Correlation of microyield behavior with silicon in X-520 and HIP-50 beryllium

Correlation of Microyield Behavior with Silicon in X-520 and HIP-50 Beryllium

K. KUMAR, J. McCARTHY, and J. B. VANDER SANDE

Significant differences were observed in the measured values of the microyield strength (MYS) for the X-520 and HIP-50 grades of beryllium. The MYS values for as-received and heat treated HIP-50 samples were in the range of 17 to 27 kpsi (117 to 186 MPa); those for similarly heat treated X-520 samples, however, ranged only from about 8 to 9 kpsi (55 to 62 MPa). Differences were also noted in the value of the strain exponent, which was measured as the slope of the line generated by plotting the stress v s residual plastic strain data on logarithmic coordinates. Strain exponent values of 0.20 to 0.34 were observed for the HLP-50 data compared to the range of 0.38 to 0.48 observed for the X-520 data. Notable differences were also observed between the X-520 and HIP-50 microstructures which appeared to explain the observed differences in microyield behavior. Second phase particles in both the beryllium grades were mainly located at the grain boundaries. However, the average size of the particles in HIP-50 was typically one-half of those in X-520, and their number density was also much higher. The particles were mainly composed of the oxides of Be in HIP-50; however, the X-520 panicles almost always contained Si, Fe, and O. X-ray and STEM results showed that the particles in X-520 were predominantly BeO containing possible reacted regions of the BezSiO4 phase. These results indicated that to produce high MYS Be it is important to keep the Si content low. Use should be made of fine impact attritioned powder, and hot isostatic pressing should be used as the densification procedure. The data indicated that Fe content (to the extent present in X-520 Be) was not very significant.

I. INTRODUCTION

MICROMECHANICAL processes leading to small mass shifts are very important to the designer of inertial instruments. These mass shifts are associated with dimensional changes that different parts of the instrument experience during different modes of operation and assembly. The most common causes of dimensional instability in structural materials usually are phase transformation, relief of residual stresses, and microplastic deformation from applied stresses. Although effects related to phase transformations and residual stresses can generally be controlled through proper alloy and process selection procedures, those related to applied stresses are more difficult to control because some minimum stresses, such as those associated with fastening operations during assembly, are in fact needed for proper functioning of the inertial instruments.

As greater demands are made on the accuracy of measuring devices, microplastic strains on the order of 10 -6 and 10 7 become significant sources of instrument error. Strains of this order of magnitude can occur at relatively low stresses in moderate strength engineering materials under the action of essential assembly operations such as shrink fit, bolt tension, or rotational stress. Since it is not possible to reduce these assembly stresses below a reasonable limit, it becomes desirable to select materials and design parts for minimal expected microplastic strain. In the absence of more pertinent microcreep data, for evaluation purposes, the designer uses the microyield strength of the material as a guideline in assessing its short-term and long-term performance inside

K. KUMAR, Chief, Materials Development Section, and J. McCARTHY, Staff Metallurgist, are with Charles Stark Draper Laboratory, Inc., Cam- bridge, MA 02139. J .B. VANDER SANDE is Professor of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139.

Manuscript submitted December 12, 1983.

the device of interest. A microyield strength measurement typically consists of a series of short-term load-unload cycles, increasing in stress level. During this period the total accumulated residual plastic strain is recorded for each in- crementally higher level of applied stress. Conventionally, that level of applied stress which is required to result in one residual microstrain (= 10 .6 strain) is referred to as the microyield strength of the material. 1.2.3

This paper deals with the measurement of the microyield behavior and characterization of the microstructure of two experimentally produced grades of beryllium, X-520 and HIP-50 beryllium. X-520 and HIP-50 are two experimental grades of low BeO content beryllium which can be fabricated to a better surface finish than is possible with 1-400 (instrument grade) beryllium which is used currently in inertial instruments. Certain component parts inside the inertial instruments, in particular those relating to gas bear- ings, require the attainment of extremely good surface finish.

