Microporosity in hot isostatically pressed Ti-6Al-4V powder compacts

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I.

INTRODUCTION

HOT isostatic pressing (HIP) of prealloyed titanium powder has been reported to produce fully dense compacts when pressed at sufficiently high temperature and pressure and for an adequate time period. ~'2'3 Typical HIP parameters for Ti-6A1-4V alloy compacts are a temperature of 900 to 950 ~ an applied pressure of 100 to 250 MPa, and a pressing time of two to four hours. Compacts prepared under these conditions appear fully dense by optical metallography. Also, scanning electron microscopy on fracture surfaces does not reveal any porosity. The microstructural details of HIP'ed Ti-6A1-4V alloy have been reported previously.4'5 The methods of density measurement and of porosity investigation have not been sufficiently sensitive to detect pores in the size range of 100 nm or less. Such microporosity may exist even though the material appears fully dense. Micropores have been found locally in the alpha phase of sinter-compacted elemental Ti-6A1-4V powder blends by transmission electron microscopy. 6 In the present paper we report experimental evidence of micropores in apparently fully dense Ti-6A1-4V HIP compacts. This finding is important with respect to the mechanical properties, high-cycle fatigue in particular, of HIP'ed titanium alloys.

II.

IlI.

RESULTS AND DISCUSSION

Figure 1 is an optical micrograph showing the thin area of a TEM specimen of HIP'ed Ti-6A1-4V. It indicates the location of the micropores in the microstructure, namely, in the boundary region between former powder particles (arrow in Figure 1). This region consists exclusively of equiaxed alpha grains. The micropores are contained in the alpha grains and are shown in the TEM micrographs in Figures 2 to 3. They decorate a boundary-like film (dark contrast) which traverses several alpha grains. Segments of this film coincide with subgrain boundaries, but other segments are not associated with a crystallographic subboundary as evidenced from the lack of fringe contrast under several imaging conditions. As expected from previous work, 6 the pore surfaces are faceted along (0001) and {131]-1} crystal planes. Energy dispersive X-ray analysis (EDX) in STEM mode did not detect any difference in composition when measuring the film (measurement includes substantial volume fraction of surrounding alpha phase) and the alpha phase on either side of the film. The chemical analysis is given in Table I. The area is slightly enriched in aluminum as expected from alpha phase. Electron energy loss spectroscopy (EELS) was tried for the detection of light elements

EXPERIMENTAL DETAILS

The Ti-6AI-4V alloy had been HIP'ed from prealloyed PSV powder (powder sprayed under vacuum) at 950 ~ at a pressure of 245 MPa for four hours. Prior to compaction an etching treatment was performed in aqueous NaOH solution to dissolve any aluminum coating that may have been present on some powder particles.3 The powder was dried for several hours at 100 ~ in air atmosphere. The surface oxide film believed to be TiO2 was of the order of 50 nm thick as estimated from

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