h e e d i n e s of SineSwedish Structural Materials SvmDosium 2007
Synthesis of TiAl-Based Intermetallic Compound by Electro-Pressure Sintering
wu y. 1 2 . 3,
(ltentral Iron & Steel Research Institute. Beijing 100081, China; 2.Advanced Technology & Materials Co. Ltd., Beijing 100081. China: 3.School of Materials Science and Engineering. Inha University, Incheon 402-751, Korea)
Abstract: Elemental powder metallurgy is an efficient processing technique to synthesize intermetallics of high melting points. TiAl-based intermetallic compound was produced by electro-pressure sintering (EPS) consolidation techniques. Through a balanced control of the heating scheme, sintering temperature and pressure, near full density state of TiAl-based intermetallic compound was obtained, in which a high hardness and absence of porosity, respectively, were realized. An optimum condition of
EPS was determined to be 98O0C/90MPd120s.Aided by interstitial carbon atoms, the intermetallic compound showed attractive compressive strength at room temperature. The roles of w b o n atoms were refinement of the iamellar microstructure and precipitation hardening as effective in the hot extruded materials.
Key words: sintering; powder consolidation; TiAI; EPS; compressive strength
1 Introduction TiAl-based intermetallic compounds are receiving a great deal of attention during the past decade since they have the potential application in aircraft and automotive engines"-']. However, they still suffer from insufficient ductility at room temperature and poor elevated temperature oxidation re~istance'~-~'. Among the synthesis methods of the intermetallic compounds, elemental powder method is an attractive route because of the homogeneity in alloy chemical composition and the fine grain siLe in the final product. An experimental alloy composition of Ti-46.6AI- 1.4Mn-2Mo-0.3C was developed by hot extrusion technique "-". From the alloy design standpoint, a microstructural refinement, particularly that of the lamellar structure, is necessary to render desirable mechanical properties such as fracture toughness in the alloys consisting of y and a2[ l o * 'I1. To optimize desired mechanical properties, therefore, it is essential to control the size of lamellar colonies and individual lamellae. A considerable amount of alloy design effort has been devoted to fine-tuning the specific traits of the lamellar structure, the interlamellar spacing being a crucial parameter['*'. In this respect, the role of interstitial alloying elements draws much attention since they can reduce the interlamellar spacing quite effectively. Electro-pressure sintering (EPS) of elemental
powder mixture has been studied by Hwang et a ~ ' ' as ~ ]a powder metallurgical approach to consolidate the Ti& -(Cu-Nb-C) compounds. Particularly, the synthesizing methods developed so far are sufficient to bring about a full density. The objective of the present work was to develop an alternative consolidation route of TiAl amenable to industrial practice. EPS method was adopted as a fast and effective means to consolidate the compound.
2 Experimental Procedure The nominal chemical compositions of the experimental alloys was Ti-46.6A1- 1.4Mn-2Mo-0.3C (at.%). Elemental powders of Ti, Al and C were 70pm, 20pm and IOpm, respectively, in the mean particle size and of higher than 99.5% in purity whereas Mn and Mo powder was in lOpm and 4pm, respectively, in the mean particle size and 99.9% in purity. Elemental powders were mixed in a stirrer for two hours under argon atmosphere. Due to agglomeration, the average particle size increased to about 100pm after mixing. Powder mixtures were further milled in an attritor filled with ethanol for 6h. Stainless steel balls of 4.75 mm in diameter and 35 times the weight of powder were used for milling the powder mixtures. Milled powders were sieved and dried for 24 hours in oven. Dried powder aggregates were further milled in a tubular shaker mixer
Proceedinns of Sino-Swedish Structural Materials SvmDosium 2007
for 30min in argon atmosphere, which yielded powders of 30 pm in average particle size. Using a double-piston compacting machine, the mixed powders were cold-compacted into round buttons and parallelepipeds. Under a compaction pressure of 700MPa, specimens weighing about log and with a relative density of 60% of the theoretical were obtained. A schematic of the EPS machine used for the present work is shown in Fig. 1. Through preliminary experiments on the applied pressure (30-100MPa) and hold time (150-600s), an optimum condition of EPS was found to be 98O0C/90MPd120s. All the specimens for analyses, therefore, have been produced by this identical processing condition. To ensure consolidation without undesirable reaction such as fire and explosion it was crucial to increase the temperature and the external pressure gradually. For this purpose, a stepwise enhancement of the two processing variables, as shown in Fig. 2, was used in the present study. After EPS, the specimens were heat treated at 1400°C for l h followed by air-cooling (AC). Phase identification of the compound was performed by X-ray diffraction (XRD) using a Cu-K,
. 1 Press
20 Power Limit
Time (sec) Fig. 2 A stepwise sequence of EPS processing route to produce Ti-46.6AI-1.4Mn-2Mo-0.3C intermetallic compound
target under an acceleration condition of 40kV and 25mA with a scanning rate of 0.04"/s. Microstructures of the as-consolidated and as-solLtion treated specimens were examined by optical microscopy (OM) and an H-800 transmission electron microscope (TEM) operated at 200kV. Energy dispersive spectroscopy (EDS) analysis was conducted in SEM under accelerated voltage of 20kV. Density of the specimens was measured using the Archimedes' method. Micro-Vickers hardness of consolidated specimens was measured with an applied load of 200g. Compression tests at a high strain rate of 1 . 6 7 ~ 1 0 ' ~ ~were '' conducted at room temperature. Cylindrical compressive specimens, with a diameter of 8mm and a height of 5mm, were used on a Gleeble- 1500 simulated test machine.
