Thermal Stability of Titanium Alloy VT8M-1 with Ultrafine-Grained Structure

The paper considers the effect of a long-dura on hea ng at a service temperature of 450oС on VT8M-1 with a coarse-grained (CG) and ultrafine-grained (UFG) microstructure. A duplex ultrafine-grained microstructure, composed of equiaxed grains of primary α-phase and an ultrafine cons tuent of αand β-phases, was processed by thermal treatment and further rotary swaging. This type of a microstructure demonstrates a best combina on of strength and duc lity at room temperature in comparison with the CG structure. A thermal stability of an UFG state was studied at 450°С for 50, 100, 200, 300, 400, and 500 hours. The evolu on of the alloy microstructure against the dura on of hea ng was considered by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM). No increase in the alloy structural elements and strength decrease resul ng from a long-term annealing (up to 500 hours) at Т=450°С have been observed. This proves a high thermal stability of the UFG structure and mechanical proper es of VT8M-1 processed via rotary swaging.


Introduc on
Two-phase (α+β) Ti alloys that perform well within a wide temperature range are used to produce a number of cri cal parts both in aircra� engineering and engine manufacturing. In par cular, VT8М-1 (Ti-5.7Al-3.8Mo-1.2Zr-1.3Sn), which is used to produce blades for a gas-turbine compressor (GTC), can be operated within a temperature range of 450-500 °С. The service condi ons of modern construc ons are currently increasingly demanding, while applied thermal or thermomechanical treatment (TMT) cannot provide the necessary strength of part [1,2]. One of the most promising technique to enhance the mechanical proper es of metallic materials is the forma on of a bulk ultrafine-grained (UFG) structure via severe plas c deforma on (SPD) methods [3]. It has been shown earlier that the forma on of UFG structure in two-phase Ti alloys (with Ti-6Al-4V as an example) leads to the increase in specific strength, fa gue resistance, and the enhancement in service proper es of parts made of such materials [4]. However, UFG metals processed via SPD o�en demonstrate a reduced thermal stability arising from a high accumulated internal energy, which leads to a fast relaxa on and decreases the recrystalliza on temperature [5]. As a result, the prac cal applica on of UFG Ti alloys at a service temperature of GTC parts is seriously limited.
It is well known that during rotary swaging billets are slightly reduced in a gradual manner, which enables to achieve a higher strain degree in the material along with a more homogeneous deforma on of a billet [6]. As a result, an UFG structure is formed in a bulk billet from VT8М-1, which is reported in [7]. The possibility to produce long-sized semi-finished parts suppor ng further shape-genera ng opera ons is another advantage of this technique. The goal of this work is to research a thermal stability of UFG structures at a service temperature and mechanical proper es of VT8М-1 processed by a rotary swaging with a view to assess the innova ve capacity of the material in terms of its usage for GTC produc on.

Materials and methods
VT8М-1 Ti alloy was taken as a study material. The chemical composi on of the alloy is shown in Table 1. Hot-rolled rods 70 mm in diameter were considered as an as-delivered state. The material was produced by VSMPO-AVISMA Company (Verkhnaya Salda, Russian Federa on). Rods were obtained by a vacuum-arc mel ng. The ini al material was annealed at 750°С for 1 hour (TT) in order to obtaining an equilibrium duplex structure. Rotary swaging took place at 750°С with a gradual reduc on along the diameter of billets from 70 to 32 mm. As a result, true strain of 1.56 was reached. Strain degree was calculated from the following ra o: e = ln(S 0 /S 1 ), where S 0 and S 1 are cross-sec on areas prior and a�er deforma on, correspondingly.
To study thermal stability, coarse-grained VT8М-1 samples processed by TT (hereina�er referred to as CG) and by TT + rotary swaging (hereina�er referred to as RS) were held in a furnace at 450 °С. Con nuous long-term annealing took place in furnaces produced by Noberterm Company. Holding me was 50, 100, 200, 300, 400, and 500 hours.
Tensile mechanical tests were performed using Instron universal tes ng machine at room temperature with a strain rate of 1×10 -3 s -1 , according to ISO 6892- . Cylindrical specimens cut out in the longitudinal direc on were tested.
The microstructure in various states was studied both in longitudinal and cross-sec onal direc ons using JEOL JSM 6390 scanning electron microscope and JEOL JEM 2100 transmission electron microscope. Samples for TEM-foils were cut out using electrical discharge machining, mechanically thinned to a thickness of 100 μm and then electro-polished using a TenuPol-5 facility with a solu on of 5% perchloric acid, 35% butanol and 60% methanol, at a polishing temperature within the range from -20 to -35 °C.
The X-ray diffrac on (XRD) analysis was conducted on a Rigaku Ul ma IV diffractometer. The samples were examined with CuКα-radia on (40 kV, 30mА) and the phase composi on of the alloy was determined using the Rietveld method.