II. PHYSICAL AND CHEMICAL PROPERTY DIFFERENCES AMONG THE DIFFERENT

GRADES OF BERYLLIUM

There are significant fabrication process and chemical composition differences among the X-520, HIP-50, and 1-400 grades of beryllium. It is unfortunate, however, that unlike 1-400 beryllium, which is available commercially in large quantities, both X-520 and HIP-50 were only experimentally produced on a laboratory scale. HIP-50 was produced by Kawecki-Berylco (now Cabot-Berylco) using hot isostatic pressing (HIP) of electrolytic flake, high purity, impact-attritioned beryllium powder. X-520 was fabricated by the Brush-Wellman Company using hot pressing of impact-attritioned, lower purity powder. Kawecki-Berylco

METALLURGICAL TRANSACTIONS A VOLUME 16A, MAY 1985--807

Table I. Comparison of Actual Properties of X-520 and HIP-50 and Specification Values of 1-400 Beryllium, as Reported by the Manufacturer

Chemical Analysis X-520" HIP-50* 1-400"

BeO 2.27 pct 1.68 pct 4.25 pct A1 200 ppm 40 ppm 1600 ppm Si 300 ppm 40 ppm 800 ppm Fe 600 ppm 500 ppm 2500 ppm C 1200 ppm 290 ppm 2500 ppm

Mg 100 ppm 20 ppm 800 ppm Ni 170 ppm

Grain size (microns) 5 5 <10 Mechanical properties

U.T.S. MPa 503 559 345 Y.S. MPa 386 434 - -

Elongation (pct) 2 to 5 pct 4 pct < 1 pct *Manufactured by Brush-Wellman Co., Elmore, Ohio 'Manufactured by Kawecki-Berylco (now Cabot-Berylco), Reading,

Pennsylvania

(now Cabot-Berylco) has since discontinued its beryllium business. 1-400, in contrast, while being consolidated using conventional hot pressing is fabricated from powder that is ball-milled and has a much higher level of impurity than either of the grades studied here.

Table I shows a comparison of several properties (actuals for X-520 and HIP-50 and specification for 1-400), as reported by the manufacturers, of the three beryllium grades. It should be noted, as stated above, that 1-400 contains the highest level of chemical impurities and HIP-50 the least. Also to be noted are the macromechanical properties of both the experimental beryllium grades provided by the manufacturers; while comparing favorably to each other, the X- 520 and HIP-50 values are considerably superior to those of 1-400 beryllium.

Table II. Details on Heat Treatments That Were Employed

Designation Description

HT1 HT2

HT3

AP + 600 ~ 100 h AP + 1055 ~ 2 h solutionize, quench + 370 ~ 24 h, furnace cool AP + 870 ~ 2 h + slow cool and step age (750 ~ 20 h + 720 ~ 20 h + 695 ~ 20 h, furnace cool)

AP = as-pressed condition.

Table III. Summary of Mieroyleld Data* on Brush Wellman X-520 and KBI HIP.50

Specimen Microyield Strength (MPa) X-520 Condition Longitudinal Transverse HIP-50

As-received 55 59 117 HT1 57 62 186 HT2 56 55 122 HT3 66 57 143 *Microyield data are for 1 • 10 -6 strain offset.

second phase particles in the STEM. This was made possible by employing an energy dispersive X-ray analyzer which detected the X-rays that were generated from these particles. Two different X-ray detectors were used, one of which had an ultra-thin window and was capable of detecting oxygen. Detailed analyses were performed on as- pressed and heat treated X-520 as well as HIP-50 samples. Analyses were performed with respect to particle size, density (number of particles per unit volume), distribution, and chemical composition.

III . E X P E R I M E N T A L P R O C E D U R E

The apparatus used for measuring the microyield behavior is described in detail in Reference 4. Briefly, the measurements were made on an [nstron tensile testing machine. The load train design permitted an acceptable level of align- ment. For a 5 kpsi (35 MPa) average level of stress, the extreme fiber bending stress was determined to be about 1.3 kpsi (9 MPa). The residual plastic strain induced in the material from the load-unload stress cycles during test was measured using resistance strain gauges and a BLH model 1200 strain indicator. The sensitivity of this strain indicator was increased by installing a BCD (binary code digital) output and printing the data with a Newport printer.

