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FABRICATION OF TiAl ALLOYS BY ALTERNATIVE POWDER METHODS - PART I


Abstract
 

Until now TiAI alloys have been produced using a variety of processing methods including forging, extrusion, casting, etc. One important problem is the variation  in  mechanical properties caused by the segregation in the composition. In order to improve these segregations, powder metallurgy is an interesting alternative for the processing of TiAI alloys. However, some drawbacks of PM have limited the practical application of TiAI alloys such as the costs of the pre-alloyed raw materials, the use of expensive processing methods like HIP, etc.
 
This work presents the activities performed at Tecnalia in the development of novel cost effective PM processing routes. Processes like Spark Plasma Sintering  and Combustion Synthesis will be presented. Microstructures, crystallographic phases, chemical analysis and mechanical properties (tensile and creep) of the different processing routes will be detailed. Main advantages of these methods are the short processing time and the possibility of using elemental powders.



1. Introduction
 

y-TiAl based intermetallic alloys have been extensively studied as potential replacements for nickel-based superalloys [1-3] for different application fields, such as turbine blades for aircraft engines, stationary turbines and space vehicles. These alloys have relatively high yield strength at elevated temperatures, good creep characteristics, and excellent oxidation/corrosion resistance. The combination of these properties, associated with their low density (3.9-4.1g/cc), make them very attractive as high-temperature structural materials for aerospace, automotive and other applications [4-5].

A number of methods have been utilized in the synthesis and processing of y-TiAI intermetallic compounds, such as Vacuum Arc Remelting (VAR) conventional melting, casting processes and hot working techniques [6-9]. Conventionally, the first step is to synthesize the alloy, which is usually done by VAR or other similar technique. The shaping and processing of the material is carried out by casting techniques (typically centrifugal or gravity casting) or hot working (forging, extrusion, etc). In these manufacturing routes, one of the main problems is the scattering in mechanical properties due to the segregation in the composition. Powder metallurgy can be an interesting alternative [10] in order to control the composition. However, main drawbacks of PM of TiAI are the costs of the pre-alloyed raw materials, the use of expensive processing methods like HIP, etc.


In recent years different alternative powder metallurgical methods have been applied to TiAI alloys, such as EBM [11], combustion synthesis [12], SPS [13,14], microwaves [151], etc. Some of them have presented very interesting results regarding mechanical propertles.
 
This work presents the results achieved by TECNALIA in both the combustion synthesis and electrical sintering methods.



2. Alternative PM methods

2.1. Combustion synthesis
 

The self-propagating High-temperature Synthesis (SHS) is based on the principle of maximum utilization of chemical energy of reacting substances (exothermicity) for obtaining inorganic compounds, such as intermetallics. The SHS method was discovered in the 1970s [16] and some useful technologies were developed on the basis of this method, several of them with the application of mechanical pressure during the reaction [17].
 

In classical SHS processes the reactants, in form of fine powders, are usually dry-mixed and cold-pressed to obtain a green body. This body is then placed in the synthesis reactor under controlled atmosphere and ignited through an electrical coil, a laser beam or an electric discharge. Other alternative method is to heat the powder until the thermal explosion of the reagents. Once ignited, there is sufficient heat release that the reaction becomes self-propagating and a combustion wave travels along the reactants converting them into the required product. A remarkable feature of the mixtures is their ability to evolve a large amount of heat during the interaction. However, due to low caloricity of reaction between Ti and Al, the classical SHS reaction between these two metals can be initiated only upon pre-heating of the green mixture [18-19].
 

However, the product of the classical SHS reaction between Ti and AI is a porous material. Applying load during the process, it is possible to obtain dense materials. As it was presented in previous works [12, 23], the diffusion of the elements after the SHS reaction is not complete and it is necessary to perform a thermal treatment after the synthesis. Figure 1 presents typical microstructures after the SHS synthesis and after the thermal treatment.
 

Figure 1.SEM images: A) Material produced by SHS,B) Material produced by SHS and posterior thermal treatment.

