Titanium and alloys based on it have unique physicomechanical and corrosive properties and are widely used in the aerospace, chemical industries, in shipbuilding, mechanical engineering, automotive, biomedicine [1–9]. These properties of the alloys acquire due to the doping of titanium with elements that affect the temperature of its polymorphic transformation (α- and β-stabilizers), and neutral reinforcers, which are introduced into the charge of smelting only in the form of master alloys. Carbon, oxygen, and nitrogen form introduction solid solutions with titanium. Like aluminum, they stabilize the α phase, increasing the transition temperature for β. These elements are considered harmful impurities that deteriorate the ductility and viscosity of titanium alloys. However, their small amounts, especially oxygen and nitrogen, greatly increase the strength of titanium [10,11]. The coefficient of titanium hardening by nitrogen is 20.0 - 25.0 MPa for every 0.01% N. Hardness, respectively, increases by HB 6.0. Every hundredth fraction of a percent for carbon increases the tensile strength of titanium by 7.0 - 10.0 MPa and hardness by HB 2.0. However, the content of nitrogen and carbon in titanium alloys, as a rule, should not exceed 0.05 and 0.20 wt. %, respectively [1, 10, 12–16].
The main producers of master alloys, based on rare refractory metals, for the titanium industry in the world are: JSC Uralredmet, Russia; Gesellschaft für Elektrometallurgie GmbH (GFE), Germany; Reading Alloys, USA; EVRAZ Stratcor, Inc./Kennametal International Specialty Alloys (ISA), USA. The production of the V-Al-N-C master alloy at Uralredmet, the main supplier of master alloys for VSMPO-AVISMA Corporation, began relatively recently. At present, work is underway to improve the smelting technology, search for optimal charge mixtures, develop new precursor nitrogen and carbon containing alloys in order to produce ingots with the equal distribution of alloying elements over their cross section and with an oxygen content not exceeding 0.1 wt. % . The chemical composition of the V-Al-N-C master alloy for the major global manufacturers is given in Table 1.
Earlier [20–24], we studied the structure and phase composition of V-Al-N, Al-V-Ti-C and V-Al-Ti-N-(C) master alloys. This paper presents information on the phase composition and microstructure of the V-Al-N-C master alloy, which may be of interest not only to manufacturers of titanium alloys but also to manufacturers of highly alloyed constructional steels.
2. Materials and Research Methods
The V-Al-N-C master alloy at JSC "Uralredmet" is produced by the method of out-of-furnace aluminothermic reduction of vanadium pentoxide [25–27]. Melting is carried out with the upper fuse of the mixture in uncooled copper molds. As a nitrogenizator, vanadium-alumina powder nitrated by the SHS (self-propagating high-temperature synthesis) method is used , and graphite is used as a carbidization agent. The chemical composition of the master alloy provided to us for research is given in Table 1. An ingot weighing 96.0 kg is melted from a mixture (184.5 kg) containing, in addition to the main components, about 17.0 wt. % of the recycled materials in the crumb form of the master alloy V-Al-N-C and 4.0 wt. % of fluxing additives (CaO, CaF ).
X-ray phase analysis (XRPA) of four samples taken from the ingot of the master alloy according to the scheme shown in Figure 1, performed on an x-ray diffractometer DRON-2.0 with automatic programmed control (Cu-Kα radiation, angle range from 10 to 90, speed 1 deg/min).
The master alloy structure was studied by electron microprobe analysis EMPA using a JSM-59000LV scanning electron microscope (Japan) and an Oxford INCA Energy 200 energy-dispersive X-ray spectrometer (United Kingdom).
|№. of point||Element composition, at. %||Phase|
|1||- 49.95||1.61||47.67||- 0.77||- AlN+V(Al)|
|2||- 8.69||52.86||38.19||- 0.26||- Al O +AlN|
|3||- 47.17||2.20||46.24||- 4.39||- Al O +V(Al)|
|4-5||- 48.48||2.68||48.57||- 0.22||0.05||AlN+Al O|
|6-9||- -||- 37.01||0.53||62.25||0.21||Matrix V(Al)|
|10, 12, 13||25.90||- -||24.13||0.08||49.89||- V Al C|
|11||31.15||- -||21.41||2.34||45.10||- V Al C +SiC|
|Figure 3 (b)|
|1-3||28.39||- -||22.83||- 48.78||- V Al C|
|4-6||- 51.62||0.26||47.68||- 0.44||- AlN|
|7||- 9.92||54.22||35.53||0.15||0.11||- Al O +AlN|
|8-10||- -||- 36.03||0.58||63.14||0.25||Matrix V(Al)|
3. Results and Discussion
According to the results of XRPA (Figure 2), the V-Al-N-C master alloy matrix is a solid solution of aluminum in vanadium V(Al) . The main nitrogen-containing phase is AlN of two modifications, with cubic type aluminum nitride prevailing. The carbide phase is identified by us as V Al C Such a phase composition of the master alloy can be traced in all four samples.
The composition of the master alloys and inclusions was determined by the EMPA method. Typical sections of the structure with scanning points printed on them are shown in Figure 3. It can be seen from the figure that dark small ( 10 μm) and large (30 100 μm) inclusions of irregular shape with rounded and acute-angled edges are present in the structure of the ingot. Large inclusions form branched conglomerates. In contrast to the V-Al-N master alloy [20,21], gray areas up to 100 μm in length, mostly in the form of whisker crystals, are found in the structure of the V-Al-N-C master alloy.
According to the results of the EMPA (Table 3), the master alloy matrix (light gray area) is a solid solution of aluminum in vanadium. Dark inclusions of irregular geometric form and conglomerates formed from them are close in composition to stoichiometric aluminum nitride. As in the case of the V-Al-N master alloy, conglomerates are formed near the particles of aluminum oxide. Gray inclusions are ternary carbide V Al C , identified by us earlier during the samples study for the V-Al-N-C master alloy by XRPA method. Single inclusions, in which Al ( 40.0 at.%), N (up to 10.0 at.%) and O (the rest of it) were determined by the EMPA method, can be considered a mixture of Al O +AlN, rather than aluminum oxonitrides. Iron and silicon, most likely, were introduced during the manufacture of thin sections.
• Using X-ray phase analysis and electron microprobe analysis EMPA, it was established that the phase composition of the V-A-N-C master alloy is a solid solution of aluminum in vanadium, in which nitride (AlN) and carbide (V Al C ) inclusions are evenly distributed.
• The carbide phase mainly has the form of whiskers up to 100 microns in length. Nitride phase is represented by small (up to 10 μm) and large (30 100 μm) inclusions of an irregular geometric shape with rounded and pointed edges forming conglomerates of branched irregular shape. Conglomerates, in turn, are formed around Al O alumina.