Samples were evaluated in the as-received (which was also the as-pressed) condition as well as after subjecting the materials to selected thermal treatments. The different thermal treatments, identified as HT1, HT2, and HT3, are described in detail in Table II. These were investigated on the basis of recommendations from an earlier, reasonably exten- sive study of the strengthening processes in beryllium. 5

A JEOL 200 CX electron microscope was used for per- forming conventional TEM (transmission electron microscope) analysis, and a Vacuum Generators HB-5 unit was used for STEM (scanning transmission electron microscope) work. Chemical composition analysis was performed on the

IV. RESULTS AND DISCUSSION

A. Microyield Behavior

As stated earlier, microyield strength measurements were made on samples both in the as-pressed as well as in the heat-treated condition. Additionally, with the X-520 material, two sets of samples were evaluated (one set each belonging to the longitudinal and the transverse directions of pressing, respectively), whereas only one set was examined for the HIP-50 material. This decision was based both on a shortage of material availability and also because HIP-50 is isotropic, having been produced by the hot isostatic pressing (HIP) technique. All of the collected MYS data is shown in Table III.

Notable differences in behavior were seen for these materials in their response to heat treatment as well as in their measured tensile properties. The HIP-50, as received, showed a microyield strength value that was 70 pct higher than what is typically observed for 1-400 (17 kpsi (117 MPa) for HIP-50 vs 10 kpsi (69 MPa) for 1-400). Of the three heat treatments that were used, an aging treatment of 600 ~ hours was the most effective, raising the microyield strength value by an additional 59 pct, to 27 kpsi (186 MPa). Heat treatments at 870 ~ followed by a step aging treatment, and at 1050 ~ followed by aging at 370 ~ gave improvements in MYS value approximately 20 pct and 2 to 4 pct, respectively. Testing of the X-520

808--VOLUME 16A, MAY 1985 METALLURGICAL TRANSACTIONS A

10 5

J ~

MJ oc I- r

U J m - - I e,_

<

104

103 A

1/81 CD22480

I I [

A HT1 [ ] HT2

<~HT3

Q AS-HIP

, I L l l 10-6

1 l I 1 I I I I I , l I I I l i 1 ]

10-5 10"4

RESIDUAL STRAIN

Fig. 1--Microyield data for HIP-50 samples.

grade beryllium in the "as-received" condition, on the other hand, showed an MYS value which barely met the 8 kpsi (56 MPa) acceptance specification for 1-400. These num- bers were virtually unaffected by any of the heat treatments, although there was an apparent slight improvement imparted by the 600 ~ hour aging exposure.

As a means of comparing sample behavior, the MYS data for these samples are plotted as log (applied stress) v s

log (residual strain) 6 and are shown in Figure 1. The data from both samples appeared well-behaved and a straight- line relationship was observed for this plot.

Based on the observations in Figure 1, for the range of low levels of strain considered here, the stress-strain relationship can be empirically expressed as:

~ r = A ~ " ( f o r e ' - e - - < e '')

or

log o r= n log e + logA

where cr = applied stress e = residual strain

' , e" : assigned values of strain in the low strain regime n = strain exponent A = proportionality constant

In the above expression n, the strain exponent, is a measure of the strain hardening effects in the material. The larger the value of n, the greater is the amount of strain hardening in the material.

