ON ATOM-PROBE ANALYSIS OF CUBIC MX-TYPE CARBIDES AND CARBONITRIDES

(1)

HAL Id: jpa-00228149

https://hal.archives-ouvertes.fr/jpa-00228149

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ON ATOM-PROBE ANALYSIS OF CUBIC MX-TYPE CARBIDES AND CARBONITRIDES

U. Rolander, H.-O. Andrén

To cite this version:

U. Rolander, H.-O. Andrén. ON ATOM-PROBE ANALYSIS OF CUBIC MX-TYPE CAR- BIDES AND CARBONITRIDES. Journal de Physique Colloques, 1988, 49 (C6), pp.C6-299-C6-304.

�10.1051/jphyscol:1988652�. �jpa-00228149�

(2)

ON ATOM-PROBE ANALYSIS OF CUBIC MX-TYPE CARBIDES AND CARBONITRIDES

U. ROLANDER and H.-0.

ANDREN

Department of Physics, Chalmers University of Technology.

5-412 96 GBteborg, Sweden

Abstract -The field evaporation behaviour of cubic MX-type carbides and carbonitrides have been investigated using four (Ti,W)(C,N)-2 vol% Ni test alloys with well known compositions. Preferential field evaporation of titanium and nitrogen may occur when the analyses are done in certain crystallographic directions. The reproducibility of the analyses is not very good and thls is mainly due to the very high tendency of the atoms to evaporate in pairs or small clusters. It is shown that as much a s 14 Oh of the total C content and 9 % of the total Ti content is lost due to pair ion formation. Since there is no simple way to correct for these losses the instrument must be modified so that either the detector dead time or the mass resolution is decreased.

1 - INTRODUCTION

Cubic MX-type carbides and carboniMdes are of great importance e.g. for the

cemented carbide industry where these are used to replace some or all of the tungsten carbide. This is done both due to economic and supply considerations and to the superior high temperature properties of e.g.

Tic

^ascompared to WC.

One severe problem when trying to understand the behavior of these materials is the difficulty of obtaining accurate measurements of variations in the chemical

composition through the carbides. Since the atom probe has both the spatial resolution and the analytical accuracy to resolve fine scale variations in carbon, nitrogen and oxygen content it .seems to be the perfect choice for this type of investigation.

However, due to the rather complex field evaporation behaviour of these materials it is necessary to start by investigating the accuracy and reproducibility of the atom probe analyses.

2

-

EXPERIMENTAL

Four [Ti.W)(C,N)-2 volYo Ni test alloys with well known compositions were studied.

The Ni is not dissolved in the carbide phase but is added in order to improve the sintering properties. The composition of the carbide phase of each material is shown in table 1.

Atom probe specimens were made by electropolishing using 5% in methanol, cooled to -30 OC. All specimens were inspected by transmission electron microscopy prior to atom probe analysis.

The atom probe used has been described in detail previously 11, 2. 31. It is equipped with a Poschenrieder type energy compensator and a microchannel plate ion detector.

The flight times of the ions are detected with 5 ns resolution and the mass to charge ratios of the ions are recorded with a precision of 0.01 atomic units. The ions are labeled so that those ions which have been evaporated by the same pulse can be distinguished. Pulses which do not render any ions are, however, not recorded.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988652

(3)

C6-300 JOURNAL DE PHYSIQUE

Atom probe analyses of all four materials were carried out at a temperature of 90 K.

using an evaporation pulse amplitude of 15% of the standing voltage. The analyses were done at standing voltages ranging from 6 to 18 kV and at a specirnen-to-probe- hole distance of 4 c m Special care was taken to keep the evaporation rate low in order to minimize losses due to the detector dead time. The reproducibility of the results was checked by repeating the analyses at least once for each material.

As shown in figure 1 the evaporation field was strongly dependent on crystalographic direction. Therefore two analyses. one close to the 4 1 11> direction and one close to the cool> direction, were made on each material. The evaporation field in these two directions was estimated to be approximately 5.0

v/A

and 3.6

v/A

respectively for all four materials.

Effects of specimen temperature and evaporation pulse amplitude were investigated by performing additional analyses of material D at 90 K, using pulse amplitudes of 10%.

1596, 20% and 25% of the standing voltage, and at 70 K and 50 K using a pulse amplitude of 15%.

3

-

^RESULTSand DISCUSSION

Table 2 shows the result of the analyses at 90 K and 15% pulse height. For materials A and B the amount of titanium detected when analysing close to the el 11> direction is significantly too low. However. in all other analyses and especially the analysis of material D close to the cool> direction the titanium content is much too high. For materials B, C and D, which contain a significant amount of nitrogen, it is seen that the nitrogen content is either correct or slightly too low for all analyses. The carbon content, of course. balances the analyses so that high titanium content means low carbon content and vice versa.

A plausible explanation for the loss of titanium and possibly also nitrogen in some analyses is that the analyses are done at the limit of preferential field evaporation. This is cofirmed by making a profile of these analyses. From such profiles it is seen that titanium is lost only in certain parts of each analysis. In the other parts the titanium content is instead somewhat too high.

When repeating the analyses it is seen that the analytical reproducibility is not very good, but the trends are the same. ntaniurn is lost in parts of those analyses done in the el1 1> direction, and more often in materials A and B where the nitrogen content is low. When preferential field evaporation does not occur the titanium content is always too Ngh. The nitrogen content is always either correct or slightly too low.

