A magnetic-disk substrate has a pair of main surfaces and arithmetic average roughnesses Ra of the main surfaces are each 0.11 nm or less. Also, in surface unevenness of the main surfaces, an average area of regions occupied by a plurality of protrusions having a height of 0.1 nm or more from an average plane of the surface unevenness is 25 nm.sup.2/protrusion or less. The arithmetic average roughness Ra and the surface unevenness are measured using an atomic force microscope with a probe having a probe tip provided with a carbon nanofiber rod-shaped member.

An aluminum alloy plate for a magnetic disc substrate according to the present disclosure includes, in mass %, Mg: 3.0 to 8.0%, Cu: 0.002 to 0.150%, Zn: 0.05 to 0.60%, Fe: 0.001 to 0.060%, Si: 0.001 to 0.060%, Be: 0.00001 to 0.00200%, Cr: 0.200% or less, Mn: 0.500% or less, and Cl: 0.00300% or less, with the balance being Al and inevitable impurities, and an abundance of a Cr oxide having a maximum diameter of 3 to 10 m observed in a metal structure is 1 or less per single side of a disc.

An aluminum alloy substrate for a magnetic disk, wherein the sum of the circumferences of second phase particles having the longest diameter of 4 m or more and 30 m or less in the metal microstructure is 10 mm/mm.sup.2 or more.

An aluminum alloy substrate for a magnetic recording medium, the substrate including: Si in a range of 9.5 to 13.0% by mass or less and Cu in a range of 0.5 to 3.0% by mass or less, wherein a content of Fe is less than 0.01% by mass, the balance is Al, the substrate has a diameter in a range of 53 to 97 mm and a thickness in a range of 0.4 to 0.9 mm or less, and the substrate satisfies at least one of the following conditions (i) and (ii): (i) Sr is contained in the substrate in a range of 0.005% by mass or more and 0.1% by mass or less; and (ii) at least a part of the Si is present as Si particles, and an average particle diameter of particles having a longest diameter of 0.5 m or more among the Si particles is 2 m or less.

An aluminum alloy disc blank for a magnetic disc made of an aluminum alloy containing Fe: 0.005 to 1.800 mass % with the balance being Al and inevitable impurities, wherein a flatness change of the aluminum alloy disc blank for a magnetic disc when the aluminum alloy disc blank for a magnetic disc is held in the atmosphere at 50? C. or lower for 336 hours is 2.0 ?m or less.

A base for a magnetic recording medium, includes a substrate made of an Al alloy and having a surface, and a film made of a NiP-based alloy and plated on the surface of the substrate. The film has a thickness of 7 m or greater, and a ratio E/ is 29 or greater, where E [GPa] denotes the Young's modulus of the substrate, and [g/cm.sup.3] denotes a density of the substrate.

Disclosed are an aluminum alloy substrate for a magnetic disc, which includes an aluminum alloy consisting of Mg:4.5-10.0 mass % (hereinafter referred to as %), Be: 0.00001-0.00200%,Cu: 0.003-0.150%, Zn: 0.05-0.60%, Cr: 0.010-0.300%, Si: 0.060% or less, and Fe: 0.060% or less, with a balance being Al and an unavoidable impurity, an amount of an Mg-based oxide being 50 ppm or less, (I.sub.Be/I.sub.bulk)(C.sub.Be)0.1000% where (I.sub.Be) is a maximum optical emission intensity of Be in a surface depth direction using a glow discharge optical emission spectrometer (GDS) prior to performing a plating pretreatment, (I.sub.bulk) is a mean optical emission intensity of Be in an interior of a base material of the aluminum alloy prior to performing a plating pretreatment, and (C.sub.Be) is an amount of the Be, and a method of manufacturing the magnetic disc aluminum alloy substrate.

A base for a magnetic recording medium, includes a substrate made of an Al alloy and having a surface, and a film made of a NiWP-based alloy and plated on the surface of the substrate. The film includes W in a range of 15 mass % to 22 mass %, P in a range of 3 mass % to 10 mass %, and Pb in a range of 0.03 mass % to 0.08 mass %. The film has a thickness of 5 m or greater.

A base for a magnetic recording medium includes, a substrate made of an Al alloy, and a film made of a NiP-based alloy and provided on the substrate. The Al alloy of the substrate includes Mg in a range of 0.2 mass % to 6 mass %, Si in a range of 3 mass % to 17 mass %, Zn in a range of 0.05 mass % to 2 mass %, and Sr in a range of 0.001 mass % to 1 mass %. An average grain diameter of Si grains in an alloy structure of the substrate is 2 m or less. The film has a thickness of 10 m or greater. The substrate has an outer diameter of 53 mm or greater, a thickness of 0.9 mm or less, and a Young's modulus of 79 GPa or higher.

A magnetic disk substrate having a flat main surface, an end face, and a chamfered face formed between the main surface and the end face. The substrate has an offset portion, present on the main surface within a range of 92.0 to 97.0% in a radial direction from a center of the substrate. A distance from the center of the substrate to the end face of the substrate in a radial direction is 100%, the offset portion being raised or lowered with respect to a virtual straight line connecting two points on the main surface, set at positions of 92.0% and 97.0%. A maximum distance from the virtual straight line to the offset portion in a direction perpendicular to the virtual straight line is a maximum offset value.