The data plotted for HIP-50 and X-520 revealed significant differences in strain exponent between the two materials and to a lesser extent among the heat treatments applied. The data are tabulated in Table IV for all the samples. Dif-

Table IV. Measured Values of the Strain Exponent

Material/Heat Treatment n

HIP-50/As HIP 0.28 HIP-50/HT- 1 0.28 HIP-50/HT-2 0.20 HIP-50/HT-3 0.34 X-520 L*/as-received 0.43 X-520 T**/as-received 0.43 X-520 L/HT-1 0.45 X-520 T/HT- 1 0.47 X-520 L/HT-2 0.38 X-520 T/HT-2 0.44 X-520 L/HT-3 0.41 X-520 T/HT-3 0.48

*Longitudinal **Transverse

ferent measured strain exponent values for the different material types and conditions imply that the perceived relative merits of any two materials or heat treatments will change depending on the strain level at which they are compared. When it is considered that the 10 -6 level is completely arbitrary, and that a completely different ranking of apparent value could be obtained by simply focusing on a different low level of strain, it is apparent that assessment of resistance to microplastic deformation must take into account such additional factors as the strain exponent. As stated earlier, the MYS value is a measure of the applied stress required to induce a permanent strain in the material at an arbitrary, low (10 6) strain value. The strain hardening exponent, on the other hand, is a measure of the incremental increase in applied stress required to induce a given level of

METALLURGICAL TRANSACTIONS A VOLUME 16A, MAY 1985 809

resulting incremental increase in plastic strain and can, therefore, be interpreted as an indicator of expected long- term microcreep effects in the material. Whereas the MYS value provides a measure of the inherent resistance to microplastic deformation of the material, the strain hardening exponent provides an indication of the continued resistance to such deformation (with increasing plastic strain) such as those that lead to permanent mass shifts with time in the inertial instruments. Both the MYS and n values should, therefore, be considered in assessing the expected long-term behavior of the material in the absence of more pertinent microcreep data. From the foregoing it appears desirable to use materials that show both high MYS as well as high n values.

With respect to the microyield data reported in this paper it should be noted that in each instance only one sample was measured, mainly because of material shortage. Even so, whereas the relative values obtained from the different heat treatment conditions for samples belonging to a given beryllium grade might be suspect (because only one sample was measured in each instance), the data clearly show that sub- stantial differences in microyield behavior do exist between the HIP-50 and the X-520 beryllium grades. HIP-50 shows a considerably larger MYS value and a substantially lower strain exponent, n, value than does X-520.

The improved long-term dimensional stability expected because of the higher MYS of HIP-50 is partly offset by the lower value of n. However, when it is considered that the lowest MYS value measured in the several HIP-50 samples was typically about twice the values obtained in the X-520 samples, it becomes apparent that for inertial applications, HIP-50 is superior to X-520 even though it shows a lower n value. It should be noted that, over the short term, for applied stress values of about 17 kpsi (117 MPa) HIP-50 will show an accumulated strain of only 10 6 whereas X-520 will typically show more than five times as much. Also, the advantages associated with an increased resistance to short- term deformation in X-520, as indicated by a higher value of n, will show up only at high stress levels (above 50 kpsi) and large (greater than 10 4) residual strain. Since applied stress levels that the parts see during fabrication and use of the inertial instruments are typically limited to values less than 1 kpsi (6.9 MPa), it is evident that HIP-50 would be the preferred choice in the absence of more pertinent microcreep data. Similar arguments would apply to any comparison of HIP-50 to conventional 1-400 also, because of the much lower (only marginally superior to X-520) MYS that is associated with the 1-400 beryllium grade. It should be noted, however, that for beryllium grades with MYS values that are substantially closer, the grade with the higher n value should be the material of choice.

B. Microstructure Investigations

Initial experimentation, which involved optical microscopy of polished surfaces and scanning electron microscopy of freshly fractured surfaces, did not show any appreciable differences among the microstructures of the several Be samples. These materials were then examined using X-ray diffraction and TEM/STEM techniques which provided significant insights. These results are discussed below.

