APFIM STUDY OF THE INITIAL STAGES OF AGING IN DILUTE URANIUM ALLOYS

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APFIM STUDY OF THE INITIAL STAGES OF AGING IN DILUTE URANIUM ALLOYS

B. Jenkins, D. Edmonds

To cite this version:

B. Jenkins, D. Edmonds. APFIM STUDY OF THE INITIAL STAGES OF AGING IN DI- LUTE URANIUM ALLOYS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-277-C6-282.

�10.1051/jphyscol:1987645�. �jpa-00226850�

Colloque C6, suppl6ment au nO1l, Tome 48, novembre 1987

APFIM S T U D Y OF THE INITIAL STAGES OF AGING IN DILUTE URANIUM A L L O Y S

B.A. Jenkins and D.V. Edmonds

Department of Metallurgy and Science of Materials, University of Oxford, Oxford OX1 3PH, U.K.

Abstract. Atom Probe Field Ion Microscopy (APFIM) has been used to characterise the microstructural and chemical changes occurring in depleted U - 0.75wt% Ti and depleted U - 2.2 wt% Mo during aging to peak strength.

Strengthening in U - 0.75 Ti was found to follow the classical GP zone -

equilibrium precipitate route. Precipitation hardening was confirmed in the U - 2.2 Mo alloy.

I - INTRODUCTION.

Dilute uranium alloys possess a range of useful mechanical properties and can be applied where a combination of high strength and high density is required. When dilute uranium - titanium and uranium - molydbenum alloys are quenched from the high temperature phase field, supersaturated martensite variants of the uranium CY phase are produced. These are designated C!'a for the acicular martensite in the U -

Ti alloy and CY'b for the banded martensite in the U - Mo alloy. On aging, a range of microstructures, with a commensurate variation in mechanical properties, are produced. A combination of high density and strength can be obtained in each of the alloys in the peak-aged condition /I/. The details of the fine scale strengthening reactions leading to peak strength in these alloys have not been completely documented. Due to uranium's low electron transparency and high oxidation rate, APFIM is one of the few techniques which can provide high resolution morphological and micro-analytical details in uranium alloys. FIM has been previously performed on pure uranium / 2 / , but this work represents the first APFIM investigation into the phase transformations of uranium and its alloys. Some aspects of this study have been reported previously /3/. This investigation uses Oxford University's VG FIM 100 Atom Probe to examine the microstructural and chemical changes occurring during the initial stages of aging, and aging up to peak strength, in depleted uranium - 0.75wt% (3.61at%) titanium and uranium - 2.2wt% (5.29at%) molybdenum alloys. The strengthening mechanisms in both the alloys are discussed in relation to previous investigations.

I1 - EXPERIMENTAL.

APFIM. Suitable rods for FIM tip preparation were machined from heat treated depleted U - 0.75 wt% Ti alloy stock. For the U - 2.2 wt% Mo alloy, rods were sliced from bulk water-quenched samples, using a slow speed diamond cutting wheel, and subsequently aged in vacuum. Compositions for each of the alloys are nominal. A two stage electropolishing technique was used to manufacture tips suitable for APFIM.

The first stage used a 25% perchloric acid - 75% acetic acid electrolyte at 20V and 20" C. Final, slow polishing was achieved with a 5% perchloric acid - methanol solution at - 60' C and at 10V.

FIM imaging and atom probing were performed with tip temperatures in the range of 60-100K using both argon and neon as imaging gases. Uranium and its alloys are particularly prone to oxidation and despite the careful measures taken, heavy oxide build up on the newly polished samples was unavoidable. To facilitate the oxide removal, samples were initially imaged in argon. However, it was necessary to remove the argon and insert neon to obtain clear, detailed FIM images once the oxide layer

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987645

JOURNAL DE PHYSIQUE

had been removed. AP analysis was performed with and without imaging gas at pulse ratios of between 15 and 20%.

I11 - ^RESULTS AND DISCUSSION

a. Mass Spectra. Mass spectra of the U-0.75 wt% Ti and U-2.2 wt% Mo alloys studied are presented in Figures la and b respectively. Uranium was found to appear predominantly in the u2+ and u3' charged states, however due to the presence of residual argon imaging gas, the species U A ~ ~ + also appeared in significant quantities. Some confusion arose as to the identity of the species at M/n ratios of between 90 and 100 in the U - Mo alloys as both ~ 0 2 ~ ' and MO' satisfy the M/n values. This situation is further complicated b,y the presence of the U A ~ ~ + peak. A comparison between the number of ions in each of the peaks with what would be expected from the 7 ionised MO+ isotopes as well as the 15 possible ~o~~~ species.

suggested that ~ 0 2 ~ ' was the dominant charged state. The presence of some quantities of Mo+ cannot be ruled out, however with the result that the bulk Mo analysis may be slightly over-estimated by a maximum of 0.6 at%.

