The Influence of Heat Treatment on the Structure and Tensile Properties of Cast Titanium Alloy Ti-5111

Transcription

1 The Influence of Heat Treatment on the Structure and Tensile Properties of Cast Titanium Alloy Ti-51 A.C. Robinson 1, E.J. Czyryca 1, and D.A. Koss 2 1 Naval Surface Warfare Center, Carderock Division, USA 2 The Pennsylvania State University, USA Abstract The influence of heat treatment on the microstructure and tensile properties of the cast Ti-51 alloy has been examined. The effects of annealing temperature and cooling rate on the microstructural evolution in this near-alpha alloy are assessed, and the relationships between microstructure and the corresponding tensile properties and fracture behavior are explored. The results indicate good strength in both the ascast condition and after six different heat treatments. The study also indicates heat treatments promoting a large prior beta grain size also result in an occasional low value of tensile ductility in which failure occurs due to crack initiation and growth. Keywords Cast titanium, microstructure, fracture, Ti-51, tensile ductility 025/1

2 Introduction The titanium alloy Ti-51 (Ti-5Al-1V-1Sn-1Zr-0.8Mo) has relatively high specific strength, is non-magnetic, weldable, and corrosion resistant, and is lower in cost than other Ti alloys having these attributes. As such, Ti- 51 is attractive for high performance marine machinery and structural components. Ti-51 is currently being used in naval service using wrought and fabricated products in several applications. More extensive use of Ti-51 is limited in large part by cost, and one way of realizing both cost and schedule savings is to utilize the Ti-51 alloy in the cast form, where near-net-shape, complex components can be produced. Although the cast version of the Ti-51 alloy was initially used for prototype unmanned undersea vehicle hulls, increased use of costeffective cast Ti-51 components is desirable. Therefore, a more thorough understanding of the influence of processing on the microstructure and properties of Ti-51 is highly desirable, and that is the purpose of this study. Based on their experience with cast Ti-6Al-4V and due to foundry reluctance to risk heat treatments outside their standard practice, the producers of investment cast Ti-51 use heat treatments developed for the Ti-6Al-4V castings. Because Ti-51 and Ti-6Al-4V alloys are chemically and structurally different, the optimum heat treatment for Ti-6Al- 4V may not be applicable to Ti-51. This paper focuses on understanding the effects of controlling heat treatments (i.e. annealing temperatures and cooling rates) on cast Ti-51 plate material. The microstructure of a cast plate is characterized for the as-cast condition and six different thermal treatments, and the effect of critical microstructure features (prior beta grain size, grain boundary alpha thickness, alpha lath thickness, and volume fraction of each phase) on the fracture behavior of tensile specimens is assessed. Background The Ti-51 alloy is considered a near-alpha alloy due to its high concentration of alpha stabilizers (5 wt% Al) and relatively low level of beta-phase stabilizers, such as V or Mo. Wrought Ti-51 products receive a β anneal at 1010 C followed by α/β anneal at 954 C. As a result, the room temperature cast microstructure typically consists of some version of a Widmanstatten α structure in which the hexagonal close packed (hcp) α phase (usually in the form of laths or plates) is the dominant phase. However, important characteristics of that microstructure will be sensitive to both cooling rates after casting and subsequent heat treatments [3,4,5,6]. For example, the alpha plates can develop in welldefined colonies (Widmanstatten α) of parallel plates/laths nucleating from the prior beta grain boundaries, or they can develop with numerous random orientations (basketweave α) independent of the grain boundaries. These laths can also range in thickness from fine to coarse and in their length scale. The α laths are typically surrounded by thin layers of retained β, which is enriched in β-stabilizing elements [7]. These layers 025/2

