Background. Titanium microstructure gen-eration has been mentioned frequently in this chapter. It should be apparent that once a chemistry has been selected, microstructures in titanium alloys usually are developed by heat treatment or other processing (wrought/cast/powder metallurgy), which of-ten uses heat and/or is followed by heat treat-ment. With the exception of CP titanium and alpha alloys, microstructural changes are in-variably produced through transformation of some or all of the alpha phase to beta phase.
The microstructure that results is a function of the way in which the subsequent changes in beta or in residual (primary) alpha occur.
Microstructural change is limited to grain re-finement and, possibly, to grain shape changes in CP titanium and all alpha alloys. Typical al-Understanding the Metallurgy of Titanium / 21
Table 3.3(c) Compositions of various beta titanium alloys
Impurity limits, wt% max
Max others, Alloying elements, wt%(a)
Designation Specifications N C H Fe O each or total Al Sn Zr Mo Others
Ti-13V-11Cr-3Al
0.05 0.05 0.025 0.15–0.35 0.17 0.4 total 2.5–3.5 … … … 12.5–14.5V,
10.0–12.0Cr MIL-T-9047;
MIL-F-83142
0.05 0.05 0.025 0.35 0.17 … 2.5–3.5 … … … 12.5–14.5V,
10.0–12.0Cr
High-toughness grade 0.015 0.04 0.008 … 0.11 (max),
0.08
0.05 0.05 0.015 1.6–2.4 0.16 0.4 total 2.6–3.4 … … 7.5–8.5 7.5–8.5V
Beta C (UNS R58640)
MIL-T-9046, MIL-T-9047, and MIL-F-83142
0.05 0.05 0.015 0.30 0.12 0.4 total 3.0–4.0 … 3.5–4.5 3.5–4.5 7.5–8.5V
Beta III AMS: 4977, 4980;
ASTM: B 348, B 265, B 337, and B 338
0.05 0.10 0.020 0.35 0.18 0.4 total … 3.75–5.25 4.5–7.5 10.0–13.0 …
Ti-10V-2Fe-3Al Forging alloy 0.05 0.05 0.015 1.6–2.5 0.13 (c) 2.5–3.5 … … … 9.25–10.75V
Ti-15-3 Sheet alloy 0.03 0.03 0.015 0.30 0.13 (c) 2.5–3.5 2.5–3.5 … … 14–16V,
2.5–3.5Cr
Ti-17(d) Engine compressor alloy 0.05 0.05 0.0125 0.25 0.08–0.13 (c) 4.5–5.5 1.6–2.4 1.6–2.4 3.5–4.5 3.5–4.5Cr
Transage 175 High-strength, elevated-temperature
0.05 0.08 0.015 0.20 0.15 (b)(e) 2.2–3.2 6.5–7.5 1.5–2.5 … 12.0–14.0V
Transage 134 High-strength alloy 0.05 0.08 0.015 0.20 0.15 (b)(e) 2.0–3.0 1.5–2.5 5.5–6.5 … 11.0–13.0V
Transage 129 … … … … … … … 2 2 11 … 11.5V
(a)Unless a range is specified, values are nominal quantities. (b) 0.1 max each, 0.4 max total. (c) 0.1 max each, 0.3 max total. (d) Alloy Ti-17 is an alpha rich near-beta alloy that might be classified as an alpha-beta alloy, de-pending on heat treatment. (e) 0.005 max Y and 0.03 max B
pha-beta and beta alloy microstructural devel-opment is covered for two selected alloys in the following sections.
Ti-6Al-4V Microstructure. Ti-6Al-4V is one of the most widely used titanium alloys. It is an alpha-beta type containing 6 wt% Al and 4 wt% V. Typical uses include aerospace appli-cations, pressure vessels, aircraft gas turbine disks, cases and compressor blades, and surgi-cal implants. Ti-6Al-4V has an excellent com-bination of strength and toughness along with excellent corrosion resistance.
The properties of this alloy are developed by relying on the refinement of the grains upon cooling from the beta region, or the alpha-plus-beta region, and subsequent low-temperature aging to decompose martensite formed upon quenching. When this alloy is slowly cooled from the beta region, alpha begins to form be-low the beta transus, which is about 980 °C (1796 °F). The alpha forms in plates, with a crystallographic relationship to the beta in which it forms. The alpha plates form with their basal (close-packed) plane parallel to a special plane in the beta phase. Upon slow cooling, a nucleus of alpha forms, and because of the close atomic matching along this com-mon plane, the alpha phase thickens relatively slowly perpendicular to this plane but grows faster along the plane. Thus, plates are devel-oped. Because there are six sets of nonparallel growth planes in a given beta grain, a structure of alpha plates is formed consisting of six noparallel sets. The Widmanstätten micro-structure developed is illustrated in Fig. 3.6.
