US3772090A

US3772090A - Alloy microstructure control

Info

Publication number: US3772090A
Application number: US00165030A
Authority: US
Inventors: R Allen; C Calhoun
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-07-22
Filing date: 1971-07-22
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13
Also published as: FR2146716A5; BE782473A; DE2219275B2; DE2219275C3; GB1386808A; IT953571B; DE2219275A1; JPS5335887B1

Abstract

A columnar grain structure and improved high temperature, high mechanical strength properties are provided in a metal article by solid state method. First the article is formed in a manner which places the metal microstructure in a condition of dislocation density which results in the metal microstructure undergoing relatively rapid transformation to relatively large grains when heated in the range of about 50 to less than 100 percent of the metal incipient melting temperature in degrees Rankine. Then the article so preconditioned is progressively and selectively heated in that temperature range but below the incipient melting temperature of the metal. This is accomplished by heating the article in a narrow zone. Such zone traverses the article, as a result of relative movement between the article and the source of heat energy, from the cooler portion and in the direction desired for growth of the principle axis of the columnar grains, with the zone having a thermal gradient of at least about 500*F/in. A preferred form of a processed wrought article is characterized by grains of at least about 2000 microns in diameter and a length to diameter ratio of at least about 10.

Description

United States Allen et al.

atet 1 Nov. 13, 1973 ALLOY MICROSTRUCTURE CONTROL [75] Inventors: Robert E. Allen; Clyde D. Calhoun,

both of Cincinnati, Ohio [73] Assignee: General Electric Company,

Cincinnati, Ohio [22] Filed: July 22, 1971 [21] App]. No.: 165,030

[52] U.S. C1. ..148/11.5 F, 148/12, 148/13, 148/31, 148/32, 148/133, 148/134, 148/154 [51] Int. Cl C22f l/l0, C22f l/l8, C2ld 7/00 [58] Field of Search 75/171, 170; 148/13, 148/l.6, 11.5 R, 11.5 F, 12,133,134, 31, 32, 154

[56] References Cited UNITED STATES PATENTS 3,677,835 7/1972 Tien et al 75/171 3,260,505 7/1966 Ver Snyder 75/171 Primary ExaminerRichard 0. Dean Att0rney-Derek P. Lawrence et al.

[57] ABSTRACT A columnar grain structure and improved high temperature, high mechanical strength properties are provided in a metal article by solid state method. First the article is formed in a manner which places the metal microstructure in a condition of dislocation density which results in the metal microstructure undergoing relatively rapid transformation to relatively large grains when heated in the range of about 50 to less than 100 percent of the metal incipient melting temperature in degrees Rankine Then the article so preconditioned is progressively and selectively heated in that temperature range but below the incipient melting temperature of the metal. This is accomplished by heating the article in a narrow zone. Such zone traverses the article, as a result of relative movement between the article and the source of heat energy, from the cooler portion and in the direction desired for growth of the principle axis of the columnar grains, with the zone having a thermal gradient of at least about 500F/in. A preferred form of a processed wrought article is characterized by grains of at least about 2000 microns in diameter and a length to diameter ratio of at least about 10.

6 Claims, 4 Drawing Figures PAIENIEnnnv 13 ms 3.772.090 sum 2 or 2 rim/ 5647066 3 INVENT zaeier f 4615/1/ F34 mgwgozd ALLOY MICROSTRUCTURE CONTROL BACKGROUND OF THE INVENTION The present invention relates to an improved solid state method for alloy microstructure and grain growth control preferably in a wrought metal article, to apparatus which provides such control and to a wrought article which is produced. The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.

Study of the effect of stress at high temperatures, such as 2,000F, upon grain structure of certain alloys has identified areas in the grain boundary of metals which show separation of cavitation. This condition has been observed in high temperature alloys after exposure to stress at high temperatures, after creep or stress rupture testing, and even after some tensile testing. The mechanical strength properties of high temperature alloys for such an application can be improved in the direction of stress in a number of ways. These include elongating the grains so as to make the length-todiameter ratio very large, increasing the size of the grains, improving the mechanical strength of the grain boundaries and improving the grain texture by orienting the grain boundaries more closely.