3 form as a result of diffusion of the β stabilizing elements ahead of the migrating α interface. In general, critical microstructure features such as prior beta grain size, alpha lath thickness, grain boundary alpha thickness, and volume fraction of beta phase are sensitive (to some degree) to the time and temperature of the heat treatment as well as the subsequent cooling rate. The resulting microstructure changes can obviously affect the mechanical properties and fracture behavior of the alloy. Materials Investigated For this study, Wah Chang cast a 3.2-cm thick Ti-51 plate (Heat R286) using a graphite mold (GM). The plate was subsequently HIPed at 899 C for 2 hours at a pressure of 103 MPa. The chemistry of this plate, along with the ASTM Standard B 265 Grade 32 for Ti-51 plate is shown in Table 1. The plate was cut into 16.5 x 10.1 x 3.2 cm plate pieces and heat treated by PCC Structurals, Inc. as outlined in Table 2. Additionally, an as-cast plate piece was included as a benchmark in determining how each of these microstructure properties change as a function of heat-treatment temperature and cooling rate. The heat-treatments in Table 2 were intended to vary the critical microstructure features by altering the solutiontreating temperature and cooling rate. Table 2 also provides resulting prior beta grain size, volume fraction of alpha phase, and Rockwell hardness, discussed later in this paper. Experimental Procedure Microstructure Characterisation Clemex imaging software was used in coordination with optical microscopy to determine the volume fraction of the alpha and beta phases in the microstructures. One hundred regions of a metallographic specimen were measured and averaged to determine the volume fraction of alpha phase with the remainder being the retained beta phase. The prior beta grain size was measured in accordance with ASTM E 2 using the Planimetric Procedure method. A region approximately 25 mm by 20 mm in size was averaged to obtain a representative grain size. Mechanical Properties Mechanical property evaluation of each casting included hardness measurements and tensile tests. The hardness measurements were conducted using a Rockwell-C indenter with a load of 150 kg. A minimum of three measurements on each specimen was taken to obtain representative hardness values. For each cast plate, three tensile specimens were tested in accordance with ASTM E 8 using 6.4 mm diameter specimens. The tests were conducted at a strain rate of 10-3 /s and ambient temperature. Results and Discussion Microstructure 025/3

4 Figure 1 shows microstructures resulting after the heat treatments detailed in Table 2. All specimens exhibited a Widmanstatten colony structure with grain boundary alpha phase decorating the prior beta grain boundaries. The as-cast specimens (GM-1) exhibit a fine colony structure with thin alpha laths and a thin grain boundary alpha phase. Some of the alpha colonies are large and cover a significant portion of the prior beta grain, Fig. 1(a). The largest difference in microstructure of the heat- treated specimens was observed as a function of cooling rate. The specimens cooled slowly at a rate of 1 C/min exhibited significantly coarser alpha laths and thicker grain boundary alpha phase. For example, specimens GM-2, Fig. 1(b), and GM-3, Fig. 1(c), were both β annealed at 1010 C and cooled at two different rates. Specimen GM-2 (14 C/min) shows a very fine lath structure with thin grain boundary alpha, while the specimen cooled at 1 C/min (GM-3) exhibits a rather coarse structure with thicker grain boundary alpha phase. Similar cooling rate differences can be noted between the specimens given a single α/β anneal or duplex anneal. The micrograph of specimen GM-7, Fig. 1(g), shows minimal grain boundary alpha and a rather fine lath structure, consistent with these features being minimally affected by an α/β heat treatment alone. Comparisons of the prior beta grain size, volume fraction of alpha phase, and the macrohardness of each heat-treated specimen that are reported in Table 2 show the following: The as-cast material has the highest hardness and with a relatively large prior beta grain size compared to the other plates. It also has a comparatively low volume fraction of fine-scale α phase and thin grain boundary alpha. The β-annealed structures have beta grain sizes smaller than the ascast material. The β anneal decreased the hardness even though the volume fractions of each phase was similar to the as-cast condition. Increasing the cooling rate typically decreased the volume fraction of α phase that formed during cooling to room temperature, as expected. Decreased cooling rates from the β-phase field increased the scale of the alpha laths and grain boundary α phase. The α/β anneal increased the volume fraction of α phase and resulted in a small increase in hardness above that of the β anneal condition. Omitting the β anneal heat treatment and relying solely on an α/β anneal after casting resulted in similar hardness levels to those specimens that had been β annealed, but with larger prior beta grains. Mechanical Properties Consistent with the hardness results (Table 2), the yield and tensile strengths of the as-cast condition exceeded the strength of heat-treated castings, as shown in Table 3. Note in Table 3 the tensile properties for all 025/4

