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International J ournal of Minerals, Metallurgy and Materials

V olume20, Number3, March 2013, Page 266

DOI: 10.1007/s12613-013-0722-7

Influence of semisolid forging ratio on the microstructure and mechani-

cal properties of Ti14 alloy

Yong-nan Chen1,2), Gang Liu1), Xue-min Zhang2), and Yong-qing Zhao3)

1) State Key Laboratory for Mechanical Behavior of Material, Xian Jiaotong University, Xian 710049, China

2) School of Materials Science and Engineering, Changan University, Xian 710064, China

3) Northwest Institute for Nonferrous Metal Research, Xian 710016, China

(Received: 19 April 2012; revised: 24 June 2012; accepted: 26 June 2012)

Abstract: The present work is focused on the microstructure and mechanical properties of Ti14 alloy with different

semisolid deformation ratios during forging tests. The results revealed that the forging ratio had a significant effect on the

precipitation of the alloy. Fewer plate-shaped Ti2Cu tended to precipitate on grain boundaries with higher forging ratios,

and finally the plate-shaped Ti2Cu formed precipitate-free zones along grain boundaries with a forging ratio of 75%. The

precipitation on grain boundaries was found to be controlled by a peritectic reaction. Large forging ratios accelerated theextrusion of liquid and resulted in less liquid along the prior grain boundaries, which reduced the peritectic precipitation

in this region and formed precipitate-free zones during re-solidification. In addition, increasing the forging ratio could

accelerate dynamic recrystallization, which is favorable for improving the semisolid formability. The tensile ductility

increased with increasing forging ratio, and a mixed fracture mode, involving both cleavage and dimple fracture, was

observed after forging with a forging ratio of 75%, which is attributed to the presence of precipitate-free zones formed

along grain boundaries during semisolid processing.

Keywords: titanium alloys; semisolid; forging; microstructure; precipitates; mechanical properties

1. Introduction

Semisolid forming is an effective net-shaped forming

process by deforming metals in the semisolid state, which

combines elements of both casting and forging and presents

many advantages over the conventional process [1]. Com-

pared with conventional forming methods, semisolid form-

ing is characterized by a number of factors including a low

deformation resistance, which facilitates the shaping pro-

cess, a reduced energy input, and lower capital costs, all

of which are beneficial to productivity [2].

On account of the above advantages, extensive works

in the semisolid deformation and processing behavior of Al,

Mg, and steel have been carried out by many scientists and

engineers. The results showed that the processing parame-

ters, such as forging/rolling temperature or forging/rolling

ratio, play a very important role in controlling the mi-

crostructural evolution during semi-solid forming (SSF)

[3-8]. Generally, the deformation temperature is associ-

ated with the volume fraction of the liquid phase present,

whereas the deformation ratio is associated primarily with

the distribution of the liquid phase in semisolid processing.

Therefore, the microstructure and mechanical properties

can be improved by controlling the processing parameters

[9-11].

During the past 5 years, our group [12-15] has inves-

tigated on the deformation and thixoforging behavior ofTi14 alloy in semisolid state and found that temperature

has a significant effect on the flow behaviors of liquid in

semisolid state, which result in different precipitation char-

acteristics and mechanical properties. However, the influ-

ences of deformation and/or forging ratio, which are con-

Corresponding author: Yong-nan Chen E-mail: frank [email protected]

c University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2013

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Y.N. Chen et al., Influence of semisolid forging ratio on the microstructure and mechanical ... 267

sidered the other important processing parameters, on the

flow behaviors of liquid and microstructure as well as me-

chanical behavior of Ti14 alloy have not been reported yet.

In this work, a comprehensive study of the microstructure

evolution, particularly the precipitation behavior on grain

boundaries with different forging ratios, was carried out

to reveal the relationship among forging ratio, microstruc-

ture, and mechanical properties.

2. Experimental

2.1. Materials

Ti14 is a new + Ti2Cu-type burn-resistant Ti al-

loy [16-17]. There are many Ti2Cu phases in the alloy.

The melting point of Ti2Cu is 990C. If the deformation

or testing temperature increases above 990C, Ti14 alloy

changes to a semisolid state. During the cooling process,

the alloy undergoes both peritectic reaction (990C) and

eutectoid reaction (790C).

2.2. Compressive deformation tests

The compressive deformation tests were conducted toinvestigate the distribution of liquid with different com-

pressive ratios. Cylindrical specimens with a diameter of

8 mm and a height of 12 mm were cut from the original

bar. Specimens were compressed to heights of 6, 5, and 3.5

mm corresponding to compression ratios of 50%, 60%, and

75%, respectively, using a Gleeble-1500 thermal simulator

at a deformation temperature of 1050C.