1. X-ray analysis

Long-term X-ray diffraction patterns were obtained on solid, roughly 1 mm diameter, beryllium samples using a Debye-Scherrer camera. The X-radiation used was Ni- filtered Cu-radiation at 35 KV and 20 mA, and the exposure times were 21 to 24 hours for each sample. Samples of both beryllium grades were examined in the as-received as well as the heat-treated condition. The effects of the heat treatments were not noticeable in the diffraction patterns. All four HIP-50 samples produced identical patterns, as did the X-520 group. There was, however, a distinct difference between the two groups with respect to the presence of diffraction lines from trace amounts of an extraneous phase (or phases). In both groups of patterns, the beryllium diffraction lines were very strong and the BeO lines were of medium intensity. Very weak diffraction patterns were obtained from extraneous phases in the form of lines and wide bands. Since no effect of heat treatment was noticeable in the patterns, only one set of diffraction data is presented for each group in Table V, along with the data on some compounds obtained from the ASTM diffraction card file.

Except for two extremely broad bands at very large d spacings, all the other lines of HIP-50 were accounted for by the Be and hexagonal BeO patterns. The X-520 patterns, however, had four additional lines. Of the several suspected compounds including F e B e . , FeAIzBe3, A1FeBe4, and Be2SiO4, the latter gave the best fit to the (observed) extra lines. The reduced level of impurity phases in HIP-50, as indicated by the absence of diffraction lines associated with extraneous phases, is in agreement with the much higher reported purity of this material. It is possible that the two extremely broad bands at very large d spacings, observed for both beryllium grades, were associated with the particular Debye-Scherrer camera that was used.

2. TEM /STEM studies

a. X-520 beryllium Figure 2 is a low magnification, annular dark-field de-

tector image from an as-received (which is also as-pressed) X-520 specimen. The micrograph clearly shows the presence of the second phase particles, which appear in dark contrast, distributed throughout the material. Most of the particles were observed at the grain boundaries; however, a significant fraction of the particles in this sample also appeared to be located elsewhere in the beryllium matrix. Similar, low magnification observations were also made on heat-treated specimens. The observations made on the heat- treated samples were similar to those in Figure 2, except that the second phase particles were less uniformly dispersed in the heat-treated specimens with a larger majority of them segregating preferentially at the grain boundaries. Using images of the type shown in Figure 2, Table VI was constructed. The data were obtained primarily for differences in the samples with regard to size, distribution, area fraction, and spatial density of the second phase particles. A reasonably large number of particles was examined for this purpose.

All of the heat-treated specimens showed an average particle size of about O. 1 /xm as compared to the as-received sample which had an average particle diameter of roughly


Table V. Diffraction Data of HIP-50, X-520 Along with Various Compounds from ASTM Card Files

HIP-50 X-520 Be BeO FeBe~, FeA12Be3 BezSiO4

d (Int) d (Int) d (Int) d (lnt) d (lnt) d (Int) d (lnt) 11.5 (?) 11.5 Band 10.7 (55) 5.7 (?) 5.7 Band 6.24 (40)

3.66 (VVW) 5.36 (65) 3.73 (5) 3.67 (75) 3.57 (100) 3.39 (25)

2.52 (W) 2.35 (M) 2.35 (M) 2.20 (M) 2.20 (M) 2.10 (M) 2.09 (M) 1.99 (VVS) 1.99 (VVS) 1.80 (VVS) 1.80 (VVS) 1.74 (VVS) 1.75 (VVS) 1.61 (W) 1.61 (W)

3.15 (VVW) 3.16 (50) 3.12 (100) 2.97 (65) 2.53 (50) 2.52 (75)

1.98 (30) 1.79 (25) 1.73 (100)

2.35 (9l) 2.19 (61) 2.06 (100)

1.6 (22)

1.54 (VW) 1.35 (W) 1.35 (W) 1.37 (30) 1.33 (S) 1.33 (S) 1.32 (11) 1.24 (W) 1.24 (W) 1.238 (29) 1.15 (S) 1.15 (S) 1.15 (9)

VVS = Very, Very Strong; S = Strong; M = Medium; W = Weak; VW = Very Weak; VVW = Very, Very Weak

2.21 (100)

1.53 (7) 1.50 (vs)

0.25 ~m. The increase in the density of particles in the heat-treated samples was interpreted as indicative of increased precipitation in the beryllium alloys. This additional precipitation might well have contributed to a lowering of the measured average particle size as well as resulting in a greater preference for the grain boundary segregation that was observed in these samples.

