Several research efforts have been directed towards in-situ fabrication of titanium
matrix composites (TMCs) from Ti and B4C powder mixtures as one of the ways to
improve the physical and mechanical properties of titanium and its alloys. In this
perspective, the present study reports the development of in-situ particulate
reinforced titanium matrix composites from TiH2-B4C and Ti-B6O powder mixtures
The relationship between densification and microstructure and mechanical properties
(hardness and fracture toughness) of pure Ti and in-situ reinforced titanium matrix
composites have been studied in detail using pressureless and hot-pressing
techniques. Titanium hydride powder was compacted into cylindrical pellets that
were used to produce pure Ti through dehydrogenation and pressureless sintering
technique. Various composition of TiH2-B4C powder mixtures were initially milled
using alumina balls in a planetary mill. The milling was to achieve homogeneous
mixing and distribution of the ceramic partially in the TiH2 powder, as well as
uniform distribution of reinforcing phases on the resulting Ti matrix.
Dehydrogenation and conversion of loose powder and compacts of TiH2 powder was
carried out in argon atmosphere and complete removal of hydrogen was achieved at
680 and 715oC for loose and compacted powder respectively. Pressureless sintering
of pure Ti from TiH2 was carried out between 750-1400oC, while pressureless
sintering and hot pressing of TiH2-B4C was carried out in the temperature
range1100-1400oC using 30MPa for hot pressed samples in argon atmosphere.
Different sintering times were considered. The microstructure and phase composition
of the sintered and hot-pressed materials were characterized using scanning electron
microscopy (SEM) and X-ray diffractometry (XRD). Densities of the sintered and hotpressed
materials were measured to determine the extent of densification, while
Vickers hardness and indentation fracture toughness were used to measure the
mechanical properties of the sintered and hot-pressed materials. Pure Ti from TiH2 showed higher densification of above 99% of theoretical density compared to literature where lower densification and swelling was observed. Its Vickers hardness is higher than that of commercial Ti sintered under the same conditions. Titanium matrix composites (TMCs) with different volume content of in-situ formed
reinforcements (TiB + TiC) were successfully produced. The amount of
reinforcements formed increases with increased amount of B4C used in the starting
powder mixtures, while the amount of needle-type TiB decrease and size and amount
of blocky-type TiB increase with increasing volume fraction of TiB. Dense materials
and improved Vickers hardness were achieved by the hot-pressed composites
especially at 1400oC compared to the pressureless sintered composites under the
same conditions and to the relevant literature. TMCs produced in this study show
higher Vickers hardness compared to available data in the literature. The hardness
was found to depend on the volume content of the reinforcing phases. However, the
fracture toughness obtained is low (5.3MPa.m1/2) in comparison to pure Ti but is
comparable with reported data in the literature.
The mechanisms leading to the achievement of improved densification and higher
hardness and the reasons for lower fracture toughness with different sintering
temperature and composition of reinforcements in the composites are critically
analysed. It has been shown that pure Ti can be pressureless sintered using TiH2 and
reinforced Ti matrix composites with improved densification and mechanical
properties can be produced from TiH2-B4C powder mixtures. Further work on the
comprehensive study of the mechanical properties of these composites would enhance
the industrial potential of using these materials and the processing route to produce
economically feasible titanium matrix composites
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/9323 |
| Date | 04 April 2011 |
| Creators | Jimoh, Abdulfatai |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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