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The incorporation of low density, high modulus ceramic particles into a steel matrix is a potential route to improve the mechanical performance of steels. A powder metallurgy, mechanical blending route has been adopted to produce a homogeneous distribution of TiB 2 particles in both pure Fe and 316L stainless steel matrices. This approach gave large increases in both the static and fatigue strength with increasing TiB 2 volume fractions, in comparison with the matrix material. Additions of TiB 2 also resulted in reduced density and increased stiffness in the composite or lightweight steel materials, giving a speci®c stiffness increase of 52% with a fraction of 30 vol.-%TiB 2 in Fe, compared with the matrix.MST/4564

Low density, high modulus, and potentially improved wear resistance are the major bene®ts of a ferrous composite material. A BS S.156 (4%NiCrMo) gear steel reinforced with 15 vol.-% titanium diboride particles has been demonstrated for possible high performance structural applications. This composite has been produced by a powder metallurgy/mechanical milling processing route, to give a homogeneous distribution of ®ne reinforcement particles. The composite tensile strength was 90% of the matrix in the fully heat treated condition, with ductility of up to 6%. However, martensite formation and hence the hardenability was found to be suppressed in the composite. A fatigue study indicated that the titanium diboride particles did not appear to contribute to crack initiation in the composite. MST/4531At the time of writing Miss Kukilowski, Mrs Godfrey, Dr Wisbey, and Dr Goodwin were in the Mechanical Sciences Sector,

The y-TiAI-based alloys are potentially very attractive low-density materials for use at elevated temperatures. In this article, a novel method of controlling the grain size of these alloys using mechanical alloying and hot isostatic pressing is presented. 390....-------------. (002) (100) (101) Figure 1. XRD trace of PBMed Ti-48AI-2Mn-2Nb after 50 hours showing the transformation to a-HCP titanium with standard peak positions and intensities (2-theta-scale).LJ 10f.lm Figure 2. BSE image of a SPEX-milled powder displaying modified (lighter surface regions) and retained (dendritic central region) microstructure after 12 hours of milling. 40 INTRODUCTIONTitanium aluminides based on the y-TiAI (LID structure) have been widely investigated for elevated-temperature applications due to their retention of good mechanical properties at elevated temperatures. 1 -3 However, these alloys, like most intermetallics, suffer from low ductility and fracture toughness at ambient temperatures; y-TiAI also suffers from greater-than-desirable oxidation at temperatures significantly lower than its melting temperature. The use of an alloying addition such as manganese for ductility and chromium and niobium for increased oxidation resistance can improve these properties. Modification of the microstructure may also increase the ductility and fracture toughness ofy-TiAI-based alloys. In this respect, nanocrystalline-equiaxed y grains and the fine lamellar y / a 2 microstructures are considered to be good candidates. 1 -3 The nanocrystalline material should also have superplastic-forming capabilities. 4 The mechanical alloying (MA) of elemental powders has been shown to be a versatile synthesis route capable of forming equilibrium and nonequilibrium phases. MA can also refine the microstructure and is capable of producing homogenous, nanocrystalline grains. Mechanical milling (MM) differs from MA in that prealloyed powders are used instead of elemental addition and, if appropriate measures are taken, oxygen and nitrogen pickup can be limited to acceptable levels. s Following milling, it is necessary to consolidate the powders; however, because of the far-from-equilibrium nature6-10 of the powder, care must be taken to avoid excessive-time temperature exposuresY At first, hot isostatic pressing (HIPing) appears to be an undesirable method of compacting far-from-equilibrium materials; however, several studies have been carried out on the HIPing of y-TiAI composition powders produced by MA/MM that have shown that nanograins can be retained with this process. 12-18 Thus, the excellent size-and shape-making capabilities ofHIPing 18 can be used in conjunction with this type of material.In this article, preliminary results on a novel method of controlling the microstructure in a milled and HIPed Ti-48AI-2Mn-2Nb prealloyed powder are presented. EXPERIMENTAL METHODSPrealloyed Ti-48AI-2Mn-2Nb powders supplied by the Interdisciplinary Research Center of Birmingham, United Kingdom, and GKSS of Germany were milled either in Fritsch Pulverisette P5 / 4...