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(1) The uniformity of ductility in the vertical direction of a Ti compact is

degraded when the sintered density increases from 92.2% to about 94%. This is because pores become combined together and crack liked voids are

generated. The appearance of these voids is accounted for in the inferior ductility of test-pieces cut from a Ti compact.

(2) Additions of 0.3-0.6wt.% SA and 0.3-1wt.% MgSt improve the sintered density distribution in Ti compacts by controlling the pore morphology with respect to the size, aspect ratio and orientation. But 1wt.% SA creates many pores ranging in size from 50-100μm both in the top and bottom regions and this leads to very bad ductility. The consistency in mechanical properties of a sintered Ti Φ40 mm compact with a H/D ratio =1 is significantly improved by adding 0.6wt.% SA.

(3) Variable mechanical properties in a large sintered Ti compact are initiated by a variation in green density. Such a variation in the horizontal direction in a compact creates uneven sintering stresses leading to pores with a high aspect ratio and a preferential orientation. An improvement in the consistency in mechanical properties from adding an internal lubricant is because of an improvement in green density distribution in the horizontal direction. (4) Additions of SA lead to additional oxygen pick-up of about 6% and this is

acceptable. By comparison MgSt leads to an additional three fold oxygen pick up compared with SA, thus MgSt is not recommended as an internal lubricant for use in P/M Ti alloy processing.

7. Chapter 7 Sintering of Titanium and Its Alloys with

Small Amounts of Rare Earth (RE) Additions

7.1 Introduction

Very recently, adding RE to sintered titanium metal and it’s alloys has attracted a lot of attention [17, 91, 138-142]. The appeal for adding RE is to assist with scavenging impurity elements, refining the microstructure and hence improving the mechanical properties of Ti metal and alloys. RE are added to the matrix in the forms of metal, intermetallic, oxides, hydrides and so on. Owing to their acute reactivity, introducing RE elements is virtually impossible [138]. The properties would not be modified until a huge mass of RE element is added [142]. The effect of introducing RE oxides on the ductility is trivial, but it can also scavenge some chlorine impurities [140, 143]. More recently attention has been given to the use of master RE alloys or RE composites [91].

This work started mid-2012, with the aim of controlling the oxygen level in titanium alloy powders and refining the sintered microstructure. Some Y and Er powders had already been purchased a few years earlier, and LaB6 powder was

purchased this year. These RE powders were mixed with Ti metal or Ti6Al4V alloy powder either by direct addition or by mechanical milling (MM).

One of the materials of interest is HDH CP Ti powder with not too high an oxygen content (0.2-0.3wt.%). Excellent mechanical properties have been

obtained in parts made using this powder. Thus the Ti-Y, Ti-Er and Ti-LaB6 were

fabricated to test the mechanical properties, to see whether there are any benefits from RE addition to a Ti alloy to improve properties. Because pure Ti metal exists at room temperature as a single α phase, more microstructural analysis was carried

However, because of the very high oxygen content of the current used HDH Ti6Al4V powder, the mechanical properties of the Ti6Al4V-Er alloy were not measured. However, for a Ti6Al4V-Y alloy, because the mechanical milling was carried out using GA Ti6Al4V powder, with lower oxygen content, the

mechanical properties in this case were investigated.

This chapter investigates the effects on microstructure and properties of adding Er, Y and LaB6 to Ti and/or Ti6Al4V alloy. The Ti(Ti6Al4V)-RE (Er, Y, LaB6)

alloy was processed as follows:

Various microstructures and quite different mechanical properties were obtained after sintering, open die forging (ODF) and heat treatment. To get a clear

understanding of the process deriving from rare-earth additions, this discussion is analysed from the aspects of microstructure and mechanical properties. The influence of RE additions on microstructural refining and oxygen scavenging are discussed. This will provide preliminary knowledge about the influence of RE additions on the microstructure and mechanical properties of Ti metal and its alloys.

7.2 Experimental details

Ti64 powder respectively. The mixing was carried out in a stainless steel container on a roller mixer for 3h at a rotational speed of 50Hz.

For the mechanical milling process using Y additions, 1wt.% of Y was added to Ti and Ti64 powder respectively. The mixture was milled at 200rpm for 6h. Only a Ti6Al4V-1Y alloy was used for microstructural and mechanical property analysis. Because compaction of MM powder is impossible, the milled powder was added with an equivalent mass of HDH Ti6Al4V powder to get a Ti64-0.5Y alloy (0.27 at %).

For an investigation of LaB6 additions, only a Ti-0.6LaB6 alloy was fabricated. The

mass ratios of La and B in the alloy are 0.41 and 0.19wt.% respectively and so it can be regarded to be a Ti-0.4La-0.2B alloy. The atom ratios of La and B are 0.14 and 0.85at.% respectively. If all the La and B atoms are combined with O and B in the form of La2O3 and TiB respectively, 0.07wt.% of O and 0.84wt.% of Ti will be

consumed. As a result, 0.48wt.% of La2O3 and 1.04wt.% of TiB are formed.

11mm diameter, 40 mm diameter and dog-bone dies were employed in present work. Cold compaction was done using a pressure of 400-700MPa and compacted samples were sintered at 1350℃ for 2.5h. An ODF process was carried out on the sintered Ti-0.4Er, Ti6Al4V-0.4Er and Ti6Al4V-0.5Y alloys. A sintered compact, made using 70 grams of powder and with a 40mm diameter, was heated under argon to 1100℃, at a heating rate of about 125℃/min, using an induction coil. The compacts were not held at the forging temperature before forging. A powder compact was open die forged (ODFed) using a 100 tonne hydraulic press, at an applied pressure of about 783MPa. The total reduction in height of the sample was about 62%. Wire cutting was used to cut the forged samples into tensile test pieces.

were encapsulated under vacuum in a quartz tube, which had been repeatedly evacuated and flushed with argon. Then the tubes were put into a muffle furnace at the pre-selected temperature. A commonly used heat treatment was used for the Ti6Al4V alloy: The regime is:

Solution treatment: 900℃ for 1h followed by water quenching; Aging treatment: 550℃ for 6h followed by furnace cooling.

The fabrication methods used and the intended use of the samples are listed in Table 7.1.SEM and EDS were used to analyse the microstructure of sintered, ODFed and heat treated samples. The fracture surfaces were also observed by SEM. OM was used to measure the grain size of the heat treated samples. TEM was also used to investigate the morphology of the precipitates formed as a result of RE and RE oxide additions to both Ti metal and Ti alloy.

Table 7.1 Fabrication methods and the usage of Ti(Ti64)-Re alloys

Alloy addition method ODF process Heat treatment Usage

Ti-0.4Er Mixing Yes - Mechanical properties Microstructural analysis Ti64-0.4Er Mixing Yes Traditional Microstructural analysis

Ti-1Y MM - - Only for XRD

Ti64-0.5Y MM Yes Traditional Mechanical properties Microstructural analysis Ti-0.6LaB6 Mixing - - Mechanical properties