2.3. Forging tests

The influence of processing deformation ratio on the

microstructure of Ti14 alloy in a semisolid state was stud-

ied by forging tests. The Ti14 alloy used in this paper

was a 30 kg ingot; after conventional ingot breakup andforging to bars of 40 mm in diameter, the material was re-

forged to final diameters of 20, 25, and 30 mm at 1050C,

corresponding to a forging ratio of 45%, 60%, and 75%,

respectively.

2.4. Test of mechanical properties

Cylindrical tensile specimens, with a diameter of 4 mm

and a gauge length of 45 mm, were cut from the forged

billets. The machined specimens were wet grinded using

waterproof emery paper down to 1500#. Tensile tests were

conducted using an Instron testing machine at room tem-

perature with a nominal strain rate of 4.2103 s1.

2.5. Analysis of microstructureMicrostructures after compression and forging tests

were analyzed by optical microscopy (OM; Olympus

GX71), scanning electron microscopy (SEM; JSM-6700),

and transmission electron microscopy (TEM; JEM-

200CX). All samples were cut from the center of the var-

ious specimens. The grain size after forging was calcu-

lated using Olympus 3M software and the mean transver-

sal method. SEM studies were carried out to evaluate the

microstructure and to determine the distribution of pre-

cipitates on grain boundaries. The fracture surface fea-

tures, after tensile testing, were also examined by SEM.

TEM studies were performed to observe the morphology

of precipitates in the grain b oundary regions. TEM foils

were prepared by twin-jet electropolishing in a solution of95vol% butyl alcohol and 5vol% perchloric acid at 15 V

and 30C.

3. Results

3.1. Effect of compression ratio on the distrib-

ution of liquid phase during compression

tests

Microstructures of samples compressed with different

compression ratios are shown in Fig. 1, in which partial

melting mainly occurred in the grain boundary regions,

and much less liquid was observed in the center of the

compression samples with an increase of compression ra-tio. The relationship of compression ratio and flow rate

can be demonstrated as follows [10]:

v

flV (1)

wherev is the flow rate, V is the strain rate,fl is the liquid

fraction, and is the compression ratio. The estimated

flow rate increases with an increase in compression ratio

and more liquid tends to flow to the surface of the sam-

ples with a higher flow rate. As a result, much less liquid

segregated to grain boundaries with a larger compression

ratio during semisolid processing (Figs.1(a)-1(c)).

3.2. Effect of forging ratio on microstructureFig. 2 shows the cross-sectional microstructures of

specimens forged at 1050C with a forging ratio of 45%,

60%, and 75%. It is obvious that the grain size of-Ti is

refined with an increase in forging ratio. The grain size of

the specimens and the reduction in grain size (compared

with the grain size of the as-received specimen, 700m) are

provided in Table 1. For each of the forging ratios, it can be

seen that the grain size decreases with an increase in forg-

ing ratio, and the grain size is reduced by about 51% with

75% forging compared with that of the as-received sample.

This suggests that dynamic recrystallization has occurred

during semisolid forging; moreover, higher forging ratios

can promote dynamic recrystallization, resulting in signif-

icant grain refinement. This recrystallization mechanism

gradually makes the grains rounder and finer, which plays

an important role in improving the semisolid formability.

A previous research [12] on the relationship between

semisolid forging temperature and microstructure revealed

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268 Int. J. Miner. Metall. Mater., V ol.20, No. 3, Mar. 2013

Fig. 1. Microstructures of the samples compressed at 1050C with different ratios: (a) 50%; (b) 60%; (c) 70%.

Fig. 2. Tri-planner micrographs of Ti14 alloy after forging at 1050C with different forging ratios: (a) 45%; (b)

60%; (c) 75%.

Table 1. Grain size of Ti14 alloy forged at 1050C

Forging ratio/ % Grain size / m Reduction in grain size/ %

45 418 21 40

60 387 15 45

75 345 15 51

that no -phase was found after forging at 1050C. In

this study, irrespective of the forging ratio, the samples

also mainly consisted of an -Ti matrix and Ti2Cu pre-

cipitate phase. Ti2Cu precipitation occurred both within

grains and along grain boundaries in the samples at each

forging ratio (Figs. 2(a)-2(c)). Fig. 3 shows the distribu-

tion of precipitates on grain b oundaries for the samples

after forging with different forging ratios. A coarse grain

boundary structure consisting of plate-shaped Ti2Cu pre-

cipitates was observed in the sample with a forging ratio

of 45% as shown by white arrows in Fig. 3(a), but less

plate-shaped Ti2Cu tended to precipitate on grain bound-

aries with higher forging ratios (Figs. 3(b) and 3(c)). As

a result, a plate-shaped Ti2Cu precipitate-free zone was

formed closed to grain boundaries as shown in Fig. 3(c)

with a forging ratio of 75%.