The formation process is shown schemati-cally in Fig. 3.10. It uses a constant-composi-tion phase diagram secconstant-composi-tion at 6% Al to illus-trate the formation of alpha upon cooling. The darker regions are the beta phase left between the alpha plates that have formed. The micro-structure consists of parallel plates of alpha de-lineated by the beta phase between them.
Where alpha plates formed parallel to one spe-cific plane of beta meet alpha plates formed on another plane, a high-angle grain boundary ex-ists between the alpha crystals and etches to re-veal a line separating them. This microstructural morphology, consisting of these sets of parallel plates that have formed with a crystallographic relationship to the phase from which they formed, is called a Widmanstätten structure.
Upon cooling rapidly, beta may decompose by a martensite reaction, similar to that for pure titanium, and form a Widmanstätten pattern.
The structure present after quenching to 25°C (77°F) depends on the annealing temperature.
Different types of martensite can form, depend-ing on the alloy chemistry and the quenchdepend-ing temperature. These are designated alpha prime and alpha double prime. Upon quenching from above the beta transus (about 980°C, or 1796
°F), the structure is all martensitic alpha prime or alpha double prime with a small amount of beta (although in some alloys the beta has not been observed).
The presence of some beta in the structure after quenching from above the beta transus is
due to the fact that the temperature for the end of the martensite transformation, Mf, is below room temperature (25°C, or 77 °F) for this
al-loy. That is because vanadium is a beta stabi-lizer, and the addition of 4% V to a Ti-6%Al alloy is sufficient to place the Mfbelow 25 °C 22 / Titanium: A Technical Guide
Fig. 3.10 Schematic of the development of a Widmanstätten structure in an alpha-beta alloy (Ti-6Al-4V) Microstructures achieved at various intermediate temperatures by slowly cooling from above the β transus.
Final microstructure consists of plates of α (white) separated by the β phase (dark).
Fig. 3.9 Main characteristics of different titanium alloy family groupings
(77 °F). Thus, upon quenching to 25 °C (77 °F), not all of the beta is converted to alpha prime or alpha double prime.
For the commercial Ti-6Al-4V alloy, there are some commonly used heat treatments.
For each of these, the following descriptions are typical of the temperatures and times used. The actual practice varies with alloy producer and user. Figure 3.11 shows some microstructures formed from Ti-6Al-4V al-loy as a function of solution temperature and cooling rate.
To place the alloy in a soft, relatively ma-chinable condition, the alloy is heated to about 730 °C (1346 °F) in the lower range of the al-pha-plus-beta region, held for 4 hours, then furnace cooled to 25 °C (77 °F). This treatment, called mill annealing, produces a micro-structure of globular crystals of beta in an alpha matrix. A typical microstructure is shown in Fig. 3.12.
Another annealing treatment is duplex anneal-ing. Several variants of this treatment are used.
Typically, the alloy is heated to 955 °C (1751
°F) for 10 minutes, then air cooled. It then is heated to 675 °C (1247 °F) for 4 hours and air cooled to 25 °C (77 °F).
With the aging treatment called solution treat-ing and agtreat-ing, typically the alloy is heated at 955 °C (1751 °F) for 10 minutes, water quenched, then aged for 4 hours at a tempera-ture between 540 and 675 °C (1004 and 1247
°F), followed by air cooling to 25 °C (77 °F).
Typical tensile mechanical properties for the three treatments are compared in Table 3.4.
The strongest alloy is the one that has been so-lution treated and aged. The mill-annealed dition is stronger than the duplex-annealed con-dition, but the difference is slight.
Ti-13V-11Cr-3Al Microstructure. The sec-ond alloy to be considered is a beta alloy, Ti-13V-11Cr-3Al. This alloy is historically im-portant as the first beta alloy to see significant use in an aircraft.
Body-centered cubic alloy elements (vana-dium and chromium have bcc crystal struc-tures) used to stabilize the beta phase in tita-nium raise the possibility of the development of a eutectoid alloy reaction. Figure 3.13 shows the two basic types of phase diagrams for bi-nary beta-stabilized titanium alloys. The hori-zontal line above the alpha-plus-gamma phase field in Fig. 3.13(b) is the eutectoid tempera-ture, and the possible transformation of the beta phase directly to alpha-plus-gamma on cooling describes the eutectoid reaction.
It might be supposed that in a system with a eutectoid reaction, rapid cooling from the beta region can lead to a martensite structure in the same way that steel martensites are formed.
However, the martensite formed in quenching or rapid cooling of the beta structure is alpha prime, which is not particularly strong. Conse-quently, as determined for alpha-beta alloys, quenching may be necessary to achieve ade-quate property levels in thicker material sec-tions, but strengths and hardness levels com-mensurate with the martensites of steels will not be achieved.