 

Typically, fully lamellar microstructures with large grain size (2-300 microns) were obtained after the thermal treatments above the-transus temperature. Regarding the propertiies of the materials, it was possible to obtain dense materials (porosity less than 1 %) with interesting creep properties. Main limitation, as usual with this kind of microstructure, was the elongation at room temperature. It is believed that the low ductility is linked with the oxygen content [24]. By improving the oxygen content of the alloy it may be possible to increase the ductility to a certain point.
 

Table 1. Summary of the properties of the materials obtained by SHS
Property Results
Density (%) ≥99%
Crystallographic phases TiAl and Ti3Al
Microstructure Fully Lamellar
Oxygen content 3000-3500 ppm
Tensile properties (RT) UTS: 450 Mpa Elongation: <0.3%
Creep properties (700 °C, 80 Mpa) Elongation <0.2% in 50 h
Minimum Creep rate 3•10-9s-1

 

By means of combustion synthesis, it is possible to obtain "near net shape" parts. However, a final machining is necessary to fulfil the tolerances. Figure 2 presents a blade obtained by combustion synthesis.

Figure 2. Blade obtained by combustion synthesis


2.2. Spark Plasma Sintering

 

Spark plasma sintering (SPS) is found to compact powders satisfactorily through the simultaneous application of direct current pulses of high intensity and pressure. The electric current induces a temperature elevation within the sample by Joule's effect.
 
Some works have studied the sintering of TiAl by means of SPS [20-22] and it is believed  that it is possible to obtain very fine microstructures. These alloys presented high strength whereas ductility was still limited.


As raw materials, normally pre-alloyed powders have been used, obtained by atomization or mechanical alloying. However, it is also possible the consolidation of elemental powders as it was presented in a previous work [14]. The consolidation of elemental powders occurs at lower temperature than for the pre-alloyed ones; however, the diffusion of some heavy elements with these short processing times is lirnited. Figure 3A presents a SEM image of one material obtained by SPS from elemental powders at 1300 °C and 8 minutes of holding time. In addition, the composition of different points was measured by EDS (points 1 to10 presented in the figure). The distance between two consecutive measurement points was 20 microns. Results of these measurements are presented in figure 3 B.

Figure 3. Sample obtained by SPS from elemental powders at 1300 °C, 8 min: A)SEM image, B) Composition

It is observed (figure 3) that the diffusion of Ti, Al and Cr is almost complete, but there are parts rich in Nb (around 10 at% in contrast to the 2 at% theoretical).
 
Regarding pre-aIloyed powders, by SPS it is possible to obtain samples with very low porosity (< 1%) and different microstructures.  For example, with the composition Ti-48AI-2Cr-2Nb from very fine double phased microstructures to fuIly lamellar ones were obtained (see figure 4).





                                                           B

Figure 4. HR-SEM images of SPS samples: A) Double phased microstructure B) Fully lamellar microstructure


Mechanical properties depend on the microstructure to a great extent.The materials with very fine microstructures presented very interesting tensile properties with ductility near to the 2 % at room temperature. In fully  lameIlar materials the ductility was clearly lower.


Table 2. Tensile properties of the materials obtained by SPS

Ref SPS-TF SPS-FL
Microstructure Double phased Fully lamellar
UTS 600 510
Elongation (%) 1.8 0.6
 

In addition, the double phased rnaterial presented very high ductility at 750 °C. Figure  5 shows the stress-strain curve for the double phased material at room temperature and 750 °C. The elongation at high temperature was around 40 %. This high plasticity was observed by other authors in titanium aluminides with very fine grain size [25-29] due to the dynamic recrystaIlization during the tensile test at low deformation rate, in this case was 10-4 s-I.



Figure 5. Strain-stress curve for double phased materials at RT and 750 °C


Regarding the CREEP properties of these double phased materials, tests were performed at 750 °C and 80 MPa (see table 3). The double phased materials presented properties below the typical fuIly lameIlar materials obtained by  SHS, with higher deformation rate at 50 and 300 h.


(Continue in PART II)