Fig. 3. SEM images of the grain boundaries after forging with forging ratios of 45% (a), 60% (b), and 75% (c).

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It was further confirmed by TEM observations on the

morphologies of Ti2Cu precipitates on grain boundaries

with different forging ratios (Fig. 4). Ti2Cu nucleated and

grew on grain boundaries with a forging ratio of either 45%

or 60% (Figs.4(a) and 4(b)), and almost no Ti2Cu pre-

cipitated on grain boundaries with a forging ratio of 75%

(Fig. 4(c)). The fraction of plate-shaped Ti2Cu on grain

boundaries decreased obviously with increasing forging ra-tio, indicating that forging ratio has significant effects on

the distribution of precipitates on grain boundaries, with

more Ti2Cu precipitation occurring on grain boundaries

under a lower forging ratio.

3.3. Mechanical properties

Table 2 shows the room temperature tensile proper-

ties of the semisolid forged alloy. It is notable that the

elongation and reduction in area decreased markedly after

forging compared with the as-received alloy and increased

with increasing forging ratio. For example, the elongation

reduced by 87.5%, 60%, and 20% after forging with forging

ratios of 45%, 60%, and 75%, respectively, compared with

that of the as-received sample. It can be reasonably ex-

plained by referring to the presence of Ti2Cu precipitation

in the grain boundary regions. However, the influence of

forging ratio on the strength is rather weak, and the yield

strength increased by 25%, 28%, and 29.5% after forgingwith the forging ratios of 45%, 60%, and 75%, respectively,

compared with that of the as-received material. These re-

sults are associated with the dependence of grain size on

forging ratio. It is well known that hexagonal close-packed

(hcp) metals such as titanium exhibit a strong grain size

dependence of strength due to the lack of slip systems [18].

Therefore, the increase in strength of the forged specimens

is associated with grain refinement induced by different

forging ratios.

Fig. 4. TEM morphologies of Ti14 after forging with forging ratios of 45% (a), 60% (b), and 75% (c).

Table 2. Room temperature tensile properties of Ti14 alloy with different forging ratios

Forging ratio / % UTS / MPa UTS / % YS / MPa YS / % El / % El / % RA / % RA / %As-received 840 680 20 40

45 955 13.6 850 25.0 2.5 87.5 11.0 72.5

60 970 15.5 870 28.0 8.0 60.0 17.5 56.3

75 975 16.1 880 29.5 16.0 20.0 30.5 23.8

Notes: (1) UTS, tensile strength; YS, yield strength; El, elongation; RA, reduction in area. (2) UTS = (UTSsemisolid

UTSas-received)/UTSas-recevied100%; YS = (YSsemisolidYSas-received)/YSas-recevied100%; El = (ElsemisolidElas-received)/

Elas-recevied100%; RA = (RAsemisolidRAas-received)/RAas-recevied100%.

Fig. 5 illustrates the SEM fractographs of the room

temperature tensile specimens after SSF with different

forging ratios. The sample with a forging ratio of 45%

displays a brittle fracture mode and a typical intergranu-

lar fracture surface (Fig. 5(a)), where the delamination ofprecipitates and the matrix is the principal mechanism for

the nucleation of microcracks. When these microcracks

propagate and reach the coarse grain boundaries, cracks

are deflected and follow the grain boundaries, causing in-

tergranular fracture. Moreover, a mixed mode of failure

involving both cleavage and dimple fracture is observed

with forging ratios of 60% and 75% (Figs. 5(b) and 5(c)).

Also, the tensile elongation increases dramatically with an

increase in forging ratio from 45% to 75% (Table 2). It

seems reasonable to assume that the low ductility is related

to the amount and distribution of these peritectic-derived

Ti2

Cu precipitates on re-solidification of the semisolid pro-cessed material.