Understanding the Metallurgy of Titanium / 23
Fig. 3.11 Effect of cooling rate on the microstructure of an alph-beta alloy (Ti-6Al-4V). (a)α'+β; prior beta grain boundaries. (b) Primaryα and α'+β. (c) Primary α and α'+β. (d) Primary α and metastable β. (e) Acicular α+β;
prior beta grain boundaries. (f) Primaryα and acicular α+β. (g) Primary α and acicular α+β. (h) Primary α and β.
(i) Plate-likeα+β; prior grain boundaries. (j) Equiaxed α and intergranular β. (k) Equiaxed α and intergranular β. (l) Equiaxedα and intergranular β. Etchant: 10 HF, 5 HNO3, 85 H2O. 250×
(a)
Water quenched
(k) (f)
(e)
Air cooled
(j)
(d)
(i)
Furnace cooled
(c) (b)
(g)
(h) (l)
Fig. 3.12 Structure of mill-annealed alpha-beta alloy (Ti-6Al-4V). Structure is globular particles of β in a matrix of α. Optical micrograph. ~500×
Beta alloys tend to be used because of the relative formability of the bcc beta structure compared to the hcp alpha structure. Sufficient alloy content allows retention of a metastable beta structure after quenching from the beta re-gion to 25 °C (77 °F) as indicated previously.
In this condition the alloy can be fabricated by plastic deformation. Then the component can be reheated below the eutectoid temperature to decompose the retained beta to a multiphase structure of beta and other phases that depend on the exact alloy chemistry, providing consid-erable strengthening over that of the retained beta. This approach is the basis of a few com-mercial alloys, and in this discussion, the physi-cal metallurgy of one of these, Ti-13V-11Cr-3A1, is examined.
The addition of chromium should maintain the desirable corrosion- and oxidation-resis-tance characteristic of titanium alloys. Table 3.5 shows the effect of several elements on the eutectoid temperature and composition and the alloy content required to lower the Mfto 25 °C (77 °F). Note that chromium is relatively effec-tive in retaining beta.
Data for vanadium and titanium-chromium alloys show that beta can be re-tained upon quenching from the beta region.
In the titanium-vanadium alloys, hardening occurs upon aging due to the formation of al-pha in the beta and the appearance of the inter-mediate omega phase. In the titanium-chro-mium alloys, hardening is associated with the formation of alpha in the beta and also, the phase TiCr2is subsequently formed. Thus, in the alloy of Ti-Cr-V, it could be expected that beta can be retained upon quenching from the beta region to 25 °C (77 °F), and that upon ag-ing, hardening associated with the formation of alpha and TiCr2, and perhaps the intermediate omega phase, occurs. These two latter phases are metastable and disappear upon prolonged aging.
The recommended commercial heat treat-ment for Ti-13V-11Cr-4A1 is to solution heat treat in the beta region from about 760 to 815
°C (l400–1499 °F) for 0.2 to 1 hour, then air cool or quench (depending on the size of the part) to retain the beta structure. Subsequent aging to precipitate alpha phase is accom-plished around 480 °C (896 °F) for a time usu-ally between 2 and 100 hours, depending on the properties desired. The use of aging tempera-tures around 480 °C (896 °F) is based on data that show that this is the optimum range to use for maximum strength for aging times up to 100 hours. Below this temperature, the rate of formation of alpha is too low to give apprecia-ble hardening. This is seen in the time-tempera-ture transformation (TTT) diagram of Fig. 3.14, which describes the transformation products of beta breaking down isothermally to alpha and other phases.
At aging (isothermal holding) temperatures above 480 °C (896 °F), the beta phase trans-forms too rapidly and the resultant structure is too coarse to attain maximum strengthen-ing.
24 / Titanium: A Technical Guide
Fig. 3.14 Time-temperature transformation diagram for a beta alloy (Ti-1 3V-11Cr-4Al). Alloy was initially solution treated in theβ region for 2 h at 760 °C (1400 °F); then air cooled at 25 °C (77 °F ); then aged.
Fig 3.13 Phase diagram schematics for beta-stabilized alloys (a) beta isomorphous and (b) beta eutectoid
Table 3.4 Ti-6Al-4V tensile mechanical properties(a)
Yield strength
Tensile strength
Elongation at
Condition MPa ksi MPa ksi fracture, %
Mill annealed 945 137 1069 155 10
Duplex annealed 917 133 965 140 18 Solution treated
and aged
1103 160 1151 167 13
(a) At 25 °C (77 °F) in the milled-annealed, duplex-annealed, and solu-tion-treated and aged conditions
Table 3.5 Effect of alloying elements in several titanium binary alloys on eutectoid temperature and composition, and content needed to retain beta
Alloying Eutectoid temperature
Eutectoid Alloy
element °C °F composition, wt% content, wt%(a)
Manganese 550 1022 20 6.5
Iron 600 1112 15 4
Chromium 675 1247 15 8
Cobalt 585 1085 9 7
Nickel 770 1418 7 8
Copper 790 1454 7 13
Silicon 860 1580 0.9 …
(a) Needed to retain beta after quenching at 25 °C (77 °F)