Besides considerations relating to size, distribution, and number density of the second-phase particles, many second- phase particles were also examined for elemental analysis using the STEM. The data that were collected for several arbitrarily selected particles in the different samples are shown in Table VII. As stated earlier, the presence of oxygen in the particles was detected using an ultra-thin-window, energy-dispersive detector. The analysis for oxygen presence was performed on the particles present in the as- received (No. 5) and in the HT2 heat-treated (No. 8) sam- pies. Huge oxygen signals were measured from all of the particles that were examined. The particles in sample Nos. 6 and 7 (see Table VII) were not examined for oxygen with the ultra-thin window, energy dispersive detector. The ND (not able to be detected) comment for oxygen in Table VII

Fig. 2--Annular dark-field image in STEM showing second-phase particles and grains in as-received material.

Table VI. Analysis of Second-Phase Particles in X-520 Beryllium

- ~ Descriptor Sample (Heat Treatment)

5 (as-received) 6 (HT1) 8 (HT2) 7 (HT3)

Image Number of Number Density Magnification Particles (Particles/cm 3)

2,000 525 0.8 x 1012 10,000 174 7.2 • 1012

20,000 41 6.8 • 1012 10,000 437 18 • 1012

Average Particle Diameter (txm)

0.26 0.11 0.10 0.12


Table VII. X-Ray Analysis of Selected Second-Phase Particles in X-520 Beryllium. Heat Treatment Conditions for Samples Were: 5 (As-Received); 6 (HTI); 7 (HT3); 8 (HT2).

Sample Element

No. Location O A1 Si P S Cr Fe

5 B1 VS S S S 5 C1 VS W W S 5 D1 VS W W S 5 E1 VS - - W 5 F1 VS VS S W VS 5 G1 VS - - W 5 HI VS - - - -

6 A ND - - W W 6 B ND - - - - W 6 C ND W S W S 6 D ND W W W 6 E ND S VS W S 6 F ND - - W W 6 G ND - - W W 6 H ND - - - - W 6 I ND - - W - - W - - W 6 J ND - - S - - W W S

7 A ND W S W S 7 C ND - - S S S - - W 7 D ND - - W W W 7 E ND - - W W 7 F ND W S S W - - S 7 G ND - - S S S - - S

8 A VS W S S W 8 C VS - - S S - - S S 8 D VS W S S W - - S 8 F VS W W W 8 G VS W S S S S S 8 I VS W VS W W S VS

N O T E S : V S = V e r y S t rong , S = S t rong , W = W e a k , - - = N o p e a k , N D = No t able to be de tec ted

for these specimens indicates that the X-ray detector used with samples 6 and 7 was not capable of analyzing for oxygen. The results on samples 5 and 8 suggest that huge oxygen signals would also be expected from both samples 6 and 7. It was noted that silicon and iron were also invariably present (in addition to oxygen) in all the particles that were analyzed. Other elements, including aluminum, phos- phorus, sulfur, and chromium, were found to be only occa- sionally present. The capability for detecting beryllium was not present with this instrumentation.

These oxygen-related observations showed that precipitates consisting of binary and ternary compositions of beryllium, aluminum, and iron such as those reported to be present in instrument grade, 1-400 beryllium 8'9 are not present in detectable quantities in the X-520 materials. The observations were in agreement with X-ray diffraction results on the as-pressed and heat-treated X-520, which had shown the presence of extra lines attributed to the Be2SiO4 phase composition. The STEM analysis, however, did show that the particle composition itself was substantially richer in oxygen than would be expected for stoichiometric Be2SiO4. The implication of these observations, therefore, is that the bulk of an individual particle must still be BeO with some reacted regions consisting of the Be2SiO4 phase. Silicon is believed to assist in the agglomeration of the BeO particles, l~ It is possible that the formation of this ortho-

silicate phase is a result of such a process. The presence of iron can be explained as being associated with a small solubility of iron in the BeO particle.

b. H1P-50 beryllium Figure 3 is representative of the as-HIPed HIP-50 micro-

structure as observed in the electron microscope. In a manner similar to what was done for X-520 beryllium, Table VIII was constructed with the data obtained on the HIP-50 samples.