Results and discussion
VT8М-1 processed by TT has a duplex microstructure (Fig. 1а). A mean grain size in a CG state of a primary α-phase is about 3 µm with a volume frac on α glob ≈ 65%. Upon RS, the material microstructure is explicitly oriented along the axial direc on (Fig. 1b), which is associated with the flow of the material during a RS. α-globules are elongated in the longitudinal sec on (Fig. 1b), and are twisted quite a lot in the cross sec on (Fig. 1c). The lamellar cons tuent upon deforma on is divided into the fragments of oval and globular shapes. However, this process is rather heterogeneous with both almost unaffected laminas and strongly fragmented areas (which used to be integral laminas) observed.
The tensile mechanical tes ng of VT8М-1 in a CG state and a�er RS at room temperature shows an increase by 20% in the ul mate tensile strength in the RS-processed state as compared to a CG one (i.e. by 200 MPa) and cons tuted 1290 MPa, and a decrease by 5% in the elonga on as compared to a CG state cons tu ng d=9%. Also, it should be noted that a uniform elonga on in the material a�er RS changed slightly making up about 3.4 % (Fig. 2). In a recent work, the ultrafine-grained structure in VT8М-1 was obtained by ECAP [7]. These specimens demonstrated a slight (about 1,2 %) uniform elonga on and rather rapid strain localiza on typical for many SPD-processed metals. It is reasonable to expect that the material subjected to RS has a higher fracture toughness compared to the ECAP-processed state.
By comparing the microstructure in CG and RS states subjected to a long-term annealing for 500 hours, it can be seen that annealing has almost no effect on the structure morphology and element dimensions ( Fig. 1 and 3). Volume frac on α р remained at the level of 65% (Fig. 4) with a similar mean size of α glob. (3 µm). The same picture was observed in an UFG state of VT8М-1 with grain size of primary α-phase and its volume frac on being at the same level a�er 500 hour hea ng ( Fig. 3  and 4). TEM method was applied to study the microstructure a�er annealing at 450°С with a holding me of 50, 300, and 500 hours more thoroughly (Fig. 5). RS processing resulted in an increased density of both grain-boundary and intragranular disloca ons leading to less dis nct interphase boundaries. The resul ng microstructure has the combina on of a lamellar cons tuent retained a�er deforma on (with a thickness of lamellas of about 150 nm) and grains as small as 300 nm of an oval shape. Such grains appeared during the forma on of a cross interphase boundary, which resembles the fragmenta on of lamellas. Local contrac ons with the forma on of cross boundaries suggest that the fragmenta on of lamellas take place by the mechanisms of groove forma on and propaga on.
The annealing for 50 hours at 450°С led to the decrease in disloca on density with clearly dis nct boundaries of lamellas and grains as a result of disloca on redistribu on and annihila on. The longer holding dura on (up to 300 and 500 hours) gave rise to recovery processes as well as to the genera on of some more perfect grain boundaries (marked with the arrow in Fig. 5). The appearance of moire contouring of such grains points to a low level of internal stress.
A mean cross lamella dimension remains at the level of 150 nm with a grain size about 300 nm. The analysis of TEM microstructure images of the annealed alloy conforms well to the X-ray results (Tab. 2). In par cular, disloca on density fell from 10.8 to 7.5×10 15 m -2 and crystallite size increased from 25 to 41 nm resul ng from the longer dura on of annealing up to 100 hours (Tab. 2).
Closer examina on of the fine microstructure of samples subjected to RS + annealing for 300h revealed some interes ng results. A diffrac on pa�ern of the refined microstructure was obtained with an area of about 3µm 2 . Both Ti α-and β-phases were observed as well as extra reflec ons forming a ring near the reflec on (Fig. 6).  It may be suggested that ageing processes caused by long-term annealing result in the precipita on of Ti-, Zr-, Si-based par cles [8]. According to the literature on the subject (Hirsch), the forma on of such ring-like reflec ons can actually point to the precipita on of a finely dispersed phase. However, we have not yet managed to accurately iden fy the precipitated phases. This will require some more me and a greater number of diffrac on pa�erns.
Upon SPD, a volume frac on of β-phase in VT8М-1 decreases (Tab. 2), which relates to β→α transforma on under the effect of large compressive stress [9].
Following long-term annealing, samples in a CG state and RS-processed were subjected to mechanical tensile tests. A long-term annealing of a CG state with a holding me of up to 300 hours resulted in a slight strengthening (Fig. 7a). At the same me, a notable reduc on in a rela ve elonga on starts only a�er annealing for 100 hours (Fig. 7b). The opposite situa on in terms of both strength and duc lity was observed a�er hea ng for 400 and 500 hours (Fig. 7a,b). This behavior can be a�ributed to β о →α+β 2 decay resul ng from a long-term hea ng at 450°С : ini al strength increase and duc lity drop is condi oned by the precipita on of secondary α-phase disperse par cles with further par cle coagula on resul ng in the opposite situa on [10]. The study of the thermal stability of a RS-processed state reveals slight fluctuations of ultimate stress, yield strength and relative elongation, which are within the limits of error. These minor variations, like an increase by 30-40 MPa, can be observed at the first point, following the annealing for 50 hours. This behavior is associated with the action of two competing mechanisms: namely, the strengthening in a RS-processed state (due to ageing during a long-term holding, like in a CG state) and the recovery in the strongly deformed material.
Thus, the rotary swaging results in both enhanced strength and ductility in the VT8M-1 alloy as compared to ECAP processing. Up to date, this technique is the most efficient way of producing rods on a commercial scale. High strength and thermal stability of the UFG VT8М-1 at 450º С open the way for successful application of the material to manufacture GTC parts.

Conclusions
The research of the effect of a duplex UFG microstructure in VT8М-1 processed via rotary swaging on the modification of its mechanical properties with an increase in a heating time up to 500 hours at 450°С leads to the following conclusions: 1. VT8М-1 processed by a rotary swaging preserves its thermal stability at 450°С after 500 hours of holding. At the same time, there has not been observed any notable change in a volume fraction of a primary α-phase of VT8М-1 samples both for a GG state and that subjected to a rotary swaging.

2.
The mechanical properties of the UFG materials change slightly due to the action of two competing mechanisms: ageing during heating at 450°С and the structure recovery. The yield strength of the UFG alloy at room temperature exceeds the characteristics of a CG state by 200 MPa and remains at the same level after long-term annealing up to 500 hours. We would like to express our gratitude to I.Ramazanov for the performance of mechanical tests in the Shared Knowledge Center of the Nanostructured Materials and High Technologies Research and Educational Center of the Ufa State Aviation Technical University.