It has been demonstrated and reported that high temperature mechanical properties of certain commercial alloys can be enhanced by casting articles made from such alloys in the manner which results in directional solidification to produce relatively large, elongated grains in the alloy. Typical of such alloys are the high temperature nickel base alloys, sometimes referred to as superalloys and widely used as turbine blades in gas turbine engines.

Such directional solidification is normally accomplished after casting the molten alloy into a mold by removing heat during the solidification process in a directional manner to elongate, enlarge and align the grains generally in the direction of principle stress. This results in very few grain boundaries oriented in the direction transverse to the intended application of principle stress. Thus, processes such as grain boundary sliding, stress-induced boundary cavitation and grain boundary shear are minimized. However, this type of structure still has a cast appearance. It can show relatively severe segregation effects, both from the center to the edge of the dendrite-like grains and also from one end of the directionally solidified article to the other.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide a solid state method, rather than a solidification method from the molten state, for providing columnar grain structure in an article so that such structure can be provided without melting the article to avoid the limitations inherent with cast structures.

A further object is to provide apparatus capable of controlling heating or cooling rates to accomplish such method.

Another object is to provide such a method which can be used to produce a columnar grain structure and improve the mechanical properties in a wrought metal article at a temperature below its incipient melting temperature and with minimal change in the article's dimensions.

Still another object is to provide an improved wrought article of unusually large columnar grain structure by, a solid state method.

These and other objects and advantages will be more clearly understood from the following detailed description, examples and the drawings, all of which are typical of rather than limiting on the scope of the present invention.

Briefly, the method form of the present invention provides a solid state method for producing in a metal article a columnar grain structure and improved high temperature, high strength mechanical properties by first preconditioning the article. Such preconditioning involves forming the article to place its metal microstructure in a dislocation density condition such that the microstructure will undergo relatively rapid transformation to relatively large grains of at least about 200 microns when heated in the temperature range of 50 to less than percent of the metals incipient melting temperature in degrees Rankine (T After such preconditioning, the microstructure is transformed to relatively large grains by progressively and selectively heating in that temperature range and below the incipient melting temperature by applying heat to the article from a heat energy source which produces in the article a high temperature zone having a thermal gradient with the metal of the article of at least 500F/in. The zone traverses the article as a result of relative movement between the article and the source in the direction desired for growth of the columnar grains.

In a more preferred form of the method for use with wrought articles, including mill forms, based on Fe, Ni or C0, the transformation occurs in the range of about 1,650 2,500F through a temperature gradient of 500 5000F/in. at the rate of 0.5 144 inches/hr. and preferably greater than about 5 inches/hr.

A wrought article resulting from practice of the method of the present invention is characterized by unusually large grains of at least about 2,000 microns in diameter and a lengthto-diameter ratio of at least about 10, the metal of the article being capable of maintaining high temperature, high mechanical strength properties at temperatures of atleast about 50 percent of the metal incipient melting temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical presentation of the improved ductility attainable in wrought bar through the present invention compared with the same bar without such processing;

FIG. 2 is a graphical presentation of the improved 0.5 percent creep properties attainable in sheet through the present invention compared with commercial sheet without such processing;

FIG. 3 is a graphical presentation of dynamic modulus for both commercial sheet and wrought bar processed according to the present invention to show no sacrifice in thermal fatigue resistance through practice of the present invention; and

FIG. 4 is a diagrammatic, sectional view of one form of the apparatus which can be used in thepractice of the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The mechanical property improvement attainable through the provision of relatively large, elongated,

aligned grains having a relatively high length-todiameter ratio have been reported and discussed, particularly in connection with reduction of grain boundaries transverse to the axis of principle stress. However, because the methods used in obtaining such a structure in high temperature alloys have been casting methods, most if not all, textbook definitions of columnar grain structure refer in one way or another to the article as a casting. Such known methods used with high temperature alloys generally involve the creation of such type of columnar grain structure as a result of cooling from the molten to the solid state.

it has now been recognized, according to the present invention, that such a columnar grain structure can be created in the solid state in a metal article at temperatures below the incipient melting temperature of such a metal. Therefore, as used in this specification, the term columnar grain structure is not limited to a cast article. Its most practical and preferred use is in connection with a wrought metal article which can include mill forms as well as more completely shaped articles.

