Ultrafine Grained Ti-47Al-2Cr (at%) Alloy Prepared by High Energy Mechanical Milling and Hot Isostatic Pressing

Abstract

Gamma TiAl based alloys are important materials with potential applications in aerospace and automotive applications due to their high specific strength and creep resistance. The major barrier for their applications is their limited ductility at room temperature and limited hot workability. One way of overcoming this barrier is to reduce the grain sizes to ultrafine grained (<500μm) or nanostructured (<100nm) level. In our present study, we attempt to produce bulk ultrafine grained Ti- 47Al-2Cr (at%) alloy using a combination of high energy mechanical milling of elemental powders to produce a very fine structured Ti/Al/Cr composite powder and consolidation of the powder using hot isostatic pressing (HIPping). It was confirmed that high energy ball milling using a planetary ball mill led to the formation of extremely fine Ti and Al layered composite structure. The thermal behaviour of the powder was studied using differential thermal analysis, and it was shown that the reactions between the Ti and Al phases in the fine structured composite powder occur at fairly low temperatures, below the melting point of the Al phase (660oC). The macrostructure and phase structure of the HIPped samples were also examined using optical and scanning electron microscopy and X-ray diffractometry (XRD). This paper is to report and discuss the results of this investigation.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF ULTRAFINE GRAINED Ti-47Al-2Cr (at %) ALLOY PRODUCED USING POWDER COMPACT FORGING

“…4a. The strength and density at the centre, of the powder forged sample are comparable to that of as HIPed sample, from our previous results 6 ductility in this case was found to be less than as HIPed sample, in which 0.5% ductility has been observed. This is expected due to the higher processing temperature (1150°C) used in the case of powder forging, as compared to that of HIPing (1000°C), resulting in a slightly bigger grain size in the former case.…”

“…The Cr rich regions are likely due to the inhomogenity of the particle composition of the initial Ti/Al/Cr composite powder used to produce the powder compacts. Our previous study 6 showed that the UFG Ti-47Al-2Cr (at %) alloy produced by hot isostatic pressing of the extremely fine Ti and Al layered composite powder, did not show any Cr rich regions. The as-milled Ti/Al/Cr composite powder used for powder compact forging in this study was found to have some particles rich in Ti (Fig.…”

“…The detailed parameters used for the production of powders and canning of the powder compacts has been explained elsewhere 6 . The canned compacts were forged at 1150°C temperature, with a heating rate of 15°C/minute.…”

“…For the thermomechanical processing of powder metallurgy γ TiAl based alloys the literature is limited 4,5 . Our previous studies 6 , showed that an UFG TiAl alloy can be produced using hot isostatic pressing (HIP) of mechanically milled Ti/Al based composite powders. In the present work, an UFG alloy of Ti-47Al-2Cr (at %) composition has been produced using direct forging of powder compacts and an attempt has been made to explain the process in detail.…”

High temperature mechanical properties of Ti-47Al-2Cr (at %) alloy produced using powder compact forging of a mechanically milled powder

“…The details of the powder preparation and consolidation process have been reported elsewhere [5]. Electro discharge wire cutting machine was used to cut the tensile and compression samples.…”