Ni-Ti system

Figure 11.1. Calculated (a) metastable phase diagrams in the Ni-Ti system, (b) some combinations of these digrams and (c) comparison of calculated and observed stable diagram (Ansara 1979).

As their name implies, shape-memory alloys are able to revert back to their original shape, even if significantly deformed (Figure 3.24). This effect was discovered in 1932 for Au-Cd alloys. However, there were no applications for these materials until the discovery of Ni-Ti alloys (e.g., NiTi, nitinol) in the late 1960s. As significant research has been devoted to the study of these materials, there are now over 15 different binary, ternary, and quaternary alloys that also exhibit this property. Other than the most common Ni-Ti system, other classes include Au-Cu-Zn, Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. 

The intermetallic Ni-Ti system has the imusual property of after being distorted, returning to its original shape when heated. This was the first of the shape memory alloys (SMAs) and was discovered by accident at the Naval Ordnance Laboratory, hence its name Nitinol. Other SMAs include Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. The shape memory mechanism depends on a martensitic solid-state phase transition that takes place at a modest temperature (50°C—150°C), depending on the alloy. The high temperature phase is referred to as austenite and the low temperature phase is called martensite (following the terminology of the Fe-FeCa system). 

Interdiffusion in Ni-Ti system splitting of inert markers to mark the Kirkendall frame of reference of each phase formed (Bastin and Rieck, 1974). 

CALPHAD calculations were made for a number of binary systems including Au-Si, (Hf,Ti,Zr)-Be and Ni-Ti (Saunders and MiodoAvnik 1983) and a series of ternaries Hf-Ti-Be, Hf-Zr-Be and Hf-Ti-Be (Saunders et al. 1985). Figure 11.6 shows such a calculation for Ni-Ti. The results were encouraging in that they predicted with reasonable accuracy the limit to glass formation when the terminal solid solutions were considered. However, there was limited success when taking into account compoimd phases. To this end the approach was extended to include the kinetics of transformation more explicitly. Remarkably good results were then obtained for a wide variety of binary and ternary system and these are reported in Section 11.3.4. 

Shape-Memory Alloys. Stoeckel defines a shape-memory alloy as the ability of some plastically deformed metals (and plastics) to resume their original shape upon heating. This effect has been observed in numerous metal alloys, notably the Ni—Ti and copper-based alloys, where commercial utilization of this effect lias been exploited. (An example is valve springs that respond automatically to change in transmission-fluid temperature.) Copper-based alloy systems also exhibit this effect. These have been Cu-Zn-Al and Cu-Al-Ni systems. In fact, the first thermal actuator to utilize this effect /a greenhouse window opener) uses a Cu—Zn-Al spring. 

By adding NiCl2 to the TiCl3—AlEt3 catalyst (Ni/Ti/Al molar ratio 1 1 3), the contents of 1-butene units in the copolymer were increased at the same comonomer feed as compared with the copolymerisation system without added... 

To date for the large-scale production of high czs-l,4-BR (cis- 1,4-contents > 90%) solution processes are applied in which catalyst systems on the basis of Co, Ni, Ti and Nd are used. In order to elucidate the specific benefits and disadvantages of Nd-technology it is compared with the three other available technologies. This comparison is performed in two respects ... 

As can be seen in Table 30 Co-, Ni-, Ti- and Nd-based catalyst systems yield BR with cis- 1,4-contents >90%. A direct consequence of high cis-1,4-contents is strain-induced crystallization of raw rubbers as well as of the respective vulcanizates. As the cis- 1,4-content is extraordinary high in Nd-BR, strain-induced crystallization is particularly pronounced for this BR-grade. As strain-induced crystallization beneficially influences the tack of raw rubbers as well as tensile strength and resistance of vulcanizates to abrasion and fatigue the high cis- 1,4-content makes Nd-BR particularly useful for tire applications. 

Cocco et al. [54] discuss the preparation of metallic glass, while copper-titanium, aluminum-titanium, and palladium-titanium systems in particular are prepared under a controlled atmosphere with hydrogen and argon. Components of Nb-Ni and Nb-Y have also been described [55]. Amorphous Ni-Ti alloys have been prepared by Schwarz et al. [56], while Ni-Ga, Ni-Ge, Ni-In, and Ni-Sn has been synthesized in supersaturated solid solutions [57]. Fe, Co, Ni-Ta-alloys are described by Lee and Yang [58], while FeSi2 doped with Co or Al for thermoelectric material is also mentioned [59]. 

Interesting results have been also obtained in [12], where the hydrogenolysis process in the Ti-Ni-H system was used in order to obtain the non-destructive (mechanically) hydrogen-absorbing composite material. 

These works were aimed at increasing the intensity of the absorption maximum at lower energy. This research led to the conclusion that dithiolene complex should contain (a) extended ti systems, with electron-donating substituents as coplanar as possible with the Ni(dithiolene) unit and (b) sterically bulky substituents in order to increase the solubility. 

In the Ni-Ti/A1203 system, as Eq 0, O-Ti clusters in Ni are very active at metal/oxide interfaces but wettability is affected also by the formation of a continuous Ti203 layer at the Ni/Al203 interface. The replacement of ionocovalent A1203 by the semi-metallic oxide Ti203 increases Wa and decreases 9, as explained in Section 6.3. The combination of these two effects, i.e., adsorption of O-Ti clusters at the liquid-side of the interface and formation of a semi-metallic oxide... 

Results for titanium-boron and several intermetallic systems (Ni-Al, Ti-Al, Ni-Ti, Co-Ti) are presented in Fig. 49. In all cases, the measured combustion tern-... 

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