As was stated, improvement of mechanical strength properties of an alloy in the stressed condition can be enhanced by minimizing grain boundaries transverse to the direction of stress, such as by making the grains long in relation to their diameter and of a relatively large size. This can be accomplished through provision ofa columnar grain structure. Additional improvement can result from a more close orientation of grain boundaries to provide a strong texture. The method of the present invention, which is particularly beneficial with high temperature alloys capable of maintaining strength when operating at temperature of at least about 0.5 T for example, alloys based on the elements Fe, Co, Ni, Ti and the refractory metals, provides all of these improvements in a procedure more rapid than that which has heretofore been attainable. According to such method, after growth of a set of grains has been initiated, further initiation is suppressed by a steep temperature gradient as the article is moved from a cooling chamber to a heating chamber. Growth competition among existing grains is promoted by moving an article through the gradient at appropriate rates. In this way, large columnar grains form in the direction of motion. In respect to texture, material processed according to the present invention has the same texture as is seen in material processed according to other processes and therefore no sacrifice of thermal fatigue properties.

EXAM PLE l A commercially available alloy which has been studied extensively in connection with the present invention consists nominally of about 80 weight percent Ni, 20 weight percent Cr with about 2 volume percent dispersed thoria. Generally, this alloy is referred. to as TD nickel chromium alloy and abbreviated TD Ni Cr alloy. Such alloy has a T of about 2,550F.

A portion of an as-extruded bar of TD Ni Cr alloy sheet billet with a rectangular l X 6 inch cross-section was machined to a it inch diameter bar after which it was passed longitudinally through a steep temperature gradient of about l,500F/in. at 0.5 inch/hr. at a temperature of about 2,400F to obtain columnar grains. The 54: inch diameter bar after processing contained unusually large, elongated grains: 4 5 grains in the crosssection and each was approximately 2 to 2 7% inches long.

A portion of the same bar was processed in accordance with the standard recommended heat treatment of 2,400F for 1 hour.

Stress rupture specimens prepared from both bar portions were tested at 2,000F. The results are shown in the following Table I:

TABLE 1 Stress Rupture Data-2000F Load Life Elong R. A. Process (ksi) (hrs) (9b) Failed Other 8 1.7 l.5 yes Invention 8 90.1 no 10 50 no 12 51.3 no 14 67.3 no l6 6.8 5.9 79 yes in the above Table l, the specimens processed in accordance with the present invention were step loaded from 8 thousand pounds per square inch (ksi) in increments to 16 ksi until failure occurred after about 7 hours. Comparison of these data show the significant advantage of the billet article prepared in accordance with the present invention over that prepared with the recommended heat treatment.

Although extensive thermomechanical processing involving large reductions under carefully controlled conditions has shown improvements in the properties of the TD Ni Cr type alloys, the present invention provides an improved method for achieving solid state grain growth without extensive thermomechanical processing. Thus, difficultto-fabricate oxide dispersion strengthened type alloys, such as those including large quantities of gamma prime or those including an embrittling rare earth addition, can be successfully produced in a wrought form by processing according to the present invention. In addition, fabricable alloys, such as the TD Ni Cr alloy, have been shown to be benefited such as in the form of complex extruded airfoil shapes, as in the following Example 2, which have been processed according to the present invention to give uniformly high properties throughout the cross section.

EXAMPLE 2 A complex airfoil article of the turbine blade type, of TD Ni Cr alloy, worked to a condition at which the material will transform to larger grains in the range of about 2,300 2,400F, when processed at about 2,425F through a gradient of about l,500F/in. at a rate of 5 inches/hr. produced the same large elongated grain structure as described above. Stress rupture testing at 2,000F after processing of such a structure resulted in no failure at 14 ksi after 300 hours. These specimens were step loaded to 15 ksi where no failure occurred after 50 hours of testing. it was not until the specimens were loaded to 16 ksi that they failed.