Preferential field evaporation of titanium is not a very dmcult problem. One can e.g.

either use a higher evaporation pulse or a lower specimen temperature, or simply do the analysis in a low field direction. A more severe problem to solve is the loss of up to 20% of the carbon, which, as we shall see, is due to the very high tendency of the atoms to evaporate in pairs or small clusters.

Figure 2 shows the number of ions which is detected after each pulse for different pulse heights. For comparison two analyses done in the austenite and ferrite phases of a 22 wtYo Cr, 5 5% Ni duplex stainless steel are included. As can be seen the fraction of pulses during which 2. 3 or 4 ions are detected is very high and does not change significantly when the pulse height is varied. Unfortunately no significant change can be seen with variations in temperature or moderate changes in evaporation rate either.

Typically as much as 20 % of the detected pulses contain more than one ion and approximately 40 % of the detected ions have at least one pair ion i.e. an ion which has been evaporated simultaneously.

(4)

special care was taken to record the evaporation voltage at short intervals so that the flight times could be calculated

.

Figure 3 shows the time difference between ions with the same mass to charge ratio which have been evaporated simultaneously. From this figure the detector dead time was estimated to approximately 9 ns.

By looking at the width of the four main peaks in the spectrum at mass to charge ratios 6, 12, 16 and 24 atomic units, (i.e. C2+. C+, 4%13+ and

48Ti2+

respectively) and using the mean evaporation voltage during the analysis to convert mass to time it was possible to estimate the amount of ion pairs which differ in time less than the detector dead time. It turns out that 96.9 % of the C2+ pairs, 94.2 % of the C+ pairs, 90.3 % of the 48Ti3+ pairs and 79.8 % of the 4 8 ~ i 2 + should be lost. For the titanium peaks these figures were checked by comparing the amount of detected pairs containing two ions of the main isotope with the amount of pairs containing one ion of the main isotope and one ion of a small isotope and then using the natural abundance of the isotopes (73.7 % for the main isotope) to calculate the amount of ions lost. This procedure leads to the result that 90.3 % of the 48Ti3+ pairs and 80.0 % of the 4 8 ~ i 2 + pairs were lost. This is in very good agreement with the results above.

It is now possible to do a fairly accurate correction of the analysis. The result is shown in table 3. As can be seen, as much as 14 O h of the total carbon content and 9 % of the total Ti content are lost due to the loss of pair ions. In the corrected analysis both the titanium content and the nitrogen content are too low. We may therefore conclude that at a specimen temperature of 90 K both these elements field evaporate preferentially but the effect is usually much smaller than the losses due to pair ion formation.

It would be tempting to use the results of this analysis to do corrections on the other analyses. However, this can only be done if it is possible to estimate how the

probability for pair ion formation of a certain ion type varies with composition and relative peak height. Unfortunately this can not be done with simple statistics since the amount of pairs, containing two specific ion types, which are detected do not follow what is expected from a random distribution. Instead the only solution to the problem is either to decrease the detector dead time considerably or to decrease the mass resolution of the atom probe.

4 - CONCLUSIONS

-The field evaporation behaviour of cubic MX-type carbides and carbonitrides have been investigated using four (Ti,W)(C,Nl-2 vol% Ni test alloys with well known compositions.

Preferential field evaporation of titanium and nitrogen may occur when the analyses are done in a crystallographic direction where the evaporation field is high.

-The reproducibility of the analyses is not very good and this is mainly due to the very high tendency of the atoms to evaporate in pdrs or small clusters.

*The detector dead time of the instrument used has been determined to 9 ns and this result has been used to show that as much as 14 % of the total C content and 9 % of the total Ti content is lost due to pair ion formation.

Since the amount of pair ions formed, containing two specific ion types, do not follow what is expected from a random distribution it is not possible to use simple statistics to do corrections.

(5)

*Instead the instrument must be modified so that either the detector dead time or the mass resolution is decreased.

Acknowledgements -This work was supported by AB Sandvik Coromant and the National Swedish board for technical development (STU).

REFERENCES

111 Andrkn. H.-0. and Norden, H.. Scand. J. Metall. 8 (1979) 147.

[ 2 ] Andrkn. H.-0.. J. Phys (Orsay) 47 (1986) C7-483.

131 Hellsing. M. Karlsson, L. Andrkn, H.-0. and Nordkn, H., J. P h y ~ . E: Sci. Instrum. 18 (1985) 920.

Table 1. Carbide compositions of the four alloys used. (at %).

Table 2. Atom probe analyses ofthe four alloys done close to the 4 1 1> direction and close to the <001> direction (at % f

d:

*

The analyses in table 1 are used to separate O+ from **3+

(6)

dead time.

C N Ti

+

0 W

uncorrected analysis

no of ions:

atom

^%:

corrected analysie:

no of ions:

atom

%:

correction:

no of ions:

%

of

peak:

%

of

C

content:

%

of

Ti

content

Flgure 1.

Neonfild ion mkmgraphs of material

B. In (b)

the voltage

^fs

decreased

so

that the large

varfatlons fn curvature over the swface are clearly seen.

loZ

3

0 1 2 3 ₄ 5

ions per pulse

10%. materlal B 15%. matcrlal B 20%. material B 25%. materlal B Austcnlte Fcrrlte

Figure 2.

The fraction of pukes for whkh

1. 2, 3

or

⁴

b n s are detected for d~fferent

pulse helghts. For comparison two analyses done in the austentte and ferrite phases

^{l j j}

a

^{22 wt96}

Cr,

5 wt96 N1

duplex stainless steel are Included.

(7)

Figure 3

The time deviations between ion pairs with the

same

mass to

charge ratios.