Only high angle grain boundaries were observed in the as-HIPed condition. Most of the second-phase particles were located at the grain boundaries with the grain interiors appearing relatively clean. A low density of particles was observed to be uniformly dispersed throughout the sample.

The interiors of the heat-treated samples were very clean. Most of the particles were located at the grain boundaries even though several boundaries were found to be reasonably clear of precipitates. The number density of the particles as shown in Table VIII was the highest for sample HT2. In the HT3 sample, many low angle grain boundaries were observed that were found to be free of particles. As indicated in Table VIII, the number density of the particles was reasonably high even for this sample.

3. Comparison of X-520 and HIP-50 microstructure data

Analysis of the second-phase particles in HIP-50 beryllium has shown these particles to be composed primarily of the oxides of beryllium. This differs from the particles in X-520, which contained reacted regions of the Be2SiO4 phase, and many of which were found to contain iron, possibly in solid solution. The HIP-50 average particle size (as shown in Table VIII) was typically one-half of what was observed for X-520, while the number density of particles in HIP-50, in general, was substantially larger than what was observed for X-520. This was generally expected because the total expected oxide levels for HIP-50 (1.7 pct BeO) and X-520 (2.3 pct BeO) are reasonably comparable, and for finer particle sizes (in HIP-50 compared to X-520) one would expect a larger number of particles in a given volume of material. Differences observed between samples belonging to the two beryllium grades were considered significant because composition, size, and number density of the second-phase particles were substantially different for one grade of beryllium with respect to the other (as indicated collectively by all the samples that were examined). These differences were deemed more significant than effects related to heat treatment (within a given grade of beryllium) for purposes of explaining the micromechanical property differences that were earlier observed between the two beryllium grades. The grain size in these samples appeared to be reasonably comparable and, therefore, not very significant.

V. C O N C L U S I O N S ON T H E C O R R E L A T I O N OF M I C R O Y I E L D

B E H A V I O R W I T H M I C R O S T R U C T U R E

MYS measurements performed in this study show that the HIP-50 grade of beryllium is substantially superior in its MYS value compared to the X-520 grade. The latter, however, possesses a substantially greater level of strain

8 1 2 - - V O L U M E 16A, MAY 1985 M E T A L L U R G I C A L TRANSACTIONS A

Fig. 3--Transmission electron micrograph of as-received HIP-50 beryllium sample.

hardening (as indicated by the measured strain hardening exponent) than the HIP-50 material. Because, as explained earlier, it appears to be quite desirable to have both a high MYS value as well as a high strain hardening exponent for developing a maximum resistance to deformation processes, it is important to understand how these properties depend on the microstructure of the material.

The TEM/STEM results show a much finer particle size and a higher number density of the second-phase particles in HIP-50 compared to the X-520 grade. In both these grades of beryllium, the second-phase particles are mainly segre- gated at the grain boundaries. The foregoing would therefore suggest that the finer particle distribution of HIP-50 is responsible for the high MYS that is measured. This would

further imply that initiation of microplastic deformation in these materials occurs primarily in the grain boundary regions, since these are expected to be strengthened (by the fine particles that are distributed) to a larger degree in HIP- 50 than in X-520. The lower level of strain hardening observed for HIP-50, in comparison with X-520, appears to be related to the presence of the very high level of stresses that are needed to initiate the microplastic flow process as indicated by the measured MYS value. Solid solution strengthening of the X-520 grain interiors cannot explain the observed differences in the strain exponent. Iron is found to exist in similar amounts in both the beryllium grades, and recent work suggests extremely low solid solubility of aluminum in beryllium. ~2