EXAMPLE 3 Additional specimens of the TD Ni Cr alloy were prepared in several different conditions. Specimens of one series were prepared from a sheet of about 0.06 inch in thickness and which had been rolled from about 1 inch thick at about 1,300F. Specimens of another series were made from a A inch diameter round bar which had been extruded at about /1 reduction at a higher temperature of 1,860F. Examination of the microstructure of both of these series of specimens by electron transmission microscopy showed that the sheet specimens had a high dislocation density estimated to be about dislocations/cm. Such dislocations which are line defect imperfections in the crystal lattice, were present as dense tangles of dislocation lines with a vague appearance of cell structure along with high hardness of about 40 Rockwell C. By way of contrast, examination of the bar specimens showed very few dislocations within a well defined cell structure and relatively low hardness of about 32 Rockwell C.

Specimens of both series were processed according to the present invention by passing through a steep thermal gradient created by an induction coil in apparatus of the type which will be described and discussed later in connection with FIG. 4. The apparatus was used to heat the specimen in a narrow zone as each specimen was moved through an induction heating coil. The thermal gradient maintained at the interface between the moving heated zone and that portion of the specimen adjacent the moving heated zone was about 2,000F/in. with the zone itself heated at a temperature of about 2,400F. It was found that the sheet specimens, transformed to large grains, as a result of the high dislocation density, at about 1,650F, at a rate of 0.1 inch/hr. However, the extruded bar specimens, havinga relatively low dislocation density, transformed at about 2,100F at an unusually rapid rate of 24 inch/hr.

Study of the grain structure of the two types of specimens, after processing, revealed that, despite the high dislocation density of the sheet specimens, their grains were increased from about 0.5 microns generally equiaxed to 14,000 X 1,000 X 200 microns. However, the grains of the bar specimens, having a much lower dislocation density, were increased significantly more from about 1 micron equi-axed to 100,000 X 3,000 microns.

Stress rupture specimens were prepared from the sheet and bar processed in this example and tested at 2,000F. Data from such tests are included in the following Table II along with standard test data for the same type of material but not processed according to the present invention.

(a) Failed on loading (b) Pin failure at 0.6% elong.

Data from these tests for the bar specimens are shown in the graphical presentation of FIG. 1. That figure shows variation in ductility with rate of processing and compares the process of the present invention with the other more conventional and normally used process of heating at 2,400F for 1 hour. It is easily seen that the practice of the method of the present invention results in a dramatic increase in ductility. These data were obtained from pieces of the same bar with different processing.

In the graphical presentation of FIG. 2, the stress rupture data for sheet were plotted for 0.5 percent creep and compared with data for commercial sheet on a Larson-Miller parameter plot. The great increase in that property, for example about 10 parameters at double stress, can be seen from FIG. 2.

From FIG. 3, it is seen that TD Ni Cr alloy processed according to the present invention has a dynamic modulus similar to that of commercial sheet. These data were generated from bar processed as in the Examples above and having a l00 texture and from commercial sheet having a 00l texture. Both are lower than randomly oriented material. Therefore, the thermal fatigue resistance properties of these oriented materials are substantially better than randomly oriented material and processing according to the present invention does not sacrifice fatigue resistance.

Thus, an important characteristic of the method of the present invention is the preconditioning of the article to be processed. This is accomplished to adjust the dislocation density such that the metal microstructure of the article will undergo transformation to grains, the largest dimension of which is at least about 200 microns, when heated sufficiently high in the temperature range of about 0.5 to less than IT That range is otherwise expressed herein as about 50 to less than 100 percent of the metal incipient melting temperature in degrees Rankine. It is preferred that the microstructure of this type of alloy be made up of well-defined cell structure prior to processing.

After processing according to the method of the present invention, the microstructure of this type of alloy, i.e., face-center cubic structure, was characterized, in addition to large columnar grains, by a very low dislocation density, substantially no change in the thoria size from that expected from ordinary processing and a very heavy density of annealing twins, sometimes called stacking faults, and which occur during recrystallization of face-center cubic structures.

EXAMPLE 4 Specimens wsr a pars frsmano h r 74 n ameter TD Ni Cr alloy extruded bar, preconditioned as was the bar in Example 3 and processed according to the present invention. Such processing involved heating the specimen at 2,400F while moving the heated zone in the specimen at the rate of 72 inch/hr. The thermal gradient between the heated zone and the adjacent portion of the specimen was about 1,500F/in. Similar large grains to those for the bar in Example 3 were generated.

