Deformation twins in single crystals of tungsten

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JOURNALOFTHELESS-COMMONMETALS

Short Communications Deformation twins in single crystalsof tungsten SCHADLER has recently observed the occurrence of twinning in single crystals of tungsten at 20°K and 70”Ki. These single crystals had been grown by the electron bombardment floating zone melting process. He also noted the existence of twins on the fracture surface of a single crystal which had been drawn at room temperature. This communication relates additional observations of twins formed by compressing, rolling, and swaging single crystals of tungsten at higher temperatures. Indeed twins were observed after mechanical deformation even at temperatures up to 1500%. Most single crystals used in these experiments were grown by the electron beam melting process. They were about gq.qg”/o W having the same purity level as has been previously reported for this process by other investigatorsl.2. Specimens rolled

Fig.

I.

Twins in tungsten.

(x

500)

as rods were encased in stainless steel after grinding and electropolishing to size. These specimens were preheated for rolling in a nitrogen atmosphere furnace to temperatures of 400°C and IIOO’C. Specimens deformed in compression at room temperature were cubes which had been ground and electropolished. Some as grown crystals were swaged in air from preheat temperatures of 7oo°C-1500°C. After deformation, structures which we identify as twins were seen in profusion (Fig. I). Evidence of their identification as twins was provided by X-ray analysis using the two-surface technique 3. This analysis showed the twin plane to be (112); this is the twin plane found in other body-centered cubic metals and also reported by SCHADLER~. The twinning direction is presumed to be . Twins can be seen as relief effects on the electropolished surfaces of compressed J. Less-Common

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specimens, but greater detail can be revealed after etching*. In some instances as reported by SCHADLER~, aligned etch pits characteristic of deformation twins were noted. Twins were seen in the body of the specimen, on the fracture surfaces, and in regions adjacent to cracks. Short thin twins adjacent to cracks were evidently produced by stress concentrations at the head of the propagating cracks. Twins in

Fig. 2. Serrated twip in tungsten. (X IOO)

Fig. 3. Twin intersecting

a boundary.

(x 500)

some instances are formed prior to fracture since their appearance shows offsetting at the crack intersection. Several general types of twins were evident. Some were long lamellar twins with irregular sides (Fig. I), which are similar to those reported for tantalum4, molybdenumh, and niobium6. Other twins had a serrated side. A twin exhibiting this * Specimens 3% H&z.

were electropolished

in

10%

Na(OH)

solution

and etched in a boiling solution

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serrated pattern is shown in Fig. 2. It is similar in appearance to those reported for tantalum4 and molybdenum5. Occasionally in this investigation twins were also seen to have a step-like structure with a tendency to veer off the (112) planes. Twins in most instances cross subboundaries easily. A twin intersecting a boundary at an angle with accompanying distortion in the adjacent grain is shown in Fig. 3. Intersections of twins with other twins appear to “pinch-out” (Fig. I). A similar type of twinned structure has also been observed in tine. Using electron microscopic techniques, these investigators have observed the nucleation of an alternate twinning system at the boundaries of existing twins.

Fig. 4. Microcracks within and between twins. (X 500)

Some twins contain microcracks (Fig. 4). In some instances, these microcracks appear to initiate within a twin band, proceed to the next twin, and then occasionally follow the twin-matrix interface. Sometimes dislocation pile-ups are evident on one side of the twin band without resulting in microcracks. From these results, it appears that microcracks are initiated by the twinning action, Deformation twins appear to be stable at high temperatures since they are still evident after vacuum annealing for one-half hour at about 2ooo°C. Although twinning was predominant at lower working temperatures, twins were evident at high temperatures. After swaging a specimen from a preheat temperature range of 7oo-15oo~C, twins were evident on the fracture surface. Although low working temperatures and deformation twins usually promote recrystallization (i.e., formation of new grains), these conditions inhibit recrystallization in single crystal tungsten. This was demonstrated by deforming a previously sectioned single crystal from two preheat temperatures of 400°C and IIOO’C to similar reductions in area of about 50%. The single crystal section deformed from a preheat temperature of IIOO’C recrystallized between 1200 and 13oo“C when vacuum annealed for one-half hour. The section deformed from a preheat temperature of 400°C contained twins, and failed to recrystallize when vacuum annealed for one-half hour.

at a temperature

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Single crystals appear to be prone to twinning, although the ability to twin appears to be dependent on the number of electron beam melting passes, or consequently residualimpurities. A single crystal specimen having two passes twinned and fractured readily with one hammer blow at -rg6”C. However, a crystal which had five passes did not fracture or twin when subjected to a similar test. On a subsequent blow with the hammer, this single crystal did, however, also fracture and twin. Twins in single crystal tungsten have characteristics of twins observed in one or another of the body-centered cubic metals. In single crystal tungsten, low working temperatures, and/or deformation twins inhibit recrystallization. The twinning action also appears to initiate microcracks. Although single crystals are prone to twin, the ability to twin appears to be dependent on the number of passes of the electron beam and consequently certain residual impurities. Twins in tungsten also appear to form at a much higher temperature than previously reported. R. H. G. H.

Metals Research,

WestinghozLseElectric Corp.,

SCHNITZEL KEITH

Lamp Division, Bloomfield,

N. J. (U.S.A.)

1 H. W. SCHADLER, Trans. AZME, 218 (1960) 649. 2 R. H. ATKINSON elal., W.A.D.D. Tech. Report. 60-37, (1960). 3 C. S. BARRETT, Structure of Metals, McGraw-Hill Book Co., New York, 1952. 4 C. ~.BARRETTAND R.BAKISH, Trans. AZME,212 (1958) 122. 5 F. MUELLER AND E. PARKER, O.N.R. Report, N. V. zzz-p-27, (1960). 6 E. T. WESSELS, L. L. FRANCE AND R. T. BEGLEY, Westinghouse Research Report, (1960). 7 J.T. FOURIE,F.WEINBERGAND

F. W. BOSWELL,

AdaMet.,

II-0103~I-PI,

(Ig6o)85.

Received August r6th, 1961 J. Less-Common

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Solubility of scandium in manganese and iron at elevated temperatures

INTRODUCTION

A limited quantity of metallic scandium being available, it has been possible to prepare a few dilute alloys of this metal in both manganese and iron. Alloys have been examined by thermal analysis in thoria refractories under an atmosphere of argon, as described elsewherei, and by microscopical and X-ray methods. The solvent metals were of the same purity as those employed in previous work of a similar nature2. Only a little over I g of the scandium was available so that the results are confined to alloys containing less than 1.5 at.% of the solute. The results are only fragmentary, but enable the forms of the phase diagrams to be deduced, and show the influence of scandium on the allotropic modifications in manganese and iron. J.

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