The precipitation sequence of Ni 3 Ti in Co-free maraging steel
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I.
INTRODUCTION
M A R A G I N G steels are a class of Fe-Ni alloys which are annealed to produce a martensitic structure then aged to precipitation harden. 1.2 The precipitation processes are not well understood. There are conflicts as to whether the intermetallic phases nucleate in the martensite v s the austenite before it transforms to martensite. There is also disagreement in the identification of the intermetallic phases in the range of alloys studied, t,3,4 Austenite reversion, 5'6 which occurs during aging, makes the problem more complex. In the early 1980's a Co-free maraging steel was developed which has high strength and hardness. 7 A TEM study 8 showed that the annealed structure consists of heavily dislocated martensite. The annealed and aged samples were found to contain precipitates identified as Ni3Ti by electron diffraction. This paper will examine the transformations in cobalt-free maraging steel.
II.
Fig. 1 - Lath martensite containing precipitates from sample annealed at 713 ~ for 1 h, aged at 482 ~ for 4 h. The line directions of the precipitates are [111], [100], and [1T1].
EXPERIMENTAL PROCEDURE
Vasco Max T-250 plate in the as-received condition was rolled, annealed at 816 ~ and air cooled. The composition is Fe-18.5 pct Ni-3.0 pet Mo-1.49 pet Ti. Small foils were encapsulated in quartz vacuum tubes before annealing. The samples were annealed at 649 or 713 ~ for one hour and quenched in ice water. The aging treatments were at 482 ~ for either 30 minutes or 4 hours. The foils were mechanically thinned before electropolishing with 90 pet methanol I0 pet perchloric acid solution. The samples were then placed in a Gatan ion beam thinner with the cold stage for 1 to 3 hours to remove the oxide layers. The transmission electron microscopy was performed on the JEM 200 CX at the Army Materials Technology Laboratory and the Vacuum Generators HB5 Scanning Transmission Electron Microscope (STEM) at Massachusetts Institute of Technology.
III.
RESULTS
The structure of the annealed samples is lath martensite (Figure 1) with some twinned martensite (Figure 2). There is austenite between some laths when the annealing temperature is 649 or 713 ~
D . M . V A N D E R W A L K E R is Material Scientist, Army Materials Technology Laboratory, Watertown. MA 02172. Manuscript submitted October 10, 1986.
METALLURGICAL TRANSACTIONS A
Fig 2 - - T w i n n e d martensite in a sample annealed for 1 h at 713 ~ for 30 min at 482 ~
aged
The samples were aged at 482 ~ for either 30 minutes or 4 hours. Figure 3 is a micrograph from a sample annealed for one hour at 713 ~ aged 30 minutes at 482 ~ The plate and disc shaped precipitates were found by microdiffraction to have an fcc crystal structure with a lattice parameter of 0.36 nm. These precipitates must be austenite. STEM X-ray analysis indicates the precipitates contain Ni and Ti. Figure 4 is a dark-field image from the same sample. Large plates of austenite are present in the upper left of the micrograph and small rods are visible at two orientations at the lo
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