16 JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 200 (2008)12-24 tering.a key challe nd noor fabricability of th in fabricating/HA composite is to pr nt HA fron ounds limit their applications.Composite strengthening decomposing.The degree of HA decomposition increases wit On t other hand,to achieve a high echniques have been veloped to produce the Ni,Al-base of the g ten off and A 90 wnat mak omposite with 50wt%of Ti and 50wt%of HA,a sinte Depending on composition,the strength of NigAl increase te hea nin and a c withtemperature up to microhardn ess.while temperature resulted ty of fabricating NiAl-based composite by MIM(Bose and n simulated。 ogical conditions revealed the complete sed in the experim ent.The preferential align ment of th lution of the secondary phase bers in the matrix was attained by a modified form of MIM hate (ICP).tet caophophae tals pre also foun interingofele nental powders under exter chan ted dur identical pre use of the p cipitation of ion thermal rocedure,and pressu as evidenced b MMCs by MIM Many issues.such as the nde ntenng mecha sms ot the stem,the mech Alman and toloff furthe expl the adaptability o al pro d to b Given sintered in controle man 1991;Alman et a han d an r im ents in the mechanical 1p03 an expecte HA-based com ynthesis foll d by hot ng.Results cate 2008 2002 testsre and wang. chanek et al,1997;Hoepfner and Case. tthe Al fbers would trengthen the ostrate,espe Oktar Weng et al.,1 a1g即ey al properties The rep rted/HA composite produc materaduetotshigh-meltnont()x ellent ox 50w de sintering.HA-HA and HA-TisAl4V would be bound togethe ever,hamper the manufacture and application of this reas the Ti6Al4V powde actually restricted der atio Adding vo of chopped for the high porosity of the composite 50%)in the ved fracture sistance and hardness (Stoloff ane tud Therefore the mechanical properti f the Alman,1991:Alm an et al..1992) te are d by the HA POW s an adv ting sh omposition of HA at lower temperature (Wang and Chal orientation of the hbre Research results show that the ng et al..1994) y n the p xpa e the will be able to advance the properties of the MIMed composite oduce a fibre alienment parallel to the flow direction thein Fig.2 9b).B molding,the mechanical pro rties along a c Ithasbeen recognized thatintermetallic compounds basedo an thus be tailored according to performance requirement w ae sity,hig which are hiehl y desirable for hig ittle effect on the degree of alignment (Alma temperature structural applications as rep owever,this met uffers from sev16 journal of materials processing technology 200 (2008) 12–24 During the high temperature sintering, a key challenge in fabricating Ti6Al4V/HA composite is to prevent HA from decomposing. The degree of HA decomposition increases with sintering temperature, and it becomes significant when the sintering temperature reaches 1100 ◦C (Thian et al., 2002b). On the other hand, to achieve a high degree of densification of the composites requires a relatively high sintering tem￾perature. Thian et al.’s results showed that for a Ti6Al4V/HA composite with 50 wt.% of Ti and 50 wt.% of HA, a sintering temperature of 1100 ◦C, a heating rate of 7.5 ◦C/min and a cool￾ing rate of 5 ◦C/min produced the highest relative density and microhardness, while higher sintering temperature resulted in higher flexural strength and modulus. Furthermore, an in vitro study of the MIMed Ti6Al4V/HA tensile bars performed in simulated physiological conditions revealed the complete dissolution of the secondary phases such as tricalcium phos￾phate (TCP), tetracalcium phosphate (TTCP), and CaO after 2-week immersion. Following that, calcium phosphate crys￾tals precipitated after 4 weeks of immersion. It was also found that the mechanical properties deteriorated during the ini￾tial immersion period and then gradually recovered to almost identical pre-immersion values because of the precipitation of an apatite layer (Thian et al., 2002c). It should be pointed out that the research on Ti/HA-based MMCs by MIM is relatively new. Many issues, such as the sintering mechanisms of the Ti/HA system, the mechani￾cal properties of the composites, and the prevention of the decomposition of HA at high temperatures, need to be elu￾cidated. Given that pure HA could be sintered in controlled conditions to achieve better mechanical properties than that reported for the Ti/HA composite (Halouani et al., 1994), fur￾ther improvements in the mechanical properties of Ti/HA composites can be reasonably expected. Also, HA-based com￾posites with such reinforcements as silver, silica, titania (Nair et al., 2008; Wang and Chaki, 1993; Gu et al., 2002; Chaki and Wang, 1994; Suchanek et al., 1997; Hoepfner and Case, 2003; Oktar, 2006; Chu et al., 2004; Weng et al., 1994) have also been sintered to get attractive mechanical and biologi￾cal properties. The reported Ti6Al4V/HA composite produced by MIM consisted of 50 wt.