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Mechanical properties of amorphous and crystalline TiNiCu thin ribbons

Name
Aleksei
Surname
Kshumanev
Scientific organization
Saint-Petersburg State University
Academic degree
Bachelor
Position
Research assistant
Scientific discipline
Mathematics & Mechanics
Topic
Mechanical properties of amorphous and crystalline TiNiCu thin ribbons
Abstract
This work is dedicated to the investigation of mechanical properties of TiNiCu-alloy. Samples were obtained from a thin melt-spun amorphous ribbon. The first objective was to examine dependance of mechanical properties of amorphous specimens on the experiment temperature.
The second objective was to obtain crystallized samples and study their mechanical behaviour depening on annealing regimes, so a few temperatures above the temperature of crystallization to prepare crystallized specimens were chosen.
Keywords
Amorphous alloys, melt-spun ribbons, shape memory alloys, thermal processing, mechanical behaviour
Summary

Introduction

Nowadays shape memory alloys become very popular among scientists and manufacturers due to their remarkable properties. And titanium nickelide (TiNi) is one of the best known representatives of such alloys.

There is a possibility to obtain some TiNi-alloys with initially amorphous state using melt-spun technique. It is known that mechanical behaviour of amorphous materials is strongly dependent on the temperature, so it is very important to investigate properties of amorphous TiNiCu melt-spun ribbons at different temperatures.

To make amorphous ribbons capable of undergoing martensite transformations it is necessary to crystallize them. This could be carried out by heating the material above the temperature of crystallization. Besides, alloy microstructure and mechanical properties could vary considerably depending on the heating temperature, annealing duration and other crystallization parameters. So this dependance is a subject of a second part of the work.

 

Objects and methods

Samples made from thin Ti50Ni50Cu25 amorphous ribbons of 1.6 mm in width and 35 µm in thickness were inversigated.

 

Three-component alloy was chosen for survey because of the requirement of cooling with critical cooling rate or higher, and this rate decrease with the increasing of alloy components quantity.

The research of mechanical behaviour was conducted using tensile machine Shimadzu AG-XD plus while the temperature was mantained in the heat-chamber Shimadzu TCE-N300.

Specimens were set in specially designed and produced grips which were fixed in machine holders.

The tape was cut into pieces of 35 mm in length. Value of the gauge length turned out to be about 8 mm accoding to standards so then the appropriate grips were made.

Some specimens in the amorphous state were subjected to tension process at different temperatures. Another set of experiments was carried out on samples subjected to annealing in a furnace to become crystallized. Heating was conducted at 5°C/min from room temperature up to temperatures of 470, 500, 550, 600 and 650°C - temperatures above the crystallization temperature. Whereafter the final temperature was maintained during one hour.

 

Results

Fig. 1. Stress-strain diagram of amorphous specimens at different temperatures.

 

Table 1

Young's modulus of amorphous specimens at different temperatures

Temperature, °C

50

150

200

250

Young's modulus, 102 MPa

318

280

266

257

 

 

Fig. 2. Stress-strain diagram of crystallized specimens annealed at different temperatures and deformed at 20°C.

Fig. 3. 3D-diagram of crystallized specimens annealed at different temperatures and deformed at 20°C.

 

Table 2

Stress corresponding to the start of inelastic deformation due to the martensite reorientation (martensite reorientation limit) for crystallized specimens annealed at different temperatures and deformed at 20°C.

Temperature of annealing, °C

470

500

550

600

650

Martensite reorientation limit, MPa

20

49

123

132

129

 

Table 3

Inelastic deformation accumulated due to the martensite reorientation at full strain of 5% for crystallized specimens annealed at different temperatures and deformed at 20°C.

Temperature of annealing, °C

470

500

550

600

650

Inelastic deformation, %

3,3

3,2

---

2,5

---


Fig. 4.  Stress-strain diagram of crystallized specimens annealed at different temperatures and deformed at 20°C.

 

Fig. 5. 3D-diagram of crystallized specimens annealed at different temperatures and deformed at 90°C.

 

Table 4

Young's modulus of austenite phase for crystallized specimens annealed at different temperatures and deformed at 90°C.

Temperature of annealing, °C

470

500

550

600

650

Young's modulus, 102 MPa

262

294

319

343

315

 

Table 5

Phase yield stress values for crystallized specimens annealed at different temperatures and deformed at 90°C.

Temperature of annealing, °C

470

500

550

600

650

Phase yield stress, MPa

240

303

356

281

282


 

Conclusions

  • Amorphous melt-spun Ti50Ni25Cu25 ribbons deform elastically until fracture within a temperature range of 20-250°C. Young's modulus decreases with the increasing of deformation temperature.
  • Crystallized melt-spun Ti50Ni25Cu25 ribbons which were subjected to annealing at different temperatures demostrate pseudoelastic behaviour at austenite phase. Phase yield stress depends on annealing temperature.
  • Crystallized melt-spun Ti50Ni25Cu25 ribbons deform inelastically due to the mechanism of martensite reorientation at martensite phase. Stress corresponding to the start of inelastic deformation and value of the accumulated inelastic deformation depend on annealing temperature.

Plans for further investigations

It would be interesting to use Digital image correlation optical method for local strain measurement in future. This method allows to obtain pictures with strain fields of the whole specimen during the test so it helps in finding shear bands and initial destruction point.

 

​Reference list

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