain glass

Abstract

We systematically investigated the variation in transition behavior and physical properties over a wide excess Ni ͑acting as defect͒ concentration range ͑x =0-2.5͒ in Ti 50−x Ni 50+x alloys. This enables the establishment of an updated quantitative phase diagram for this important system. The phase diagram shows not only the well-known parent phase and martensite phase but also a premartensitic state and a strain glass state. Our experiments were able to determine quantitatively the borders of these states, the latter two having been unclear so far. The new phase diagram shows that a crossover from martensite to strain glass occurs at x = 1.3, and the appearance of a "premartensitic phase" below a critical temperature T nd for defect-containing compositions ͑x Ͼ 0͒. We propose that point defects ͑excess Ni here͒ play two roles in a ferroelastic/martensitic system: ͑i͒ changing the thermodynamic driving force for the formation of long-range strain order ͑martensite͒ and ͑ii͒ creating random local stress that favors a premartensitic nanostructure and strain glass. Our work enables a simple explanation for several long-standing puzzles, such as the appearance of premartensitic nanostructure, the vanishing of transition latent heat with increasing Ni content and the anomalous negative temperature coefficient of electrical resistivity in Ni-rich Ti-Ni alloys.

Modeling Abnormal Strain States in Ferroelastic Systems: The Role of Point Defects

“…Point defects have been known to play a central role in altering and controlling the properties of ferroelastic materials [1]. In addition to the well-known paraelastic and ferroelastic strain states, it was found that point defects can generate two abnormal strain states: a ''precursory strain state'' (or tweed) characterized by a cross-hatched nanosized strain domain structure [2,3] imbedded in a dynamically disordered paraelastic matrix [4] and a new strain-glass state that is a frozen state of local strain order [5][6][7][8], a ferroelastic analogue to ferroelectric relaxors [9] and cluster spin glasses [10,11].Recent experimental studies have yielded strikingly similar ''phase diagrams'' of strain states for many different doped ferroelastic systems [12][13][14]. An example is given in Fig.…”

Composition Dependences of Entropy Change and Transformation Temperatures in Ni-rich Ti–Ni System

“…All the determined characteristic temperatures, T 0 , T Ms , and T Af , are listed in Table 1 and plotted in Fig. 2 as a function of x together with data from some previous reports [1,7,[10][11][12][13][14][15][16][17], T 0 being approximated as the average of T Ms and T Af in this study. T Ms in 49.98 Ni (338 K) well accords with that in the slowly cooled stoichiometric TiNi (333 K) [16].…”

“…Meanwhile, from the scientific viewpoint, Ti-Ni and its derived SMA systems continue to engender fundamental interest in the nature of B2/B19 0 martensitic transformation (MT). In the Ni-rich portion of the Ti-Ni system, it is well known that the MT start temperature (T Ms ) suddenly disappears at around 51.4 at.% Ni [1]. Such a suppression of MT has only been explained so far by the concept of strain glass, which is one of the class of glassy states in transformation strain essentially similar to spin glass and relaxor [2,3].…”

Recent progresses in the understanding of the elastic and anelastic properties of H-free, H-doped and H-contaminated NiTi based alloys

“…As anticipated in a preceding subsection these authors attributed the modulus dip and the IF peak to a local strain glass transition, which was subsequently further investigated by X-rays, electron microscopy, zero-field cooling/field cooling and static stress-strain experiments (Refs. [24][25][26][27][28][29][30][31]35). Their X-ray diffraction patterns are shown in figure 18(a) while the IF and the Young's modulus results (Refs.…”

“…7,9,14,15,17,19,23,30,[45][46][47][48][49][50][51][52][53]. Remaining questions to be answered are: a) the origin of the non-thermally activated peaks P 150K and P 200K and b) the role of H in the local strain glass transition.…”