3 Results and Discussion
Fig.1 Schematic diagram of EPS apparatus used for densification of Ti-46.6A1-1.4Mn-2Mo-0.3C intermetallic compound
Many trials were made prior to successful densification of the experimental specimens. For poreless densification of the elemental powder compact, it was crucial to optimize the EPS prccess variables. The major obstacle for densification was the porosity induced by an explosive reaction among elemental powder particles. Rapid heating occurred through a synergistic effect of the exothermic reaction heat among elemental powders in addition to the external heat applied to the EPS apparatus. Attempts to control the reaction rate by
bxeedines of Sino-Swedish SUvctural Materials Svmmsium 2007
varying the average particle size of powders, the mixing time in the atiritor or the temperature and pressure of EPS were all unsuccessful. The most effective means of controlling the reaction rate was to decrease the heating rate, particularly near 980°C, to 80% of the maximum rate, which was done by manually controlling the power input unit. In this way, a heating rate as low as 1.5"C/s was obtained which resulted in satisfactory densification. The analysis results of XRD showed the major diffraction peaks corresponding to y-TiAl and az-Ti3AI for all three specimens consolidated by EPS. As shown in Fig.3. clear peaks of y-TiAl and a2-Ti3Al for the specimen proceed by 90MPa/98O0C were detected, indicating that TiAl-based compound containing two phases successfully obtained by suitable processing parameters. No peaks originated from carbides were observed in the present study, which was probably due to low amount.] The density was improved with increasing the sintering temperature from 700 to 980°C with a constant pressure of 9OMPa. All the specimens from 700 to 900°C had low relative densities, but high relative densities were obtained for the specimens at sintering temperature from 950 to 1000°C,in this case, relative densities higher than 95% of the theoretical were obtained. Fig.4 shows the microstructures of TiAl-Mn-
intermetallic compound consolidated by EPS method of WMPa/9801:
Mo-C compound consolidated by different EPS processing. For the specimen applied for WMPd950"C in Fig.4 (a), all elements was not fully consolidated and large porosities with black contrast were observable. As shown in Fig.4 (b) and (c), both specimens exhibited dense microstructure, particularly for the case of 90MPaf98O0C, in which the isolated regions with light contrast were enriched Al by EDS in SEM, indicating the white regions were TiAl-forming areas, but the black regions belonged to Ti3Al. By microscopy, it was confirmed that porosity in this specimen was insignificant. Therefore, it was concluded that the present EPS method, starting from elemental powders, yielded TiAl-based compound of a near full-density. Although the specimen consolidated at 90MPa11000"C showed high density, the microstructure was slight overheated. Therefore, the excellent processing in the present study for EPS was 90MPa/98O0C. Especially to be noted that carbon addition decreased the density of the compounds. Assuming no change in crystal structure, the density would not have decreased if carbon atoms were present as interstitial solid solutes. Thus, the density decrease was attributed to carbon atoms participating in forming a third phase such as carbide, which was confirmed by TEM analysis. The specimens consolidated by 90MPd980"C and 90MPdlOOO"C presents the microstructures of TiAl-Mn-Mo-C intermetallic compound after heat treatment of 1400"C/lh/AC as shown in Fig.5 (a) and (b), respectively. Regardless of the different EPS processing, the microstructures of both specimens were lamellar feature. As shown in Fig.5 (a), uniform colonies of aligned a2+y platelets within the grains, i.e. fully lamellar microstructure, were observed. However, the microstructure for the specimen consolidated by 90MPa/1000"C showed a nearly lamellar feature. The nearly lamellar microstructure consisted of predominantly large lamellar grains of approximately 150pm in diameter coexisting with minor amounts of fine gamma grains of about 80pm in size located at lamellar boundaries or prior a grain boundaries. A recrystallization probably occurred due to higher sintering temperature at 1OOO"C.
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Fig.4 Optical micrographs showing isolated regions (light contrast) of Al-enrichment of Ti-46.6A1-1.4Mn-2Mo-OX intermetallic compound consolidated by Merent EPS methods: (a) 9oMpa1950°C, (b) 9OMPa/98O0C and (c) 9oMPd1000°c.
