15. - 17. 5. 2013, Brno, Czech Republic, EU

NEW TI-ALLOYS WITH SUPERIOR SPECIFIC-STRENGTH

Mohamed Abdel-Hady GEPREEL a, Mitsuo Niinomi b

a Department of Materials Science and Engineering, Egypt-Japan University of Science and Technology (E-

JUST), Alexandria, 21934, Egypt.

b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.

Abstract

In this study, new ultra-high strength Ti-alloys composing low cost common metals are developed. In the

present new alloys, the beta-phase was the predominant phase in the annealed condition and showed

moderate tensile strength (UTS) in the range of 800-950 MPa where the fracture strain was below 4%. After

careful thermomechanical treatments, the UTS of the alloys were greatly increased and achieved values

higher than 1900 MPa where the elongation was less than 1%. Further increase in the ductility of the alloys

is possible with the proper treatments on the expense of the strength. The ultra-high strength of the present

new Ti-alloys make them competitive to high specific strength alloys because of its good cold workability and

low cost.

Keywords: Ti-alloys, high strength, low cost, thermomechanical treatments

1. INTRODUCTION

Broad applications of metastable -titanium alloys have been found in aircraft and automotive industries

because of their highly attractive mechanical properties, such as high strength, low density, and high fracture

toughness[1]. For example, the ultimate tensile strength (UTS) of the -21S titanium alloy exceeds 1400

MPa along with good ductility [2]. Alloys designers have directed their efforts towards increaseing the

strength and/or decreasing the density of the materials used in manufacturing the automotive parts, in order

to increase their specific strength. New alloys with higher specific strength are the candidate for future

automotive industry as they show the greatest potential to reduce weight, save fuel, enhance performance of

the future vehicle and therefore will lessen environmental impact.

Also, great attention has been directed towards the design of novel type Ti-based alloys for biomedical

applications aimed at providing structural materials with a good corrosion resistance, high strength, good

ductility, low Young’s modulus, excellent wear resistance, low cytotoxicity, and negligible tendency to

provoke allergic reactions. For example, TNTZ alloy [3] and GUM metals [4] were developed. These alloys

performed high strength, moderate elongation, and hardness by controlling the chemical composition and

microstructure. However, these alloys contain a large amount of high melting point and high cost rare metals

such as Nb, Ta, V, and Mo leading to the fabrication difficulty.

Therefore, it is desired to develop high strength type titanium alloys composing low cost common metals

such as Mn, Fe, Al or Sn so as to be applicable as high strength structural materials and at the same time for

the biomedical applications [5].

In this study, new ultra-high strength type Ti alloys composing of low cost common metals are developed.

15. - 17. 5. 2013, Brno, Czech Republic, EU

2. EXPERIMENTAL WORK

Three type Ti-based alloys composing

of low cost common metals are studied

(namely; alloy1, 2, and 3). The specimens

were annealed at 1173K for 2 ks (AN

hereafter). Some of the AN specimens

were cold rolled to 30%, 60%, and 90%

reduction ratio (CR hereafter). The alloys

were then aged at different temperatures

and times. Phase identification was

performed by X-ray diffraction (XRD) on

bulk samples at room temperature, using

the copper K radiation. The

microstructures of the studied alloys were

examined by scanning electron

microscopy (SEM) and optical

microscopic. The sample density, , was

measured by the hydrostatic weighting

technique inside distilled water using an

analytic scale with 10−4

g of precision. The Vickers micro-hardness of specimens were measured under 1kg

load. The tensile test was carried out with the specimens where the cross head strain rate fixed at 1.6x10-4

.

3. RESULTS AND DISCUSSION

The alloys have different contents of the low cost -stabilizing elements in them. The Molybdenum equivalent

index ([Mo]eq) [3] is used as a measure of the degree of -phase stability of the alloys,. The [Mo]eq of alloy 1,

alloy 2 and alloy 3 are 20.4, 23.8, and 27.4, respectively. Therefore, it is expected that the degree of β-phase

stability is increasing in the order of alloy3˃ alloy2 ˃ alloy1. Shown in figure 1 are the RXD patterns of the

new Ti-alloys. As shown in this figure, the -phase retained by quenching from the -phase zone (i.e.,

1173K) and the -phase became predominant in both alloys. Neither -phase nor athermal -phase were

detected by XRD. This confirms that the present alloys are classified as -type Ti-alloys.