b. U - 0.75 wt% Ti. Recently, electron diffraction evidence has been presented to suggest that a decomposition reaction, consistent with spinodal decomposition, occurs during the quench immediately after martensite formation in an uranium - 0.83 wt% (4 at%) titanium alloy /4/. The FIM image and associated concentration profile generated by random area AP analysis of U - 0.75 wt% Ti are given in Figures 2a and b respectively. The FIM image reveals no modulated contrast and statistical 'auto- correlation' analysis of the composition profile, Figure 2c, reveals no evidence for composition modulations. Also, sample distribution and titanium spacing analyses confirm that no transformation has occurred. Thus, the micro-analytical evidence achieved by APFIM was not found to be consistent with the proposition that decomposition occurs during the quench.

The effect on Vickers hardness of aging for 5 hours at temperatures between 300' and 450' C is shown in Figure 3. Aging at 375-C produces a small increase in hardness, with no detectable change in TEM images. Neon FIM images contain small bright regions which can be interpreted as being due to clusters of two or three titanium atoms. This is confirmed by spacing analysis of the detected titanium ions which reveals that titanium atoms are not uniformly or randomly distributed in the alloy.

Such fine scale clustering has been suggested as being a precursor to GP zone formation /5/. Titanium concentration profiles generated are similar to those of the as-quenched material with no periodic titanium concentration variation, which would be promoted if a hypothesis for spinodal decomposition was correct.

After aging for 5 hours at 400°C, changes in the structure as a result of the aging process appear with large numbers of small, black dots visible in bright field TEM images, but without diffraction evidence for another phase. After aging for 5 hours at 425'C both small dots, and some larger precipitates, which produce streaking of matrix spots in electron diffraction patterns, are present. Neon FIM images of material aged at 425OC for 5 hours, taken at below Best Imaging Voltage (B.I.V.) to preserve contrast between the precipitates and matrix, suggest a duplex structure, Figures 4a and b. The brightly imaging zones visible, which correspond to the small, black dots in the TEM images, can be interpreted as being coherent GP (11) zones characterised by two titanium or titanium-rich planes, whose separation is no more than one or two atomic planes. These follow what are thought to be the rings of the (O1O)q pole. These zones are approximately 2.5 - 4 nm in size and their average composition, as estimated from ladder diagrams, is 8 at% Ti, although the small Size of these zones makes accurate determination of the composition difficult. Their spacing of -5nm corresponds to the spacing of the small peaks in the random area AP titanium concentration profile Figure 5. In addition to the GP zones, larger, approximately 10 nm precipitates are also present. One such particle is indicated with an 'XI in Figures 4a and b. Selective analysis of these larger precipitates is possible, Figure 4c, and it is conclusively shown that they contain 33.3at % Ti,

hardening systems /6/.

Various authors have studied aging in this system and have attributed peak strength to be due solely to coherent U2Ti precipitation /7/ or coherent GP zone formation /1,8,9/. This investigation has shown that aging follows the classical GP zone -

equilibrium precipitate route with the structure in the peak aged condition ( 5 hours at 425" C) containing both GP (11) zones and small, incoherent U2Ti precipitates.

The hardness of individual martensite plates continues to rise with increased aging, a result of the production of a fine dispersion of incoherent U2Ti precipitates, but ductility is limited by the embrittling precipitation of coarse U2Ti on CY'a plate boundaries and the onset of cellular decomposition on prior 7 grain boundaries.

c . U

-

reveals an essentially random distribution of both molybdenum and interstitial hydrogen.

The effect on Vickers hardness of aging for 5 hours at temperatures between 200" and 400" C is shown in Figure 3. The mechanisms of strengthening during aging to peak strength have not been conclusively determined by optical or electron microscopy as little change in the microstructure is apparent amidst the finely twinned CY'b structure /9,10,11,12,/.

The neon FIM images of U - 2.2 wt% Mo aged 5 hours at 350" C, reveal the presence of small, approximately 8nm diameter, molydbenum clusters, Figure 6a, and larger, approximately 20nm diameter molydbenum rich precipitates, Figure 6b. The molybdenum content of these larger precipitates is 29 at%. Mo, which compares with May's x-ray analysis for

6'

6

Neon FIM images of material aged for 5 hours at 400-C clearly shows a two phase structure, Figure ?'a, a result of the continued precipitation. Selective analysis of these precipitates revealed that they contain 29% molybdenum, Figure 7b. A solute depleted regionlalso exists adjacent to the precipitates. The fine scale lamellar two phase cX +

6

Previous investigations into the structural changes occurring during early aging in dilute uranium - molybdenum alloys have variously attributed the strengthening to be due to Mo rich zones or coherent precipitates /lo/, GP zone formation /12/, spinodal decomposition / 7 / and the immobilisation of twin boundaries with interstitials /11/.

