Microstructural control in a gamma-TiAl alloy by MA and HIPing

Abstract

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...

Controlling Residual Stress in Metal Matrix Ceramic Fiber Composite

“…Pre-alloyed powders were milled in SPX800 mills [5]. The alumina fiber has a diameter of 40-50 µm compared to 184.26µm and 84.57µm for Sapphikon and Tiboride fibers, respectively.…”

Residual stress development during fabrication and processing of gamma-titanium based composites

“…Both the alumina polycrystalline fiber and Sapphikon fibers had an average diameter of 50 m. The composite material was fabricated at the McDonnell-Douglas Research Center using hot isostatic pressing (HIP'ing) of powdered matrix material over the different fiber types. Pre-alloyed powders were milled in SPX800 mills [7]. All powder handling and milling operations were carried out in argon glove boxes and Titanium lined vials to minimize contamination.…”

“…The fundamental equation of x-ray strain determination is [5]: In this equation, is the material rotation angle, is the material tilt angle, d is the inter planar spacing at angles of and in the stressed material, d o is the inter planar spacing of the unstressed material, and ⑀ ij are the strain components. The subscripts 1 and 2 denote strains in the surface plane, while 3 indicates strains perpendicular to the surface plane.…”

“…Like most intermetallics, ␥-Ti alloy suffer from low ductility and fracture toughness at ambient temperature and low oxidation resistance at temperatures significantly lower than its melting temperature. The use of an alloying addition such as manganese for ductility and niobium for increased oxidation resistance can improve these properties while maintaining the high modulus and creep resistance of the gamma system [5]. Residual stresses in composites are typically thermal in nature and are the result of the difference in coefficients of thermal expansion (CTE) between the fibers and the matrix.…”