% Ti6Al4V and 50 wt.% HA, with HA coated onto the Ti6Al4V powder in the feedstock. During sintering, HA–HA and HA–Ti6Al4V would be bound together, whereas the Ti6Al4V powder actually restricted densification of the composite (Thian et al., 2002c). That is one of the rea￾sons for the high porosity of the composite (over 50%) in the study. Therefore, the mechanical properties of the compos￾ite are mainly contributed by the sintered HA powders and Ti6Al4V/HA interfaces. Also, sintering in vacuum would cause decomposition of HA at lower temperature (Wang and Chaki, 1993; Weng et al., 1994), especially with the presence of Ti. Altering the composition and revising the sintering conditions will be able to advance the properties of the MIMed composite. 2.3. Intermetallics based MMCs It has been recognized that intermetallic compounds based on aluminium have appealing characteristics of low density, high strength at elevated temperatures, along with good corrosion and oxidation resistance, which are highly desirable for high temperature structural applications as replacement for super￾alloys. However, the brittleness and poor fabricability of the compounds limit their applications. Composite strengthening offers a way to improve the mechanical properties. Among the intermetallic compounds, nickel aluminide Ni3Al is one of the most widely studied materials and many techniques have been developed to produce the Ni3Al-based composites (Stoloff and Alman, 1990). What makes this mate￾rial unique is its anomalous thermal hardening behaviour. Depending on composition, the strength of Ni3Al increases with temperature up to approximately 600–900 ◦C. Bose and German may be among the first to investigate the feasibil￾ity of fabricating Ni3Al-based composite by MIM (Bose and German, 1988a,b; German et al., 1990; German and Bose, 1989a; Bose et al., 1992). Both prealloyed and elemental powders were used in the experiment. The preferential alignment of the fibers in the matrix was attained by a modified form of MIM, where the feedstock was extruded through a special nozzle. After debinding, the consolidation was attained by reactive sintering of elemental powders under an imposed external stress via hot isostatic compaction (HIP). Full densification was possible by appropriate selection of particle sizes, com￾position, thermal procedure, and pressure. As evidenced by the investigation, it is of general applicability to use relatively inexpensive elemental powder to fabricate hard-forming com￾pounds Alman and Stoloff further explored the adaptability of MIM for production of other intermetallic matrix (NiAl, MoSi2 and TaTiAl2) composites (Alman and Stoloff, 1990, 1991a,b,c,d, 1994, 1995; Stoloff and Alman, 1991; Alman et al., 1991). The Al2O3 fibres were chopped and dispersed into the powders by mixing in alcohol. MIM was used to form the shape and the consolidation of the composites was conducted by reactive synthesis followed by hot isostatic pressing. Results indicated superior alignment was achieved with small spherical pow￾ders after injection molding. Microhardness tests revealed that the Al2O3 fibers would strengthen the substrate, espe￾cially when aligned by MIM (Alman and Stoloff, 1991d). MoSi2 is another attractive high-temperature structural material due to its high-melting point (2030 ◦C), excellent oxi￾dation resistance and low density. Its extreme brittleness at temperature below 1000 ◦C and the low creep resistance, however, hamper the manufacture and application of this material. Adding 20 vol.% of chopped Al2O3 fibres to the MoSi2 PIM feedstock produced a MoSi2–Al2O3 composite with much improved fracture resistance and hardness (Stoloff and Alman, 1991; Alman et al., 1992). A distinct advantage of PIM in fabricating short-fibre reinforced composites is the capability of controlling the orientation of the fibres. Research results show that the expanding flow of the feedstock tend to line the fibres per￾pendicular to the flow direction, while the contracting flow will produce a fibre alignment parallel to the flow direction, as schematically shown in Fig. 2 (German and Bose, 1989b). By properly controlling the flow of the feedstock during injection molding, the mechanical properties along a certain direction can thus be tailored according to performance requirements. The key to successful alignment has been confirmed to be the size of the starting powder while the morphology of the powders has little effect on the degree of alignment (Alman and Stoloff, 1991a). However, this method suffers from sev-