4. Discussion

4.1. Effect of semisolid forging ratio on

precipitation

The results of compression experiments and Eq. (1)

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Fig. 5. Room temperature tensile fractograghs of Ti14 alloy after forging with forging ratios of 45% (a), 60% (b),

and 75% (c).

indicated that the compression ratio has great effect on

the distribution of liquid on grain boundaries for Ti14 al-

loy. A larger compression ratio causes a higher liquid flow

rate, and this accelerates liquid flow from the center to

the surface of the sample by grain boundaries and finally

results in less liquid segregates at grain boundaries. A

similar trend was obtained during the forging test, more

liquid segregates to the surface of the forging bar via grain

boundaries with increasing forging ratio, and this leads to

less liquid distributed at grain boundaries in the center of

the forging bar. It is well accepted that the volume fraction

and distribution of liquid are associated with temperature

and deformation ratio, respectively. In the present work,

the volume fraction and distribution of the liquid phase

has a significant influence on the amount of precipitation

of Ti2Cu on grain boundaries during the re-solidification.

The re-solidification and phase transformation pathway for

the alloy studied in our tests are described as follows: L +

Ti2Cu (peritectic) + + Ti2Cu (eutectoid) +

Ti2Cu (peritectic). The liquid phase forms a thin film on

the grain boundary during the forging, and the transfor-

mation of L + Ti2Cu (990C) occurs where the liquid

is present at the /L interface via a peritectic reaction.

The distribution and amount of peritectic Ti2Cu signifi-

cantly depend on the distribution and the volume fraction

of liquid present during semisolid processing. In this study,

more liquid segregates to the surface of the forging bar via

grain boundaries, leaving less liquid in the grain bound-

ary regions in the center of the forged bar with increasing

forging ratio, and this results in the formation of less peri-

tectic Ti2Cu precipitates, as schematically shown in Fig. 6.

This is the main reason for the formation of precipitate-

free zones along grain boundaries for forging ratios of 75%

and greater.

Fig. 6. Simulated diagrams of Ti2Cu precipitation on grain boundaries with increasing forging ratio.

4.2. Effect of semisolid forging ratio on tensile

properties

The differences in properties for the as-received alloy

and the hot forged one can be explained by the microstruc-

ture change resulting from semisolid forging. Grain refine-

ment caused by forging suggests that dynamic recrystal-

lization occurs during forging. All the semisolid forged ma-

terials showed a higher strength and lower ductility than

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that of the as-received alloy at room temperature. The

increase in strength is due to the finer grain size caused

by dynamic recrystallization. The decreased ductility is

believed to be associated with the precipitation of Ti2Cu

phase in the grain boundary regions [19]. Forging ratio has

a weak effect on the strength due to the fact that the grain

size is not altered greatly with increasing forging ratio,

at least over the range investigated, which can be clearlyseen in Table 1. However, forging ratio has a significant

influence on the ductility due to the precipitation at grain

boundaries. It is found that the distribution of Ti2Cu pre-

cipitates on grain boundaries caused by peritectic reaction

is the main reason for the difference in ductility of samples

with different forging ratios. More Ti2Cu tended to nucle-

ate and grow on grain boundaries via peritectic reaction

under low forging ratios, which resulted in a coarse struc-

ture in the grain boundary regions. The coarse struc-

ture is considered to be the main principal mechanism for

the nucleation of microcracks and subsequent intergranular

fracture. The increase in ductility with increasing forging

ratio suggests that increasing the forging ratio reduces theamount of liquid undergoing the peritectic reaction and

therefore produces less Ti2Cu precipitates, resulting in the

improvement in ductility.

In general, the above experimental results and discus-

sion indicate that the method of semisolid forging can be

employed to improve the tensile properties of Ti14 alloy,

especially on ductility, by controlling forging ratio. In ad-

dition, within the test regions, the increase of forging ratio

can facilitate dynamic recrystallization, which is favorable

to formability during semisolid processing.

5. Conclusions

The effect of semisolid forging ratio on the microstruc-

ture and mechanical properties of Ti14 alloy was investi-

gated by forging tests with the ratios ranging from 45% to

75%. The conclusions are as follows.

(1) Forging ratio affects the distribution of Ti2Cu pre-

cipitates in the grain boundary regions. As the forging ra-

tio increases, much less Ti2Cu tends to precipitate on and

near grain boundaries via the peritectic reaction during

post-forging solidification, which can lead to precipitate-

free zones along grain boundaries for large forging ratios

(i.e., 75%).

(2) High strengths are obtained for all forged alloys,

which are attributed to grain refinement caused by dy-

namic recrystallization.

(3) The differences in ductility are associated with the

distribution of Ti2Cu precipitates resulting from different

forging ratios. The extensive formation of precipitates in

the grain boundary regions with low forging ratio leads to

a significant reduction in ductility at room temperature

and causes intergranular fracture.

Acknowledgements

This work was financially supported by the Major

State Basic Research Development Program of China (No.

2007CB613807), the State Key Laboratory for Mechanical

Behavior of Materials (No. 0111201), and the National

Natural Science Foundation of China (No. 51201019).

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