5 castings consistently exceeded the minimum yield and tensile strength for Ti-51 wrought plate by ASTM B 265, Grade 32. Table 3 shows the large variation of ductility values (% elongation and % reduction of area after fracture) that resulted both within a single casting set and between castings. In some cases, both elongation to failure and reduction of area values varied by a factor of two within the three specimen test set. Several specimens were below the 10% minimum elongation benchmark for adequate ductility in shock applications. As in previous studies of the Ti-6Al-4V alloy [4,9], low ductility resulted mainly from tests where microstructures exhibited large prior beta grain sizes. In to understand the large variation in ductility, the fracture surfaces of tensile specimens from casting GM-3 showing extreme differences in % elongation were analyzed. Figure 2 and Figure 3 contrast the betaannealed specimen (subject to a slow 1 C/min cooling rate) exhibiting 13% elongation with a companion specimen that failed after only 7% elongation. Figure 3 shows the presence of a large, relatively flat facet on the fracture surface of the low ductility test. The facet, which is outlined in Figure 3a and extends ~1.8 mm along the specimen surface, suggests a microstructural feature on the scale of 1-2 mm may deform and crack in a coordinated manner such to create a large, relatively planar crack that subsequently propagates and limits ductility. The comparatively small difference between percent elongation (7%) and reduction of area (%) in this case supports the hypothesis that this tensile specimen fractured by crack initiation and crack growth. Thus, we believe the large facet fractured first and initiated a large crack, whose crack-tip stress field tended to confine the crack plane promoting a smoother overall fracture surface. In contrast, the fracture surface of the higher ductility specimen shows a rough, tortuous fracture path with no large, smooth fracture facets evident. Therefore, it can be concluded that the high ductility specimen deformed without a single, dominating crack. To relate the large, flat facet on the fracture surface to its corresponding microstructure, the specimen was cross-sectioned perpendicular to the fracture surface, and subsequently ground, polished, and etched, as shown in Figure 3c & d. This figure shows the large facet on the fracture surface directly corresponds to a large prior beta grain with a linear intercept of 1.5 mm at the fracture surface. This grain was oriented in such that a crack nucleated and propagated through the entire grain with minimal resistance. The fracture surfaces, such as those shown in Figure 2 and Figure 3, suggest that the presence of a large microstructural feature on the scale of 1-2 mm can fracture and initiate a large crack that then significantly limits ductility by subsequent crack growth. As an example, Figure 4 shows two adjacent Widmanstatten colonies that share an orientation relationship 025/5

6 allowing neighboring, planar slip bands to extend 2 mm across the boundary between two large colonies with no apparent deviation. Such long slip band distances are known to promote crack initiation [8], and in this case, the result is a planar fracture surface and comparatively low tensile ductility (percent elongation of this specimen was 8%). It is likely the flat fracture facet outlined in Figure 3a is the result of such a planar slip deformation/fracture process that extends across an entire beta grain. Thus, decreasing the prior beta grain size should improve ductility, and Tables 2 and 3 suggest such a relationship where castings given a β anneal at 1010 C followed by α/β anneal showed smallest prior beta grain size and highest ductility. Conclusions A graphite mold cast Ti-51 plate was examined in the as-cast condition as well as after six different heat treatments. Specimens given a β anneal (either solely or in conjunction with an α/β anneal) had prior beta grain sizes smaller then either the as-cast plate or those given solely an α/β anneal after casting. Specimens given only an α/β anneal performed the lowest in ductility, while the as-cast specimens had some of the best properties. The ductility of cast Ti-51 is controlled by the size and orientation of the prior beta grains. Large grains oriented unfavorably to the tensile axis can nucleate a slip-initiated crack of sufficient size that limits ductility by crack growth. Therefore, reducing the prior beta grain size improves ductility and reduces scatter within groups of specimens given the same heat treatment. References 1 Stauffer, A.C., Czyryca, E.J., and Koss, D.A., The Influence of Processing on the Microstructure and Properties of the Titanium Alloy Ti-51, 3rd Inter. Conf. for Adv Mat and Proc, (2005) Gaies, J.G., Stauffer, A.C., and Czyryca, E.J., Fracture Toughness of Cast Titanium Alloy Ti-51 and Cast Commercially Pure (CP- 2), NSWCCD-61-TR-2004/25 (2004). 3 Weinem, D., Kumpfert, J., Perters, M., and Kaysser, W.A., Processing Window of the Near-α Titanium Alloy TIMETAL-00 to Preclude a Fine-grained β-structure, Mat. Sci. and Eng. A206 (1996) Lütjering, G., Influence of Proc. on Microstructure and Mechanical Properties of (α+β) Ti Alloys, Mat. Sci. and Eng., A243 (1998) Greenfield, M.A., Pierce, C.M., and Hall, J.A., The Effect of Microstructure on the Control of Mechanical Properties in Alpha- Beta Titanium Alloys, Tit Sci and Tech, 3 (1973) Rogers, D.H., The Effects of Microstructure and Composition on the Fracture Toughness of Titanium Alloys, Ti Sci and Tech, Ed. by R.I. Jaffee and H.M. Burte, 3 (1973) /6