Stress rupture testing of such specimens at 2,000F and 10 ksi showed no failure after 366 hours and an elongation of only 0.4 percent. These data and their comparison with normal stress rupture and 0.5 percent creep data for this material shows the present invention to increase such properties about 2 times normal.

EXAMPLE 5 An iron base alloy consisting nominally by weight of 15% Cr, 5% Al, 1% Ch, 1% Y with the balance Fe and incidental impurities and including 4 volume percent of Al O as an oxide dispersion strengthener was precon 2,425F while moving the heated zone in the specimen at the rate of about 1 inch/hr. The thermal gradient between the heated zone and the adjacent portion of the specimen was about l,500F/in. Grains of the specimens were transformed to l6,000 X 4,000 X 500 microns.

The specimens processed according to the present invention were stress rupture tested at 2,000F by first loading them at 4 ksi. After 1 hours there was no failure and they were step loaded to 4.5 ksi where failure occurred after 10.2 hours.

These data show that the present invention has provided this high temperature iron base alloy with at least a 30 50 percent improvement in rupture stress over data for sheet otherwise processed in the normal, commercially used manner. For example, sheet rolled in the same manner to a total reduction of 75 percent failed in stress rupture at 2,000F and a lower load of 3 ksi in only 0.8 hours. The size of grains of this specimen were duplex being 100 and 1,000 microns.

The data of these examples, typical of the transition triad elements Fe, Ni and Co, shows the significant property improvement attainable through the present invention. Similar grain transformation may be achieved in the alloys based on Ti and the refractory metals.

Another significant advantage of the present invention recognized through evaluation of the type described in the examples above is the fact that limits of initial processing according to this invention are more flexible and can tolerate more variation than can mate rial processed in other normal ways. Because of the criticality of processing according to previously known methods, it is not uncommon to be required to scrap material because of too great a variation in initial processing conditions. Evaluation of the present invention has shown that the grains of poorly processed specimens can be transformed to the desirable large grains the same as more accurately processed specimens, and even if the two conditions exist in the same specimen.

A diagrammatic, sectional view of one form of apparatus used to process specimen articles according to the method of the present invention is shown in FIG. 4. Such apparatus comprises a heat energy source such as water cooled copper, flat induction coil 10 which cooperates with a heat sink or cooling means such as water cooled copper chill block 12. In FIG. 4, the flat induction coil is separated from the chill block by an electrical insulator 14 which can act as a spacer as well. One form of such an insulator used in the practice of the present invention, is polytetratluoroethylene plastic, one form of which is commercially available as Teflon material. A heating chamber 16, within heat energy source 10 and electrical insulator 14, is contiguous with cooling chamber 18, within cooling means 12, in a manner which allows an article or a specimen such as shown generally at to pass through both chambers. The chambers are generally centrally located but can be of any shape desired to receive an article to be processed. When a chill block of the type shown at 12 in FIG. 4 is used, intimate contact between walls of the specimen and walls of the cooling chamber are desirable for efficient cooling.

Heat energy source 10 and cooling means 12 are controlled and coordinated to create the desired thermal gradient 22 in specimen 20. For example, energy to induction coil 10 is controlled from a high frequency generator 24 through an optical temperature controller 26. Flow rate and temperature of water through chill block core 28 is adjusted for the degree of cooling desired. Then the degree of heating and cooling is coordinated to provide the desired thermal gradient 22 at the interface between high temperature heated zone or first portion 30 and the cooler second portion 32 of specimen article 20.

Motion control means, such as a variable speed motor, shown by arrow 34, moves specimen article 20 at the desired rate for gain transformation from the cooling chamber 18 through the heating chamber 16. Thus narrow high temperature heated zone 30 and its thermal gradient interface 22 traverses the article as a result of relative movement between the article and the heated zone. In this way, grains are transformed in the directions of motion as the thermal gradient traverses the article, the grain size being substantially fixed once the grains have been transformed and have passed through the heated zone. The thermal gradient is further established through use of the cooling chamber through which the article first passes.

If desired, the heating apparatus of other portions of the apparatus can be enclosed in an atmosphere control chamber, not shown, to control oxidation of the heated article as it passes through and emerges from the heating chamber. For example, argon can be used to protect such emerging processed article.