Fig.5 Optical micrographs showing fully lamellar (a) and nearly lamellar (b) microstructures of Ti46.6A1-1.4Mn-2Mo-0.3C corresponding to different EPS methods of 9OMPa/98O0C and 9oMPa/lOOOC, respectively. specimens were heat treated at 140O0C/lh/AC.
As reported in the hot extruded TiAl-Mn-Mo-C alloy^[^-^', carbides formed in the microstructure. An example of the carbides located along the grain
boundaries of y and a2in TiAl-Mn-Mo-C intermetallic compound consolidated by 90MPd98O"C with solution treatment at 14Oo"C/lh/AC is shown in Fig.6. These precipitates were acicular in shape, and formed at the az/y boundaries owing to dissolution of the a 2 laths and grew as the holding time during the heat treatment. The microstructures can be interpreted in relation to the isothermal section and the vertical section of the Ti-Al-C ternary phase diagram reported by Schuster et al. 14]. Ti3AlC (P-phase) is in equilibrium with Tic, a2-Ti3A1 and a-Ti, and Ti2AlC (H-phase)
with Tic, a2-Ti3Al and y-TiAl. The results of selected area diffraction patterns showed that the acicular precipitate was of perovskite type crystal structure in which the carbon atoms occupy the octahedral site of Ll2 lattice. Therefore, these acicular precipitates were determined to be carbides of the Ti3AlC type. The lattice parameters for this carbide precipitate calculated from the diffraction patterns were a=0.415nm, similar to the value published by Gouma et al."51. It was also found from the microscopic analysis that the long axis of Ti3AlC was parallel to the [Ool] direction of the y-TiAl matrix and had just a single variant. As was indicated in our previous paper"], the orientation relationship between the Ti3AlCand y-TiAl phase was 113
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sintering temperature is shown in Fig. 8. The specimen consolidated by 90MPd98O"C showed the highest compressive yield strength among three specimens, the value of 712MPa was obtained, which was in agreement with the results of hardness. A decrease of the values of the compressive strength, by a significant margin, 33 and 19% for the specimens consolidated by 90MPa/950°C and 90MPa/1000°C, respectively, were determined.
4 Conclusions Fig. 6 TEM micrograpb showing precipitates of carbides along
Ti-46.6AI-1.4Mn-2Mdl3C intermetallic compound consolidated by WMPa/980 C
From the study of an EPS method of synthesizing Ti-46.6Al- 1.4Mn-2Mo-0.3C intermetallic compounds, the following conclusions were drawn: (1) Near-full-density compound as large as 25 mm in diameter and 5 mm in height were obtained by EPS
treatment at 1400°C/lh/AC
confirmed to be of a cubic relationship, namely, (Ool), // (OOI), and [OlO], //, in agreement with the results of Tian and Nemoto The hardness of experimental compounds showed an absolute dependence on the sintering temperature. Fig.7 compares the variation of hardness of Ti-46.6A1-1.4Mn-2Mo-0.3C intermetallic compound under different processing conditions. All the specimens in the consolidation state showed higher the values of hardness than the solution treated ones, the gap between two values is about 60-100Hv, which was attributed to fully solution of the elements in the latter. Corresponding to the microstructure for each by specimen, the specimens consolidated 90MPa/95O0C and 90MPd1000"C showed the lower hardness. 359.5 and 385.2Hv, respectively. The lowest hardness was due to unconsolidated microstructure for the former, and due to the nature of the coarse microstructure for the laner. As expected, the specimen consolidated by 90MPd1000"C exhibited the highest the value of hardness (419Hv) among all specimens. which resulted form fine fully lamellar microstructures, see FigS(a). The identical trend was confirmed by compressive tests at room temperature. The variation of the compressive strength as a function of the 114
Fig.7 Effect of consolidation temperature on hardness of
'Ii-46.6AI-1.4Mn-2Mo-03C intermetallic compound under different processing conditions.
Fig8 Effect of consolidation temperature on compressive strengtb of Ti-46.6A1-1.4Mn-2Mo-03Cintermetallic compound solution treated at 14OOC/lh/AC
Proceedings of Sino-Swedish StructuralMaterials Symposium 2007
processing of elemental powder mixtures. By controlling the processing parameters such as the heating scheme, sintering temperature and pressure, TiAl-based intermetallic compound with a high hardness and absence of porosity was obtained. (PfTheoptimum processing was determined to be 980°C for 120s under an external load of 90MPa. The compound exhibited a dense microstructure after EPS, and a fully lamellar microstructure after solution treatment at 1400°C/lh/AC was obtained, in which a good consolidation of the elements was realized. (3) Carbon addition improved the compressive strength of TiAl-based intermetallic compound by refining the lamellar microstructure and precipitation hardening as the same efficient as the hot extruded TiAl-based alloys.
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