7 7 Hammond, D. and Nutting, J., The Physical Metallurgy of Superalloys and Titanium Alloys, Metal Science, October (1977) Chestnutt, J.C., Relationship Between Mechanical Properties, Microstructure, and Fracture Topography in α+β Titanium Alloy, Fractography ASTM STP600 (1976) Cotton, J.D. and Johanson, B.M., unpublished research, The Boeing Company, Acknowledgements The work discussed in this paper was conducted in support of the Office of Naval Research through the Seaborne Materials Technology Program under Dr. Julie Christodoulou and in support of the In-house Laboratory Independent Research (ILIR) program at the Carderock Division, Naval Surface Warfare Center under John Barkyoumb. The authors also acknowledge Mr. David Lee and Mr. Boyd Mueller from the Howmet Research Center for helpful discussions. Tables Table 1: Chemical Composition (wt. %) of the Graphite Mold Cast Ti-51 Element Al Sn V Zr Mo Fe Si O C N H ASTM B 265 Grade cmthick castings * * 0.08* 0.03* 0.015* ** < *max **not determined Table 2: Heat Treatments for As-Cast Ti-51 Plates, along with Prior Beta Grain Size, Volume Fraction of Alpha Phase, and Rockwell Hardness Plate ID Phase(s) at Temp. Temp. ( C) Cooling Rate ( C/min) AS-CAST + HIP Prior Beta Grain Size (µm) Vol. Frac. α Phase Hardness (HRC) GM-1 AS-CAST AS-CAST + HIP + HIP GM-2 β GM-3 β GM-4 GM-5 β α/β β α/β GM-6 α/β GM-7 α/β *All times at temperature are 1 hour 025/7

8 Table 3: Tensile Properties of Ti-51 Castings Plate ID GM-1 GM-2 GM-3 GM-4 GM-5 GM-6 GM-7 ASTM B 265, Grade 32 Ti-51 wrought plate (typical range) Ultimate Tensile Strength (MPa) minimum 0.2% Yield Strength (MPa) minimum Elongation (%) minimum Reduction of Area (%) x to to to to 40 - Figures alpha colony within prior beta grain grain boundary alpha (a) GM-1 as cast 025/8

9 (b) GM-2 β anneal, cool 13.6 C/min (c) GM-3 β anneal, cool 1 C/min (d) GM-4 β, α/β anneal, cool 13.6 C/min (e) GM-5 β, α/β anneal, cool 1 C/min (f) GM-6 α/β anneal, cool 13.6 C/min (g) GM-7 α/β anneal, cool 1 C/min Figure 1: Micrographs of each heat-treat condition of Ti-51 casting Figure 2: Fracture surface of specimen GM-3-4 exhibiting 13% elongation 025/9

10 (a) GM-3-3 (%EL = 7%) facet (b) Flat fracture facet same grain boundary polished microstructure (c) stereo micrograph correlating (d) optical micrograph showing large facet on fracture surface with the prior β grain correlating to large facet underlying microstructure Figure 3: The fracture surface and corresponding microstructure of tensile specimen GM-3-3 that exhibited low ductility slip lines and slipplane cracking Figure 4: The cross-section of as-cast tensile specimen (GM-1-5) with low 8% elongation showing slip bands parallel to planar fracture surface that extend across two large colonies 025/10