The present invention, which has been described in certain embodiments as examples, can be applied to a variety of articles. One important application is to a wrought airfoil shaped member such as a turbine blade for gas turbine engines. The present invention can provide in such a wrought airfoil member large columnar grains substantially uniformly across a section of the airfoil. This structure is unattainable by known forming methods in that they produce smaller grains of lower strength properties generally at the trailing edge where the airfoil is worked to a greater degree.

What is claimed is:

1. A solid state method for producing in a high temperature metal alloy article a columnar grain structure and improved high temperature, high mechanical strength properties, comprising the steps of:

forming the article in a manner which places the alloy microstructure of the article in a condition of dislocation density such that the microstructure will undergo transformation to large grains, the largest dimension of which is at least about 200 microns when heated at a temperature in the range of about 50 percent to less than percent of the alloy incipient melting temperature in degrees Rankine; and then transforming the alloy microstructure to the large grains through progressive and selective heating of the article by heating a first portion of the article in the said temperature range below the alloy incipient melting temperature while cooling a second portion of the article adjacent the first portion to produce in the article a high temperature zone having a thermal gradient at the interface between the first and second portions of at least about 500F/in., the thermal gradient traversing the article from the second portion to the first portion in the direction desired for growth of the principal axis of the columnar grains.

2. The method of claim 1 in which:

the article is wrought;

the alloy of the article is based on an element selected from the group consisting of Fe, Ni, Co, Ti and the refractory metals, the alloy being capable of maintaining high temperature, high mechanical strength properties when operating at temperatures of at least about 50 percent to less than 100 percent of the metal incipient melting temperature in degrees Rankine;

the thermal gradient is in the range of 500 5,000F/ini; and

the rate of traverse of the thermal gradient is 0.5

l44 inch/hr.

3. The method of claim 2 in which:

the alloy is based on on element selected from the group consisting of Fe, Ni and Co; and,

the temperature of the high temperature zone is in the range of about 1,650 2,450F.

4. The method of claim 3 in which:

the alloy is one strengthened by elements which produce strengthening selected from the group consisting of gamma prime, oxide dispersion, rare earth addition and their combinations;

the thermal gradient is at least about 1,500F/in.; and

the temperature of the high temperature zone is in the range of about 2,000 2,450F.

5. The method of claim 4 in which:

the alloy is a nickel base oxide dispersion strengthened alloy; and,

the temperature of the high temperature zone is about 2,400 2,450F.

6. A wrought alloy article made by the method of claim 1:

having a columnar grain structure;

being capable of maintaining high temperature, high mechanical strength properties when operating at temperatures of at least about 50 percent of the alloy incipient melting temperature in degrees Rankine; and,

further characterized by having grains of at least about 2,000 microns in diameter and a length-todiameter ratio of at least about 10.

l IF k

Claims

2. The method of claim 1 in which: the article is wrought; the alloy of the article is based on an element selected from the group consisting of Fe, Ni, Co, Ti and the refractory metals, the alloy being capable of maintaining high temperature, high mechanical strength properties when operating at temperatures of at least about 50 percent to less than 100 percent of the metal incipient melting temperature in degrees Rankine; the thermal gradient is in the range of 500* - 5,000*F/in.; and the rate of traverse of the thermal gradient is 0.5 - 144 inch/hr.
3. The method of claim 2 in which: the alloy is based on en element selected from the group consisting of Fe, Ni and Co; and, the temperature of the high temperature zone is in the range of about 1,650* - 2,450*F.
4. The method of claim 3 in which: the alloy is one strengthened by elements which produce strengthening selected from the group consisting of gamma prime, oxide dispersion, rare earth addition and their combinations; the thermal gradient is at least about 1,500*F/in.; and the temperature of the high temperature zone is in the range of about 2,000* - 2,450*F.
5. The method of claim 4 in which: the alloy is a nickel base oxide dispersion strengthened alloy; and, the temperature of the high temperature zone is about 2,400* -2,450*F.
6. A wrought alloy article made by the method of claim 1: having a columnar grain structure; being capable of maintaining high temperature, high mechanical strength properties when operating at temperatures of at least about 50 percent of the alloy incipient melting temperature in degrees Rankine; and, further characterized by having grains of at least about 2,000 microns in diameter and a length-to-diameter ratio of at least about 10.