This APPIM study has demonstrated that clustering of molydbenum and precipitation of 6'precipitates accounts for at least some of the strengthening occurring during early aging. No evidence was found to support the hypotheses that spinodal decomposition /7/ or the stabilisation of twin boundaries by interstitials /11/ are responsible for strengthening.

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IV - CONCLUSIONS.

APFIM has been successfully applied to the study of the strengthening reactions occurring during early aging of U - 0.75 wt% Ti and U - 2.2 wt% Mo alloys. The following conclusions can be drawn:

1. No evidence was found for a decomposition reaction occurring during the quench in U - 0.75wt% Ti.

2. Strengthening in U - 0.75wt% Ti follows the classical GP zone - equilibrium precipitate route. Pre-precipitation clustering was detected and the GP (11) zones observed after aging at 425O C are characterised by two titanium or titanium rich layers.

3. In U - 2.2wt% Mo aging proceeds by the formation of molydbenum clusters and larger molydbenum rich precipitates.

ACKNOWLEDGEMENTS.

This research is supported by the Royal Armament Research and Development Establishment, Fort Halstead, U.K.. Acknowledgement is made to Professor Sir Peter Hirsch FRS for the provision of laboratory facilities and to the Procurement Executive, Ministry of Defence for financial support. Dr G.D.W. Smith is thanked for use of APFIM equipment and Dr A. Cezero, Dr C.R.M. Grovener and Dr M.G. Hetherington for help with APFIM techniques. One of us (BAJ) wishes to thank The Institute of Metals (U.K.), British Vacuum Council, and the Commemorative Association for the Japan World Exposition for funds to attend this conference.

REFERENCES.

/1/ Eckelmeyer, K.H. and Zanner, F.J., J. Nucl. Mat. 62(1976) 37.

/2/ Carroll, J.J. and Melmed A.J., Surface Science lX(1982) 225.

/3/ Jenkins, B.A. and Edmonds, D.V., Proc. Conf 'Phase Transformations '87', Cambridge, England, July 1987, ('to be published).

/4/ Landau A., Kimmel G. and Talianker M., Scripta Met. 2 (1986) 1313.

/5/ Yoshlda H., Proc. Conf 'Decomposition Sonnenberg, Germany, September 1983, Scripta Met., page 191.

/ 6 / Yoshida H., Proc. Int. Conf. 'Solid->Solid Phase Transformations', Pittsburgh,

Pa., August 1981, Met. Soc AIME, page 363.

/7/ Eckelmeyer K.H., ASMGatlinburg TN., 1981.

/8/ Ammons A.M., 'Physical Metallurm of Uranium Alloys', J.J. Burke et al. eds., Chestnut Hill, Mass. (Brook Hill), (1976), 511.

/9/ Speer J.G., D. Phil Thesis, Oxford University, (1983).

/lo/ May G.H., J. Nucl. Mat. r(1962) 72.

/11/ Eckelmeyer K.H., in 'Physical Metallur~v of Uranium Alloys', J.J. Burke et al.

eds., Chestnut Hill, Mass. (Brook Hill), (19'76). 463.

/12/ Drolet J.P, Erickson W.H. and Tardiff H.P., in 'Physical Metallurpy of Uranium Alloys', J.J. Burke et al. eds., Chestnut Hill, Mass. (Brook Hill), (1976), 725.

/13/ Eckelmeyer K.H., J. Nucl. Mat. 62(1977) 92.

u-a~s(~a)~i

U-2.2(e,)~o

a

b

Fig. 1 - Mass spectra of ( a ) U-0.75wt% Ti and (b) U-2.2wt% Mo.

(17.4kV,80K), (b) titanium concentration profile (sample size 100 ions) and (c) auto-correlation analysis: water-quenched U-0.75wt%

Ti.

micrographs (16.9kV. 80K) and (c) selective titanium analysis of particle X: U-0.75wt% Ti aged 5h at 425-C.

Fig. 3 - The effect of aging temperature on Vickers hardness for U-0.75wt% Ti (x) and U-2..2wt% Mo

( 0 ) .

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0 mffIMS

Fig. 5 - Titanium concentration profile (Sample size 200 ions):

U-0.75wt% Ti aged 5h at 425' C.

&to-correlat~on: iia

+ 1

Wean- 0.0055 1-0.0032) Variance- 0.0039 1 O.OOP) R kl

0

I

=I

I

Fig. 6 - (a) Neon FIM micrograph of molydbenum clusters (14.3kV,85K), (b) neon FIM micrograph (13.0kV. 85K) of molybdenum rich precipitates, (c) molydbenum concentration profile (Sample size 100 ions) and (d) associated auto-correlation analysis: U-2.2 wt% Mo aged 5h at 350- C.

associated molybdenum concentration profile and ladder diagram and (c) neon FIM micrograph (12.0kV,85K) of lamellar transition structure: U - 2.2 wt% Mo aged 5 h at 400" C.