Tab. 1 Mechanical and physical properties of the new alloys 1-3 in comparison with Ti, Al, and steel alloys [3][8]

Alloys 1-3

(AN) Ti5111 Ti64 Al 7075-T6 HSLA100

Yield Strength (MPa) 800-926 690 827 503 690

Density (gm/cm3) 4.6-4.9 4.4 4.4 2.8 7.8

Elongation at Break 2-4% 15% 14% 11% 11-13%

Strength/Density 166-192 156 187 179 88

The average mechanical properties and density of the present alloys in comparison with some selected

commercial alloys (namely; Ti-6Al-4V (Ti64),Ti-5Al-1V-1Sn-1Zr-0.8Mo (Ti5111), Al-5.8Zn-2.3Mg-1.4Cu

(Al7075-T6), and high strength low allow steel, Fe-3.5Ni-1.5Cu-1Mn-0.6Cr-0.6Mo, (HSLA100) are presented

in table 1.

Fig. 1 XRD patterns of alloys1-3 in the AN condition

15. - 17. 5. 2013, Brno, Czech Republic, EU

The density of Alloys1-3 is a typical level of -type Ti-alloys; however, it is slightly higher than the average

of -type Ti-alloys due to higher content of alloying elements. The strength of the present new alloys is higher than the other high strength steel, Ti-, and Al-alloys, as shown in table 1. Therefore, the specific strength (Strength/Density) is high and comparable to Ti64 and Al7075-T6, those commercial high strength alloys, whereas the elongation of Alloys1-3 is low, which limits its range of applications.

Most of -type Ti-alloys are cold workable and show remarkable hardening by cold deformation. Interesting is that Alloys1-3 are also cold workable and showed extensive hardening effect by cold deformation. Shown in figure 2 is the change in hardness of Alloys1-3 by cold rolling.

It is observed that the hardness of the alloys is greatly increased with cold rolling. 90%cold rolling (90CR) almost doubled the hardness of alloy 2. This is a unique work hardening effect

observed in the present alloys. Also, it is observed that the hardness of the alloys is

increasing with increasing the -phase stability presented by its [Mo]eq index. This goes in accordance with what was reported in a

previous work for other single -phase alloys[9]. It was reported that the strength and hardness

of single -phase alloys are increasing with

increasing the -phase stability.

The extensive increase in work hardening in the present alloys by cold working will greatly increase its tensile strength after sever plastic deformation. For example, after 60% cold rolling, Alloy 1 showed ultimate tensile strength (UTS) of 1599 MPa and elongation of 1.8%. Also, after 90% cold rolling, alloy2 showed UTS of 1910 MPa and elongation of 1.4%. The UTS of the present alloys have raised so much to levels above any other Ti-alloys. However, the elongation of the present alloys is very low that limits its application, therefore, the elongation should be improved without much deterioration of the strength. Aging those b-type alloys after sever plastic deformation was reported to improve the ductility and elongation of the alloys [10].

The aging process of the present alloys was conducted under vacuum. The elongation of the alloys was much improved without much deterioration in its strength. For example, Alloy 2 showed elongation of 9% while the UTS was kept at high value of 1620 MPa when aged at 873K/1.8ks after 90% cold rolling.

The aging process after sever cold deformation results is stress relief that improves the elongation and at

the same time leads to precipitation of very fine -phase which plays the rule of precipitation hardening, which therefore keeps the strength at high level.

The present new alloys intrinsically show high strength besides its high affinity for work hardening by cold

Fig. 2 Change of hardness of Alloys1-3 with cold rolling

reduction ration. 30%, 60%, and 90% cold rolling

reduction are 30CR, 60CR, and 90CR, respectiviely.

Fig.3 Relationship between specific strength and

tensile strength of the new type Ti-alloys alloys

compared to selected ultra high strength

commercial alloys of each type [2].

15. - 17. 5. 2013, Brno, Czech Republic, EU

deformation. The ultra-high strength of the present new type Ti-alloys locate them as good candidates for the other high specific strength alloys because of their good cold workability and low cost, as shown in Figure 3. However, further investigations are needed to improve the elongation of these alloys while keeping the strength to its highest level.

4. CONCLUSION

New cold workable type Ti-alloys with ultra high strength are developed in this study. The superior specific strength of the present alloys, 300-390 MPa/gm.cm

3, will open a lot of room for industrial and

biomedical applications.

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