Table VIII. Analysis of Second-Phase Particles in HIP-50 Beryllium

Descriptor

Image Number of Number Density Average Diameter Magnification Particles (particles/cm 3) (/xm)

3A (as-received) 15,000 96 0.29 x 10 '4 0.05 HIP-50* (HT1) 24,000 273 0.52 x 10 ~4 0.03 2A (HT2) 25,000 136 1.13 x 10 ~4 0.05 3B (HT3) 35,000 64 1.05 x 1014 0.05

*Sample from a different lot of HIP-50

The coarser distribution of the second-phase particles observed in X-520 is believed to have resulted from: (1) the presence of an excessive amount of silicon, which has ap- parently aided the agglomeration of the oxide, thereby ac- counting for the existence of reacted Be2SiO4 regions in the BeO particles, and (2) the possible use of a higher temperature during consolidation with hot pressing. Since much higher pressures are available during hot isostatic pressing, the HIP-50 materials were possibly densified at a lower consolidation temperature.

The implication of the foregoing appears to be to keep the level of silicon low in the material. Silicon is a desirable constituent in beryllium produced by hot pressing because it aids in the densification process. However, if the process of HIP is used for consolidation, the silicon can be dispensed with and the densification performed at temperatures substantially lower than are required for hot pressing. All of this should result in the production of beryllium with a fine particle distribution in the grain boundary regions similar to what was observed for HIP-50 beryllium. The lower temperatures of HIP would also inhibit thermally activated grain grOwth in addition to minimizing particle agglomeration. The use of impact attritioned powder, such as that used for fabrication of HIP-50 and X-520, is also expected to impact favorably on the overall properties. Surface cracks and fissures will occur in powder particles produced by this method, which will develop a surface oxide film, and when these flaws heal under the effects of temperature and pres- sure, during consolidation, fine particle distributions are also expected to occur deeper in the grain interiors.

A C K N O W L E D G M E N T

This work was performed under Office of Naval Research Contract N00014-77-C-0388. The X-520 samples used in this study were obtained from Dr. J. Smugeresky of Sandia Laboratories. This project was monitored by Dr. G. London of the Naval Air Development Center on behalf of ONR.

REFERENCES

1. H. Muir, B. L. Averbach, and M. Cohen: Trans. ASM, 1955, vol. 47, p. 380.

2. T.G. Hughel: Report MR-120, General Motors Corp., Warren, MI, 1960.

3. C.W. Marschall and R.E. Maringer: Dimensional Instability--An Introduction, 1st ed., Pergamon Press, Oxford, 1977, p. 66.

4. J. McCarthy and F. Petri: Report No. R-1388, The Charles Stark Draper Laboratory, Inc., Cambridge, MA, June 1980.

5. R.M. Paine and A.J. Stonehouse: Report BW-TR-549, Brush- Wellman, Inc., Cleveland, OH, 1974.

6. G.M. Roberts and N. Brown: Trans. TMS-A1ME, 1960, vol. 218, p. 454.

7. C.W. Marschall and R.E. Maringer: Journal of Materials, 1971, vol. 6, p. 374.

8. R.S. Polvani, C. J. Bechtoldt, J. Orban, and B. W. Christ: Society of Engineering Science, Inc., 16th Annual Meeting, Evanston, IL, Sept. 5-7, 1979.

9. A.J. Stonehouse: Beryllium Science and Technology, D. Webster and G. J. London, eds., Plenum Press, New York, NY, and London, 1979, vol. 1, p. 181.

10. J.W. Butcher and V.D. Scott: Powder Metallurgy, 1971, vol. 14, pp. 1-21.

11. H.M. Lindsay and V. D. Scott: Journal of Nuclear Materials, 1970, vol. 36, pp. 29-40.

12. S.M. Myers and J. E. Smugeresky: Metall. Trans. A, 1976, vol. 7A, pp. 795-802.


Documents

Correlation of microyield behavior with silicon in X-520 and HIP-50 beryllium